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, 2018	Mar. 31, 2017
Basic
Net income	$ 187,032	$ 169,736
Basic weighted average number of shares outstanding (in shares)	144,664	144,249
Basic EPS (in dollars per share)	$ 1.29	$ 1.18
Diluted
Net income	$ 187,032	$ 169,736
Basic weighted average number of shares outstanding (in shares)	144,664	144,249
Diluted potential common shares (in shares)	1,275	1,483
Diluted weighted average number of shares outstanding (in shares)	145,939	145,732
Diluted EPS (in dollars per share)	$ 1.28	$ 1.16