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           Stock repurchase programs (Details) (USD $) 
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        0 Months Ended | 12 Months Ended | 1 Months Ended | |||
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           Nov. 16, 2012 
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           Sep. 30, 2012 
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           Sep. 30, 2011 
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           Sep. 30, 2010 
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           Sep. 30, 2010
 
          2008 and 2010 Plan 
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           Nov. 30, 2011
 
          2011 Plan 
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| Stock Repurchase Programs | ||||||
| Stock repurchase programs, authorized amount | $ 175,000,000 | $ 125,000,000 | ||||
| Common shares repurchased | 168,500 | 306,000 | 1,597,458 | 1,501,528 | ||
| Common shares repurchased, cost | 9,700,000 | 12,796,000 | 57,484,000 | 40,217,000 | ||
| Amount remaining available for future stock repurchases | $ 127,400,000 | |||||
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- Details 
          
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- Definition 
          The amount authorized by an entity's Board of Directors under a stock repurchase plan. No definition available. 
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- Definition 
          The remaining repurchase amount authorized by an entity's Board of Directors under a stock repurchase plan. No definition available. 
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- Definition 
          Number of shares that have been repurchased during the period and are being held in treasury. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition 
          Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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