Stockholders' Equity and Earnings Per Share (Earnings Per Share Calculation) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic:
Net income	$ 150,380	$ 172,349	$ 314,404	$ 359,381
Basic weighted average number of shares outstanding (in shares)	145,546	144,899	145,402	144,782
Basic EPS (in dollars per share)	$ 1.03	$ 1.19	$ 2.16	$ 2.48
Diluted:
Net income	$ 150,380	$ 172,349	$ 314,404	$ 359,381
Basic weighted average number of shares outstanding (in shares)	145,546	144,899	145,402	144,782
Diluted potential common shares (in shares)	506	1,192	614	1,264
Diluted weighted average number of shares outstanding (in shares)	146,052	146,091	146,016	146,046
Diluted EPS (in dollars per share)	$ 1.03	$ 1.18	$ 2.15	$ 2.46