Parent company only financial statements - Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Activities
Net income	$ 83,814	$ 80,236	$ 52,398
Adjustments to reconcile net income to net cash provided by operating activities:
Loss (gain) on other assets	104	(328)	664
Stock-based compensation expense	7,089	7,207	6,760
Net cash provided by operating activities	63,905	212,208	37,610
Investing Activities
Net cash used in investing activities	(182,913)	(586,501)	(267,168)
Financing Activities
Payment of dividends	(10,045)	(6,137)	0
Net proceeds from sale of common stock	804	1,196	153,356
Net cash provided by financing activities	226,333	379,898	212,982
Cash and cash equivalents at beginning of the period	125,356
Cash and cash equivalents at end of the period	232,681	125,356
Supplemental cash flow information:
Dividends declared not paid on restricted stock units	149	226	0
Noncash dividend from Bank	0	572	0
Parent Company
Operating Activities
Net income	83,814	80,236	52,398
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed income of subsidiary bank	(85,750)	(83,285)	(54,713)
Loss on investments	0	0	945
Loss (gain) on other assets	16	(297)	0
Stock-based compensation expense	7,089	7,207	0
Decrease (increase) in other assets	1,056	(441)	(2,439)
Decrease in other liabilities	(9,711)	(7,737)	(551)
Net cash provided by operating activities	(3,486)	(4,317)	(4,360)
Investing Activities
Proceeds from sale of other assets	0	869	0
Net cash used in investing activities	0	869	0
Financing Activities
Equity contribution to Bank	0	0	(154,200)
Payment of dividends	(10,045)	(6,137)	0
Net proceeds from sale of common stock	804	1,196	153,356
Net cash provided by financing activities	(9,241)	(4,941)	(844)
Net change in cash and cash equivalents	(12,727)	(8,389)	(5,204)
Cash and cash equivalents at beginning of the period	17,400	25,789	30,993
Cash and cash equivalents at end of the period	$ 4,673	$ 17,400	$ 25,789