Common Stock (Details) - USD ($)	3 Months Ended	12 Months Ended			75 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	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 (in shares)	58,798,428	58,798,428	59,946,984		58,798,428
Dividend paid (in dollars per share)	$ 0.08
Stock repurchased (in US dollars)		$ 90,821,000	$ 88,568,000	$ 112,328,000
Common Stock
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)					12,800,000
Stock repurchased (in US dollars)		10,000	$ 15,000	$ 24,000	$ 575,200,000
Remaining authorized repurchase amount (in US dollars)	$ 174,800,000	$ 174,800,000			$ 174,800,000
Common Stock | RepurchaseProgramSept262011
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)										$ 100,000,000
Common Stock | RepurchaseProgramDec52013
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)									$ 100,000,000
Common Stock | RepurchaseProgramOct232014
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)								$ 250,000,000
Common Stock | RepurchaseProgramOct282015
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)							$ 200,000,000
Number of shares repurchased (in shares)		1,400,000
Stock repurchased (in US dollars)		$ 90,800,000
Common Stock | RepurchaseProgramOct252017
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)						$ 100,000,000