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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic:
Net income	$ 171,416	$ 171,302	$ 485,820	$ 530,683
Basic weighted-average number of shares outstanding (in shares)	145,559	144,921	145,455	144,829
Basic EPS (in dollars per share)	$ 1.18	$ 1.18	$ 3.34	$ 3.66
Diluted:
Net income	$ 171,416	$ 171,302	$ 485,820	$ 530,683
Basic weighted-average number of shares outstanding (in shares)	145,559	144,921	145,455	144,829
Diluted potential common shares (in shares)	561	1,252	633	1,329
Diluted weighted-average number of shares outstanding (in shares)	146,120	146,173	146,088	146,158
Diluted EPS (in dollars per share)	$ 1.17	$ 1.17	$ 3.33	$ 3.63