Common Stock (Details) - USD ($)	3 Months Ended		54 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 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	60,681,075		60,681,075	61,067,868
Common stock, issued	198,127	178,928
Stock repurchased (in US dollars)	$ 26,076,000	$ 21,148,000
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	$ 421,900,000
Remaining authorized repurchase amount (in US dollars)	$ 228,100,000		$ 228,100,000
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)	$ 26,100,000
Common Stock [Member] | RepurchaseProgramOct282015
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)					$ 200,000,000.0