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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic
Net income	$ 172,349	$ 118,330	$ 359,381	$ 288,066
Basic weighted average number of shares outstanding (in shares)	144,899	144,485	144,782	144,368
Basic EPS (in dollars per share)	$ 1.19	$ 0.82	$ 2.48	$ 2.00
Diluted
Net income	$ 172,349	$ 118,330	$ 359,381	$ 288,066
Basic weighted average number of shares outstanding (in shares)	144,899	144,485	144,782	144,368
Diluted potential common shares (in shares)	1,192	1,255	1,264	1,406
Diluted weighted average number of shares outstanding (in shares)	146,091	145,740	146,046	145,774
Diluted EPS (in dollars per share)	$ 1.18	$ 0.81	$ 2.46	$ 1.98