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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic:
Net income	$ 173,018	$ 171,302	$ 172,349	$ 187,032	$ 84,898	$ 132,660	$ 118,330	$ 169,736	$ 703,701	$ 505,624	$ 431,677
Basic weighted average common shares outstanding (in shares)	144,960	144,921	144,899	144,664	144,542	144,498	144,485	144,249	144,862	144,444	144,087
Basic EPS (in dollars per share)	$ 1.19	$ 1.18	$ 1.19	$ 1.29	$ 0.59	$ 0.92	$ 0.82	$ 1.18	$ 4.86	$ 3.50	$ 3.00
Diluted:
Net income									$ 703,701	$ 505,624	$ 431,677
Basic weighted average common shares outstanding (in shares)	144,960	144,921	144,899	144,664	144,542	144,498	144,485	144,249	144,862	144,444	144,087
Diluted potential common shares (in shares)									1,307	1,469	1,085
Diluted weighted average common shares outstanding (in shares)	146,133	146,173	146,091	145,939	146,030	145,882	145,740	145,732	146,169	145,913	145,172
Diluted EPS (in dollars per share)	$ 1.18	$ 1.17	$ 1.18	$ 1.28	$ 0.58	$ 0.91	$ 0.81	$ 1.16	$ 4.81	$ 3.47	$ 2.97