Net Income Per Common Share - Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic
Net income applicable to common stock	$ 44,420	$ 31,964	$ 36,710	$ 33,826	$ 16,511	$ 26,421	$ 25,284	$ 23,972	$ 146,920	$ 92,188	$ 90,930
Average common shares outstanding (shares)									52,492,104	46,874,502	41,737,636
Net income per common share - basic (usd per share)	$ 0.76	$ 0.61	$ 0.74	$ 0.69	$ 0.33	$ 0.54	$ 0.57	$ 0.54	$ 2.80	$ 1.97	$ 2.18
Diluted
Net income applicable to common stock	$ 44,420	$ 31,964	$ 36,710	$ 33,826	$ 16,511	$ 26,421	$ 25,284	$ 23,972	$ 146,920	$ 92,188	$ 90,930
Average common shares outstanding (shares)									52,492,104	46,874,502	41,737,636
Effect of dilutive stock-based compensation (shares)									134,746	127,014	251,819
Average common shares outstanding - diluted (shares)									52,626,850	47,001,516	41,989,455
Net income per common share - diluted (usd per share)	$ 0.76	$ 0.61	$ 0.74	$ 0.68	$ 0.33	$ 0.53	$ 0.57	$ 0.54	$ 2.79	$ 1.96	$ 2.17