Parent Company Condensed Financial Statements (Condensed Statement of Cash Flows) (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
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 84,898	$ 132,660	$ 118,330	$ 169,736	$ 110,734	$ 110,143	$ 103,284	$ 107,516	$ 505,624	$ 431,677	$ 384,677
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income tax expense									33,856	26,966	261,214
Gains on sales of available-for-sale investment securities									(8,037)	(10,362)	(40,367)
Net change in other assets									46,005	23,685	(117,046)
Net change in other liabilities									(1,966)	15,353	16,785
Net cash provided by (used in) operating activities									696,906	641,856	469,624
CASH FLOWS FROM INVESTING ACTIVITIES
Net increase in investments in tax credit and other investments, net									(161,661)	(87,860)	(95,074)
Purchases of available-for-sale investment securities									(828,604)	(2,396,199)	(3,547,193)
Proceeds from the sale of available-for-sale investment securities									832,844	1,275,645	1,669,334
Net cash used in investing activities									(2,500,455)	(1,792,660)	(3,627,881)
Proceeds from:
Issuance of common stock pursuant to various stock compensation plans and agreements									2,280	2,982	2,835
Payments for:
Repurchase of vested shares due to employee tax liability									(12,940)	(3,225)	(5,964)
Other net financing activities									0	1,055	3,291
Net cash provided by financing activities									2,068,460	1,680,360	3,490,306
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS									296,089	517,616	321,002
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR				1,878,503				1,360,887	1,878,503	1,360,887	1,039,885
CASH AND CASH EQUIVALENTS, END OF YEAR	2,174,592				1,878,503				2,174,592	1,878,503	1,360,887
Parent Company
CASH FLOWS FROM OPERATING ACTIVITIES
Net income									505,624	431,677	384,677
Adjustments to reconcile net income to net cash provided by operating activities:
Undistributed earnings of subsidiaries, principally bank									(245,262)	(330,006)	(387,966)
Amortization expenses									6,158	14,094	22,870
Deferred income tax expense									940	6,349	17,555
Gains on sales of available-for-sale investment securities									(326)	0	(20)
Net change in other assets									(3,341)	39,929	(42,292)
Net change in other liabilities									(560)	794	(15,887)
Net cash provided by (used in) operating activities									263,233	162,837	(21,063)
CASH FLOWS FROM INVESTING ACTIVITIES
Net increase in investments in tax credit and other investments, net									(9,777)	(6,554)	(35,633)
Purchases of available-for-sale investment securities									(9,000)	0	0
Proceeds from the sale of available-for-sale investment securities									18,326	0	20
Net cash used in investing activities									(451)	(6,554)	(35,613)
Proceeds from:
Issuance of common stock pursuant to various stock compensation plans and agreements									2,280	2,081	2,835
Payments for:
Repayment of long-term debt									(15,000)	(20,000)	(20,000)
Repurchase of vested shares due to employee tax liability									(12,940)	(3,225)	(5,964)
Cash dividends on common stock									(116,820)	(115,828)	(115,641)
Other net financing activities									0	1,055	3,291
Net cash provided by financing activities									(142,480)	(135,917)	(135,479)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS									120,302	20,366	(192,155)
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR				$ 39,264				$ 18,898	39,264	18,898	211,053
CASH AND CASH EQUIVALENTS, END OF YEAR	$ 159,566				$ 39,264				$ 159,566	$ 39,264	$ 18,898