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Common Stock (Details) - USD ($)	3 Months Ended		6 Months Ended			81 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Oct. 25, 2017	Oct. 28, 2015	Oct. 23, 2014	Dec. 05, 2013	Sep. 26, 2011
Equity, Class of Treasury Stock [Line Items]
Common stock, outstanding	58,173,108		58,173,108			58,173,108	58,798,428
Common stock, issued	72,777	76,347	170,930		193,381
Stock repurchased (in US dollars)			$ 60,508,000	[1]	$ 63,430,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased						13,600,000
Stock repurchased (in US dollars)					$ 10,000	$ 635,800,000
Remaining authorized repurchase amount (in US dollars)	$ 114,200,000		$ 114,200,000			$ 114,200,000
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			796,250
Stock repurchased (in US dollars)			$ 60,500,000
Common Stock [Member] | RepurchaseProgramOct252017
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)								$ 100,000,000
[1] Beginning June 1, 2017, shares of common stock repurchased are held as treasury stock by the Company.

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

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