Income Taxes - Reconciliation from statutory federal income taxes (Details) - USD ($) $ 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
Reconciliation from statutory federal income taxes to the company's effective income taxes
Statutory federal income tax provision									$ 34,803	$ 35,778	$ 22,863
State taxes, net of federal income tax effect									12,724	6,720	4,135
Cash surrender life insurance									(582)	(645)	(407)
Tax exempt interest									(1,135)	(1,660)	(764)
Merger costs									375	824	533
LIHTC investments									(761)	(1,031)	(909)
Effect of the Tax Act									(1,441)	5,633	0
Excess tax benefit of stock-based compensation									(1,811)	(1,995)	0
Prior year true-up									0	(1,108)	0
Other									68	(390)	(236)
Total income tax provision	$ 15,376	$ 7,798	$ 10,182	$ 8,884	$ 19,370	$ 10,619	$ 7,521	$ 4,616	$ 42,240	$ 42,126	$ 25,215