Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic
Average common shares outstanding (in shares)	93,339,564	92,550,559
Net income	$ 250,127	$ 239,152
Basic net income per common share (in dollars per share)	$ 2.68	$ 2.58
Diluted
Average common shares outstanding (in shares)	93,339,564	92,550,559
Stock options and other contingently issuable shares (in shares)	2,138,874	1,931,574
Non-vested restricted stock grants (in shares)	67,714	59,726
Average common shares outstanding assuming dilution (in shares)	95,546,152	94,541,859
Diluted net income per common share (in dollars per share)	$ 2.62	$ 2.53
Stock options and other contingently issuable shares with anti-dilutive effects (in shares)	0	40,074