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Common Stock (Details) - USD ($)	3 Months Ended		9 Months Ended		57 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2016	Dec. 31, 2015	Oct. 28, 2015	Oct. 23, 2014	Dec. 05, 2013	Sep. 26, 2011
Equity, Class of Treasury Stock [Line Items]
Common stock, outstanding	60,786,064		60,786,064			61,067,868
Common stock, issued	47,050	66,223	353,116	347,713
Stock repurchased (in US dollars)			$ 29,028,000	$ 22,972,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased					10,500,000
Stock repurchased (in US dollars)			6,000	$ 5,000	$ 424,900,000
Remaining authorized repurchase amount (in US dollars)	$ 225,100,000		$ 225,100,000
Common Stock [Member] | RepurchaseProgramSep262011
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)										$ 100,000,000
Common Stock [Member] | RepurchaseProgramDec052013
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)									$ 100,000,000
Common Stock [Member] | RepurchaseProgramOct232014
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)								$ 250,000,000
Number of shares repurchased			600,000
Stock repurchased (in US dollars)			$ 29,000,000
Common Stock [Member] | RepurchaseProgramOct282015
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)							$ 200,000,000

X

- Definition

Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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