Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic:
Net income available to common shareholders	$ 35,715	$ 27,017
Average common shares outstanding (in shares)	34,261	34,070
Basic net income per common share (in dollars per share)	$ 1.04	$ 0.79
Diluted:
Net income available to common shareholders	$ 35,715	$ 27,017
Average common shares outstanding (in shares)	34,261	34,070
Adjustment for common share equivalents (in shares)	145	214
Average common shares outstanding-diluted (in shares)	34,406	34,284
Diluted net income per common share (in dollars per share)	$ 1.04	$ 0.79
Anti-dilutive shares (in shares)		7