Parent Company Only Financial Information (Income Statements) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating revenues:
Securities gains, net									$ 7,659	$ 1,085	$ 6,973
Operating expenses:
Interest									80,600	51,866	33,350
Salaries and employee benefits									200,541	196,118	171,407
Professional fees									50,022	43,510	36,474
INCOME BEFORE INCOME TAXES									184,119	145,213	119,092
Income tax benefit	$ (5,155)	$ (7,941)	$ (13,584)	$ (8,310)	$ (6,685)	$ (8,956)	$ (7,451)	$ (5,123)	(34,990)	(28,215)	(43,820)
NET INCOME	37,851	34,612	45,169	31,497	32,141	33,710	27,879	23,268	149,129	116,998	75,272
Preferred dividends					0	(13)	(13)	(13)	0	(39)	(58)
Interest expense on convertible preferred debt									0	0	12
NET INCOME AVAILABLE TO COMMON STOCKHOLDERS	$ 37,851	$ 34,612	$ 45,169	$ 31,497	$ 32,141	$ 33,697	$ 27,866	$ 23,255	149,129	116,959	75,226
Parent
Operating revenues:
Dividends from subsidiaries									137,000	85,000	70,850
Securities gains, net									0	0	3,021
Other									893	493	2,292
Total operating revenues									137,893	85,493	76,163
Operating expenses:
Interest									15,044	14,371	13,269
Salaries and employee benefits									4,072	3,639	3,146
Professional fees									3,029	2,841	2,379
Other operating expenses									15,559	12,510	7,889
Total operating expenses									37,704	33,361	26,683
Equity in undistributed earnings									34,307	52,570	16,212
INCOME BEFORE INCOME TAXES									134,496	104,702	65,692
Income tax benefit									14,633	12,296	9,580
NET INCOME									149,129	116,998	75,272
Preferred dividends									0	(39)	(58)
Interest expense on convertible preferred debt									0	0	12
NET INCOME AVAILABLE TO COMMON STOCKHOLDERS									$ 149,129	$ 116,959	$ 75,226