Condensed Financial Information (parent company only) - Statement of Income and Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income
Gain on the sale of insurance subsidiary											$ 8,265
Gain on cancellation of subordinated debentures											1,250
Net realized gains on sales of available for sale securities									2,631	3,389	2,670
Other income									2,970	3,594	1,778
Expenses
Interest expense	$ 11,583	$ 10,427	$ 8,367	$ 7,235	$ 6,631	$ 6,694	$ 6,742	$ 6,772	37,612	26,839	24,794
Salaries and employee benefits									119,812	102,552	101,908
Other expenses									15,543	15,470	15,187
INCOME BEFORE INCOME TAX	39,593	32,297	31,343	30,361	30,143	28,528	26,722	23,267
Income tax benefit (expense)	(15,210)	(7,939)	(7,207)	(7,168)	(7,850)	(7,469)	(6,716)	(5,574)	(37,524)	(27,609)	(25,665)
NET INCOME AVAILABLE TO COMMON STOCKHOLDERS									96,070	81,051	65,384
Net income	$ 24,383	$ 24,358	$ 24,136	$ 23,193	$ 22,293	$ 21,059	$ 20,006	$ 17,693	96,070	81,051	65,384
Other comprehensive income (loss)									10,673	(12,219)	268
Comprehensive income									106,743	68,832	65,652
Parent
Income
Dividends from subsidiaries									33,014	33,546	39,032
Gain on the sale of insurance subsidiary											8,265
Gain on cancellation of subordinated debentures											1,250
Net realized gains on sales of available for sale securities									(50)
Other income									350	143	197
Total income									33,314	33,689	48,744
Expenses
Interest expense									7,572	7,185	6,661
Salaries and employee benefits									4,118	639	3,023
Net occupancy and equipment expenses									797	832	464
Other outside services									1,810	967	1,455
Professional services									442	279	636
Other expenses									(385)	(470)	1,234
Total expenses									14,354	9,432	13,473
INCOME BEFORE INCOME TAX									18,960	24,257	35,271
Income tax benefit (expense)									5,946	3,243	(974)
Income before equity in undistributed income of subsidiaries									24,906	27,500	34,297
Equity in undistributed income of subsidiaries									71,164	53,551	31,087
NET INCOME AVAILABLE TO COMMON STOCKHOLDERS									96,070	81,051	65,384
Net income									96,070	81,051	65,384
Other comprehensive income (loss)									10,673	(12,219)	268
Comprehensive income									$ 106,743	$ 68,832	$ 65,652