| 
           Acquisitions (Details 2) (USD $) 
        In Thousands, unless otherwise specified  | 
        3 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 1 Months Ended | 1 Months Ended | 1 Months Ended | |||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 
           Sep. 30, 2012 
         | 
        
           Jun. 30, 2012 
         | 
        
           Mar. 31, 2012 
         | 
        
           Dec. 31, 2011 
         | 
        
           Sep. 30, 2011 
         | 
        
           Jun. 30, 2011 
         | 
        
           Mar. 31, 2011 
         | 
        
           Dec. 31, 2010 
         | 
        
           Sep. 30, 2012 
         | 
        
           Sep. 30, 2011 
         | 
        
           Sep. 30, 2010 
         | 
        
           Apr. 30, 2012
 
          Policy Studies, Inc 
         | 
        
           Sep. 30, 2012
 
          Policy Studies, Inc 
         | 
        
           Feb. 28, 2010
 
          DeltaWare 
         | 
        
           Feb. 10, 2010
 
          DeltaWare 
         | 
        
           Apr. 30, 2012
 
          Customer relationships
 
          Policy Studies, Inc 
         | 
        
           Apr. 30, 2012
 
          Customer relationships
 
          Policy Studies, Inc
 
          Minimum 
         | 
        
           Apr. 30, 2012
 
          Customer relationships
 
          Policy Studies, Inc
 
          Maximum 
         | 
        
           Apr. 30, 2012
 
          Tradename
 
          Policy Studies, Inc 
         | 
        
           Feb. 28, 2010
 
          Tradename
 
          DeltaWare 
         | 
        
           Feb. 10, 2010
 
          Tradename
 
          DeltaWare 
         | 
        
           Apr. 30, 2012
 
          Technology-based intangible assets
 
          Policy Studies, Inc 
         | 
        
           Apr. 30, 2012
 
          Technology-based intangible assets
 
          Policy Studies, Inc
 
          Minimum 
         | 
        
           Apr. 30, 2012
 
          Technology-based intangible assets
 
          Policy Studies, Inc
 
          Maximum 
         | 
        
           Feb. 28, 2010
 
          Technology-based intangible assets
 
          DeltaWare 
         | 
        
           Feb. 10, 2010
 
          Technology-based intangible assets
 
          DeltaWare 
         | 
        
           Feb. 10, 2010
 
          Customer contracts and relationships
 
          DeltaWare 
         | 
        
           Feb. 28, 2010
 
          Customer contracts and relationships
 
          DeltaWare
 
          Minimum 
         | 
        
           Feb. 28, 2010
 
          Customer contracts and relationships
 
          DeltaWare
 
          Maximum 
         | 
        
           Feb. 28, 2010
 
          Non-compete arrangements
 
          DeltaWare 
         | 
        
           Feb. 10, 2010
 
          Non-compete arrangements
 
          DeltaWare 
         | 
      |
| Intangible assets acquired | |||||||||||||||||||||||||||||||
| Useful life | 6 years 9 months 18 days | 8 years 7 months 6 days | 7 years | 8 years | 5 years | 10 years | 1 year | 5 years | 8 years 6 months | 8 years | 10 years | 4 years | |||||||||||||||||||
| Fair value | $ 22,183 | $ 6,060 | $ 16,600 | $ 3,800 | $ 614 | $ 1,783 | $ 3,733 | $ 1,474 | $ 239 | ||||||||||||||||||||||
| Fair value of the future estimated obligation related contract | 3,015 | ||||||||||||||||||||||||||||||
| Revenue and net income | |||||||||||||||||||||||||||||||
| Revenue | 300,736 | 266,353 | 243,452 | 239,603 | 250,107 | 238,296 | 227,116 | 214,114 | 1,050,145 | 929,633 | 831,749 | 60,100 | |||||||||||||||||||
| Operating income | $ 39,973 | $ 33,299 | $ 27,398 | $ 26,905 | $ 34,836 | $ 30,779 | $ 29,494 | $ 27,292 | $ 127,575 | $ 122,401 | $ 107,406 | ||||||||||||||||||||