Stockholders' Equity and Earnings Per Share (Earnings Per Share Calculation) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Basic:
Net income	$ 164,024	$ 187,032
Basic weighted average number of shares outstanding (in shares)	145,256	144,664
Basic EPS (in dollars per share)	$ 1.13	$ 1.29
Diluted:
Net income	$ 164,024	$ 187,032
Basic weighted average number of shares outstanding (in shares)	145,256	144,664
Diluted potential common shares (in shares)	665	1,275
Diluted weighted average number of shares outstanding (in shares)	145,921	145,939
Diluted EPS (in dollars per share)	$ 1.12	$ 1.28