Parent Company Only Financial Statements - Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities
Net income	$ 52,398	$ 40,591	$ 47,856
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	6,760	4,693
Financing Activities
Dividends paid		(69,300)	(25,350)
Net proceeds from sale of common stock	153,356	116,054
Net change in cash and cash equivalents	(16,576)	38,604	47,769
Cash and cash equivalents at beginning of the period	136,327	97,723	49,954
Cash and cash equivalents at end of the period	119,751	136,327	97,723
Supplemental cash flow information:
Conversion of cash-settled to stock settled compensation		5,388
Parent Company
Operating Activities
Net income	52,398	40,591	47,856
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed income of subsidiary bank	(54,713)	(26,859)	(23,879)
Loss (gain) on investments	945	(417)
Stock-based compensation expense		4,693
(Increase) decrease in other assets	(2,439)	(427)	1,292
Increase (decrease) in other liabilities	(551)	(5,251)	23
Other, net		7	(1)
Net cash provided by operating activities	(4,360)	12,337	25,291
Investing Activities
Other investments		724	761
Net cash provided by investing activities		724	761
Financing Activities
Equity contribution to Bank	(154,200)	(20,000)
Dividends paid		(69,300)	(25,350)
Payment of subordinated debt		(10,075)
Net proceeds from sale of common stock	153,356	116,054
Net cash (used in) provided by financing activities	(844)	16,679	(25,350)
Net change in cash and cash equivalents	(5,204)	29,740	702
Cash and cash equivalents at beginning of the period	30,993	1,253	551
Cash and cash equivalents at end of the period	$ 25,789	30,993	$ 1,253
Supplemental cash flow information:
Conversion of cash-settled to stock settled compensation		5,388
Forgiveness of intercompany debt		$ 6,024