Parent company only financial statements - Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Activities
Net income	$ 80,236	$ 52,398	$ 40,591
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on sale of other assets	(328)	664	103
Stock-based compensation expense	7,207	6,760	4,693
Net cash provided by (used in) operating activities	212,208	37,610	(245,201)
Investing Activities
Net cash used in investing activities	(586,501)	(267,168)	(32,741)
Financing Activities
Payment of dividends	(6,137)	0	(69,300)
Net proceeds from sale of common stock	1,196	153,356	116,054
Net cash provided by financing activities	379,898	212,982	316,546
Net change in cash and cash equivalents	5,605	(16,576)	38,604
Cash and cash equivalents at beginning of the period	119,751	136,327	97,723
Cash and cash equivalents at end of the period	125,356	119,751	136,327
Supplemental cash flow information:
Dividends declared not paid on restricted stock units	(226)	0	0
Noncash dividend from Bank	572	0	0
Conversion of cash-settled to stock settled compensation	0	0	5,388
Parent Company
Operating Activities
Net income	80,236	52,398	40,591
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed income of subsidiary bank	(83,285)	(54,713)	(26,859)
Loss (gain) on investments	0	945	(417)
Gain on sale of other assets	(297)	0	0
Stock-based compensation expense	7,207	0	4,693
Increase in other assets	(441)	(2,439)	(427)
Increase (decrease) in other liabilities	(7,737)	(551)	(5,251)
Other, net	0	0	7
Net cash provided by (used in) operating activities	(4,317)	(4,360)	12,337
Investing Activities
Proceeds from sale of other assets	869	0	0
Other investments	0	0	724
Net cash used in investing activities	869	0	724
Financing Activities
Equity contribution to Bank	0	(154,200)	(20,000)
Payment of dividends	(6,137)	0	(69,300)
Payment of borrowings	0	0	(10,075)
Net proceeds from sale of common stock	1,196	153,356	116,054
Net cash provided by financing activities	(4,941)	(844)	16,679
Net change in cash and cash equivalents	(8,389)	(5,204)	29,740
Cash and cash equivalents at beginning of the period	25,789	30,993	1,253
Cash and cash equivalents at end of the period	17,400	25,789	30,993
Supplemental cash flow information:
Conversion of cash-settled to stock settled compensation	0	0	5,388
Forgiveness of intercompany debt	$ 0	$ 0	$ 6,024