Parent Company Condensed Financial Statements (Condensed Statement of Cash Flows) (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
Statement of cash flows
Net income	$ 188,215	$ 171,416	$ 150,380	$ 164,024	$ 173,018	$ 171,302	$ 172,349	$ 187,032	$ 674,035	$ 703,701	$ 505,624
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income tax (benefit) expense									(21,604)	(16,470)	33,856
Gains on sales of AFS debt and equity investment securities											(8,037)
Net change in other assets									(170,819)	(60,791)	45,354
Net change in other liabilities									7,012	88,070	(1,965)
Net cash provided by operating activities									735,829	883,172	703,275
CASH FLOWS FROM INVESTING ACTIVITIES
Net increase in investments in tax credit investments									(146,902)	(132,605)	(173,630)
Proceeds from distributions received from equity method investees									9,502	5,185	8,387
Other investing activities									2,560	4,035	(19,969)
AFS investment securities:
Proceeds from sales available-for sale debt securities									627,110	364,270
Proceeds from sales of available-for sale securities											832,844
Purchases of available-for-sale debt securities									(2,303,317)	(888,673)
Purchases of available-for-sale securities											(828,604)
Net cash (used in) provided by investing activities									(2,571,176)	(3,832,412)	(2,506,824)
Common stock:
Proceeds from issuance pursuant to various stock compensation plans and agreements									3,383	2,846	2,280
Stocks tendered for payment of withholding taxes									(14,635)	(15,634)	(12,940)
Cash dividends paid									(155,107)	(125,988)	(116,820)
Net cash (used in) provided by financing activities									2,124,962	3,800,808	2,068,460
NET INCREASE IN CASH AND CASH EQUIVALENTS									259,772	826,785	296,089
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR				3,001,377				2,174,592	3,001,377	2,174,592	1,878,503
CASH AND CASH EQUIVALENTS, END OF YEAR	3,261,149				3,001,377				3,261,149	3,001,377	2,174,592
Parent Company
Statement of cash flows
Net income									674,035	703,701	505,624
Adjustments to reconcile net income to net cash provided by operating activities:
Undistributed earnings of subsidiaries, principally bank									(498,569)	(554,070)	(245,262)
Amortization expenses									8,703	671	6,158
Deferred income tax (benefit) expense									(10,132)	3,517	940
Gains on sales of AFS debt investment securities									0	0
Gains on sales of AFS debt and equity investment securities											(326)
Net change in other assets									10,246	(595)	(3,341)
Net change in other liabilities									(18)	(45)	(560)
Net cash provided by operating activities									184,265	153,179	263,233
CASH FLOWS FROM INVESTING ACTIVITIES
Net increase in investments in tax credit investments									(292)	(1,049)	(11,591)
Proceeds from distributions received from equity method investees									2,577	1,491	1,814
Investments in and advances to nonbank subsidiaries									(3,314)	0	0
Other investing activities									(157)	0	0
AFS investment securities:
Proceeds from sales available-for sale debt securities									0	0
Proceeds from sales of available-for sale securities											18,326
Purchases of available-for-sale debt securities									0	0
Purchases of available-for-sale securities											(9,000)
Net cash (used in) provided by investing activities									(1,186)	442	(451)
CASH FLOWS FROM FINANCING ACTIVITIES
Repayment of long-term debt									0	(25,000)	(15,000)
Common stock:
Proceeds from issuance pursuant to various stock compensation plans and agreements									3,383	2,846	2,280
Stocks tendered for payment of withholding taxes									(14,635)	(15,634)	(12,940)
Cash dividends paid									(155,107)	(125,988)	(116,820)
Net cash (used in) provided by financing activities									(166,359)	(163,776)	(142,480)
NET INCREASE IN CASH AND CASH EQUIVALENTS									16,720	(10,155)	120,302
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR				$ 149,411				$ 159,566	149,411	159,566	39,264
CASH AND CASH EQUIVALENTS, END OF YEAR	$ 166,131				$ 149,411				$ 166,131	$ 149,411	$ 159,566