Common Stock (Details) - USD ($)	3 Months Ended		9 Months Ended		72 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Oct. 25, 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	58,806,509		58,806,509		58,806,509		59,946,984
Common stock, issued	19,679	47,050	213,060	353,116
Stock repurchased (in US dollars)			$ 88,599,000	$ 29,028,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased					12,800,000
Stock repurchased (in US dollars)			10,000	$ 6,000	$ 573,000,000
Remaining authorized repurchase amount (in US dollars)	$ 77,000,000		$ 77,000,000		$ 77,000,000
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			1,400,000
Stock repurchased (in US dollars)			$ 88,600,000
Subsequent Event [Member] | Common Stock [Member] | RepurchaseProgramOct252017
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)						$ 100,000,000