Net Income Per Common Share - Basic and Diluted Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic
Net income applicable to common stock	$ 36,710	$ 25,284	$ 70,536	$ 49,256
Average common shares outstanding (shares)	49,413,754	44,415,423	49,385,244	44,390,021
Net income per common share - basic (usd per share)	$ 0.74	$ 0.57	$ 1.43	$ 1.11
Diluted
Net income applicable to common stock	$ 36,710	$ 25,284	$ 70,536	$ 49,256
Average common shares outstanding (shares)	49,413,754	44,415,423	49,385,244	44,390,021
Effect of dilutive stock-based compensation (shares)	136,007	108,118	136,801	110,259
Average common shares outstanding - diluted (shares)	49,549,761	44,523,541	49,522,045	44,500,280
Net income per common share - diluted (usd per share)	$ 0.74	$ 0.57	$ 1.42	$ 1.11