Income Taxes - Differences between provision for income taxes and amount computed as statutory rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Differences between the provision for income taxes and statutory federal income tax rate
Pretax income at statutory rates	$ 50,130	$ 45,344	$ 60,492
Add (deduct):
State taxes, net of federal benefit	7,168	6,765	4,139
Bank owned life insurance earnings	(1,127)	(747)	(1,141)
Adjustment to reserve for uncertain tax positions	84	80	59
Tax-exempt interest revenue	(1,827)	(1,229)	(1,199)
Equity compensation	(375)	(892)	(799)
Transaction costs	16	78	408
Tax credit investments	(464)	(29)	(89)
Change in state statutory tax rate	0	583	81
Increase in valuation allowance	0	0	413
Release of disproportionate tax effects related to de-designated cash flow hedges	0	0	3,400
Expense due to enactment of federal tax reform	0	0	38,198
Other	(614)	(138)	1,051
Total income tax expense	$ 52,991	$ 49,815	$ 105,013