Stockholders' Equity - Summary of the Shares Repurchased under the Stock Repurchase Program (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended				132 Months Ended	180 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2014
Summary of the shares repurchased under stock repurchase Program
Shares repurchased	733	500	823	750	30,797	33,603
Total cost	$ 19,106	$ 10,129	$ 13,349	$ 9,930	$ 733,562	$ 786,076
Weighted-average price per share	$ 26.05	$ 20.23	$ 16.23	$ 13.24	$ 23.82	$ 23.39