Income Taxes - Reconciliation from statutory federal income taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation from statutory federal income taxes to the company's effective income taxes
Statutory federal income tax provision									$ 45,729	$ 34,803	$ 35,778
State taxes, net of federal income tax effect									15,764	12,724	6,720
Cash surrender life insurance									(565)	(582)	(645)
Tax exempt interest									(1,503)	(1,135)	(1,660)
Non-deductible merger costs									0	375	824
LIHTC investments									(1,570)	(761)	(1,031)
Effect of the Tax Act									0	(1,441)	5,633
Excess tax benefit of stock-based compensation									(728)	(1,811)	(1,995)
Prior year true-up									0	0	(1,108)
Other									908	68	(390)
Total income tax provision	$ 13,109	$ 15,492	$ 14,168	$ 15,266	$ 15,376	$ 7,798	$ 10,182	$ 8,884	$ 58,035	$ 42,240	$ 42,126