Parent Company Only Financial Information (Income Statements) (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
Operating revenues:
Securities gains, net									$ 1,085	$ 6,973	$ 11,340
Operating expenses:
Interest									51,866	33,350	31,813
Salaries and employee benefits									196,118	171,407	163,547
Professional fees									39,811	32,879	27,676
Income (loss) before income taxes									145,213	119,092	116,905
Income tax benefit	$ (6,685)	$ (8,956)	$ (7,451)	$ (5,123)	$ (21,506)	$ (8,725)	$ (8,059)	$ (5,530)	(28,215)	(43,820)	(36,556)
NET INCOME	32,141	33,710	27,879	23,268	13,672	21,632	21,958	18,010	116,998	75,272	80,349
Preferred dividends	0	(13)	(13)	(13)	(13)	(13)	(13)	(19)	(39)	(58)	(292)
Interest expense on convertible preferred debt					0	3	4	5	0	12	51
NET INCOME AVAILABLE TO COMMON STOCKHOLDERS	$ 32,141	$ 33,697	$ 27,866	$ 23,255	$ 13,659	$ 21,622	$ 21,949	$ 17,996	116,959	75,226	80,108
Parent
Operating revenues:
Dividends from subsidiaries									85,000	70,850	55,250
Securities gains, net									0	3,021	54
Other									493	2,292	1,712
Total operating revenues									85,493	76,163	57,016
Operating expenses:
Interest									14,371	13,269	13,840
Salaries and employee benefits									3,639	3,146	3,044
Professional fees									2,841	2,379	2,487
Other operating expenses									12,510	7,889	2,664
Total operating expenses									33,361	26,683	22,035
Equity in undistributed earnings									52,570	16,212	37,926
Income (loss) before income taxes									104,702	65,692	72,907
Income tax benefit									12,296	9,580	7,442
NET INCOME									116,998	75,272	80,349
Preferred dividends									(39)	(58)	(292)
Interest expense on convertible preferred debt									0	12	51
NET INCOME AVAILABLE TO COMMON STOCKHOLDERS									$ 116,959	$ 75,226	$ 80,108