Stockholders’ Equity and Earnings Per Share (Earnings Per Share Calculations) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic
Net income	$ 84,898	$ 132,660	$ 118,330	$ 169,736	$ 110,734	$ 110,143	$ 103,284	$ 107,516	$ 505,624	$ 431,677	$ 384,677
Less: earnings allocated to participating securities									0	0	(3)
Net income allocated to common stockholders									$ 505,624	$ 431,677	$ 384,674
Basic weighted average common shares outstanding (in shares)	144,542	144,498	144,485	144,249	144,166	144,122	144,101	143,958	144,444	144,087	143,818
Basic EPS (in dollars per share)	$ 0.59	$ 0.92	$ 0.82	$ 1.18	$ 0.77	$ 0.76	$ 0.72	$ 0.75	$ 3.50	$ 3.00	$ 2.67
Diluted
Net income allocated to common stockholders									$ 505,624	$ 431,677	$ 384,680
Basic weighted average common shares outstanding (in shares)	144,542	144,498	144,485	144,249	144,166	144,122	144,101	143,958	144,444	144,087	143,818
Diluted potential common shares (in shares)									1,469	1,085	694
Diluted weighted average common shares outstanding (in shares)	146,030	145,882	145,740	145,732	145,428	145,238	145,078	144,803	145,913	145,172	144,512
Diluted EPS (in dollars per share)	$ 0.58	$ 0.91	$ 0.81	$ 1.16	$ 0.76	$ 0.76	$ 0.71	$ 0.74	$ 3.47	$ 2.97	$ 2.66