Net Income Per Share Calculation of Net Income Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net income	$ 84,053	$ 85,184	$ 75,140	$ 61,551	$ 66,885	$ 66,358	$ 63,089	$ 55,863	$ 305,928	$ 252,195	$ 209,942
Income allocated to participating securities-basic									0	3	1,081
Net income available to common stockholders-basic									305,928	252,192	208,861
Basic weighted average shares									134,358	136,153	138,201
Basic net income per share	$ 0.63	$ 0.64	$ 0.56	$ 0.45	$ 0.49	$ 0.49	$ 0.46	$ 0.41	$ 2.28	$ 1.85	$ 1.51
Income allocated to participating securities-diluted									0	3	1,075
Net income available to common stockholders—diluted	$ 84,053	$ 85,184	$ 75,140	$ 61,551	$ 66,885	$ 66,358	$ 63,088	$ 55,861	$ 305,928	$ 252,192	$ 208,867
Basic weighted average shares									134,358	136,153	138,201
Dilutive effect of potential common shares									1,183	1,436	1,208
Diluted weighted average shares									135,541	137,589	139,409
Diluted net income per share	$ 0.62	$ 0.63	$ 0.55	$ 0.45	$ 0.49	$ 0.48	$ 0.46	$ 0.40	$ 2.26	$ 1.83	$ 1.50