Net Income Per Common Share - Basic and Diluted Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic
Net income applicable to common stock	$ 33,826	$ 23,972
Average common shares outstanding (shares)	49,356,417	44,364,337
Net income per common share - basic (usd per share)	$ 0.69	$ 0.54
Diluted
Net income applicable to common stock	$ 33,826	$ 23,972
Average common shares outstanding (shares)	49,356,417	44,364,337
Effect of dilutive stock-based compensation (shares)	146,533	116,162
Average common shares outstanding - diluted (shares)	49,502,950	44,480,499
Net income per common share - diluted (usd per share)	$ 0.68	$ 0.54