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           Stock repurchase program (Details) (USD $) 
        In Millions, except Share data, unless otherwise specified  | 
        1 Months Ended | 9 Months Ended | 1 Months Ended | |||
|---|---|---|---|---|---|---|
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           Jun. 30, 2014 
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           May 31, 2014 
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           Nov. 30, 2011 
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           Jun. 30, 2014 
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           Jun. 30, 2013 
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           Aug. 08, 2014
 
          Subsequent Event [Member] 
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| Stock Repurchase Programs | ||||||
| Stock repurchase programs, authorized amount | $ 150 | $ 125 | ||||
| Amount remaining available for future stock repurchases | 193.7 | 43.7 | 187.3 | |||
| Common shares repurchased | 1,406,667 | 828,898 | 403,433 | |||
| Common shares repurchased, cost | $ 60.9 | $ 27.0 | $ 17.0 | |||
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- Details 
          
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- Definition 
          Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition 
          Equity impact of the value of stock that has been repurchased during the period and has not been retired and is not held in treasury. Some state laws may mandate the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition 
          Amount of a stock repurchase plan authorized by an entity's Board of Directors. No definition available. 
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- Definition 
          Amount remaining of a stock repurchase plan authorized by an entity's Board of Directors. No definition available. 
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