Net Income Per Share - Calculation of Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic net income per share:
Net income	$ 93,443	$ 96,725	$ 89,706	$ 77,922	$ 84,053	$ 85,184	$ 75,140	$ 61,551	$ 357,796	$ 305,928	$ 252,195
Income allocated to participating securities-basic									0	0	3
Net income available to common stockholders-basic									$ 357,796	$ 305,928	$ 252,192
Basic weighted average shares (in shares)									131,749	134,358	136,153
Basic net income per share (in USD per share)	$ 0.71	$ 0.74	$ 0.68	$ 0.59	$ 0.63	$ 0.64	$ 0.56	$ 0.45	$ 2.72	$ 2.28	$ 1.85
Earnings Per Share, Diluted [Abstract]
Income allocated to participating securities-diluted									$ 0	$ 0	$ 3
Net income available to common stockholders—diluted									$ 357,796	$ 305,928	$ 252,192
Basic weighted average shares (in shares)									131,749	134,358	136,153
Dilutive effect of potential common shares (in shares)									1,181	1,183	1,436
Diluted weighted average shares (in shares)									132,930	135,541	137,589
Diluted net income per share (in USD per share)	$ 0.71	$ 0.73	$ 0.67	$ 0.58	$ 0.62	$ 0.63	$ 0.55	$ 0.45	$ 2.69	$ 2.26	$ 1.83