Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic
Net income applicable to common stock	$ 16,511	$ 26,421	$ 25,284	$ 23,972	$ 23,635	$ 23,179	$ 22,900	$ 21,216	$ 92,188	$ 90,930	$ 68,014
Average common shares outstanding (shares)									46,874,502	41,737,636	35,971,877
Net income per common share - basic (usd per share)	$ 0.33	$ 0.54	$ 0.57	$ 0.54	$ 0.56	$ 0.55	$ 0.54	$ 0.53	$ 1.97	$ 2.18	$ 1.89
Diluted
Net income applicable to common stock	$ 16,511	$ 26,421	$ 25,284	$ 23,972	$ 23,635	$ 23,179	$ 22,900	$ 21,216	$ 92,188	$ 90,930	$ 68,014
Average common shares outstanding (shares)									46,874,502	41,737,636	35,971,877
Effect of dilutive stock-based compensation (shares)									127,014	251,819	255,562
Average common shares outstanding - diluted (shares)									47,001,516	41,989,455	36,227,439
Net income per common share - diluted (usd per share)	$ 0.33	$ 0.53	$ 0.57	$ 0.54	$ 0.55	$ 0.55	$ 0.54	$ 0.52	$ 1.96	$ 2.17	$ 1.88