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Common Stock (Details) - USD ($)	3 Months Ended		9 Months Ended			84 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018	Oct. 23, 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	57,751,630		57,751,630			57,751,630		58,798,428
Common stock, issued	47,125	19,679	218,055		213,060
Stock repurchased (in US dollars)			$ 95,639,000	[1]	$ 88,599,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased						14,100,000
Stock repurchased (in US dollars)					$ 10,000	$ 670,900,000
Remaining authorized repurchase amount (in US dollars)	$ 79,100,000		$ 79,100,000			$ 79,100,000
Common Stock [Member] | RepurchaseProgramSep262011
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)													$ 100,000,000.0
Common Stock [Member] | RepurchaseProgramDec052013
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)												$ 100,000,000.0
Common Stock [Member] | RepurchaseProgramOct232014
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)											$ 250,000,000.0
Common Stock [Member] | RepurchaseProgramOct282015
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)										$ 200,000,000.0
Number of shares repurchased			1,264,853
Stock repurchased (in US dollars)			$ 95,600,000
Common Stock [Member] | RepurchaseProgramOct252017
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)									$ 100,000,000.0
Subsequent Event | Common Stock [Member] | RepurchaseProgramOct232018
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)							$ 200,000,000.0
[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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