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Common Stock (Details) - USD ($)	3 Months Ended		9 Months Ended		96 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Sep. 26, 2011
Equity, Class of Treasury Stock [Line Items]
Common stock, outstanding	56,147,875		56,147,875		56,147,875	55,983,763
Common stock, issued	18,651	47,125	164,112	218,055
Common stock, dividend, per share			$ 0.08
Stock repurchased (in US dollars)		$ 35,131,000		$ 95,639,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased			0		15,900,000
Stock repurchased (in US dollars)					$ 791,500,000
Remaining authorized repurchase amount (in US dollars)	$ 158,500,000		$ 158,500,000		158,500,000
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] | RepurchaseProgramSep2011toSep2019
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)	$ 950,000,000.0		$ 950,000,000.0		$ 950,000,000.0

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Aggregate dividends paid during the period for each share of common stock outstanding.

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