Parent Company Financial Information - STATEMENTS OF OPERATIONS (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income
Interest income	$ 136,021	$ 128,876	$ 92,699	$ 90,827	$ 85,684	$ 70,161	$ 68,733	$ 45,427	$ 448,423	$ 270,005	$ 166,605
Noninterest income	6,970	8,240	8,151	7,666	9,451	8,221	8,759	4,683	31,027	31,114	19,602
Expense
Interest expense	18,475	16,163	11,528	9,546	7,514	5,870	5,395	3,724	55,712	22,503	13,530
Noninterest expense	67,239	82,782	50,076	49,808	49,886	39,612	48,455	30,005	249,905	167,958	98,063
Net income before income taxes									165,580	102,226	65,318
Income tax	15,376	7,798	10,182	8,884	19,370	10,619	7,521	4,616	42,240	42,126	25,215
Net income	$ 39,643	$ 28,392	$ 27,303	$ 28,002	$ 16,171	$ 20,232	$ 14,176	$ 9,521	123,340	60,100	40,103
Corporation
Income
Interest income									57	36	31
Noninterest income									0	0	0
Total income									57	36	31
Expense
Interest expense									6,715	4,720	3,844
Noninterest expense									6,139	8,956	3,769
Total expense									12,854	13,676	7,613
Net income before income taxes									(12,797)	(13,640)	(7,582)
Income tax									(3,678)	(5,417)	(2,785)
Net loss (parent only)									(9,119)	(8,223)	(4,797)
Equity in net earnings of subsidiaries									132,459	68,323	44,900
Net income									$ 123,340	$ 60,100	$ 40,103