Stockholders' Equity and Earnings Per Share (Earnings Per Share Calculation) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic
Net income	$ 171,302	$ 132,660	$ 530,683	$ 420,726
Basic weighted average number of shares outstanding (in shares)	144,921	144,498	144,829	144,412
Basic EPS (in dollars per share)	$ 1.18	$ 0.92	$ 3.66	$ 2.91
Diluted
Net income	$ 171,302	$ 132,660	$ 530,683	$ 420,726
Basic weighted average number of shares outstanding (in shares)	144,921	144,498	144,829	144,412
Diluted potential common shares (in shares)	1,252	1,384	1,329	1,437
Diluted weighted average number of shares outstanding (in shares)	146,173	145,882	146,158	145,849
Diluted EPS (in dollars per share)	$ 1.17	$ 0.91	$ 3.63	$ 2.88