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Common Stock (Details) - USD ($)	3 Months Ended		6 Months Ended		69 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Dec. 31, 2016	Oct. 28, 2015	Oct. 23, 2014	Dec. 05, 2013	Sep. 26, 2011
Equity, Class of Treasury Stock [Line Items]
Common stock, outstanding	59,171,463		59,171,463		59,171,463	59,946,984
Common stock, issued	76,347	107,939	193,381	306,066
Stock repurchased (in US dollars)			$ 63,430,000	$ 26,076,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased					12,400,000
Stock repurchased (in US dollars)			10,000	$ 6,000	$ 547,900,000
Remaining authorized repurchase amount (in US dollars)	$ 102,100,000		$ 102,100,000		$ 102,100,000
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
Common Stock [Member] | RepurchaseProgramOct282015
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)							$ 200,000,000
Number of shares repurchased			1,000,000
Stock repurchased (in US dollars)			$ 63,400,000

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
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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Gross number of shares (or other type of equity) issued during the period as a result of any equity-based compensation plan other than an employee stock ownership plan (ESOP). Shares issued could result from the issuance of restricted stock, the exercise of stock options, stock issued under employee stock purchase plans, and/or other employee benefit plans.

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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-Subparagraph (SX 210.5-02(28))
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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.

+ References

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