Income Taxes - Reconciliation from statutory federal income taxes (Details) - USD ($) $ 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
Income Tax Disclosure [Abstract]
Federal statutory rate (percent)									35.00%
Reconciliation from statutory federal income taxes to the company's effective income taxes
Statutory federal income tax provision									$ 35,778	$ 22,863	$ 14,253
State taxes, net of federal income tax effect									6,720	4,135	2,886
Cash surrender life insurance									(645)	(407)	(483)
Tax exempt interest									(1,660)	(764)	(742)
Merger costs									824	533	447
LIHTC investments									(1,031)	(909)	(871)
Effect of the Tax Act									5,633	0	0
Excess tax benefit of stock-based compensation									(1,995)	0	0
Prior year true-up									(1,108)	0	0
Other									(390)	(236)	(281)
Total income tax provision	$ 19,370	$ 10,619	$ 7,521	$ 4,616	$ 7,238	$ 5,877	$ 6,358	$ 5,742	$ 42,126	$ 25,215	$ 15,209