Parent Company Condensed Financial Statements (Condensed Statement of Cash Flows) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of cash flows
Net income	$ 567,797	$ 674,035	$ 703,701
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income tax expense (benefit)	(41,515)	(21,604)	(16,470)
Net change in other assets	(339,868)	(170,819)	(60,791)
Net change in other liabilities	170,403	7,012	88,070
Net cash provided by operating activities	693,325	735,829	883,172
CASH FLOWS FROM INVESTING ACTIVITIES
Net increase in investments in tax credit investments	(154,887)	(146,902)	(132,605)
Distributions received from equity method investees	15,901	9,502	5,185
Other investing activities	6,563	1,336	(449)
Net cash provided by (used in) investing activities	(6,848,716)	(2,571,176)	(3,832,412)
Common stock:
Proceeds from issuance pursuant to various stock compensation plans and agreements	2,326	3,383	2,846
Stocks tendered for payment of withholding taxes	(8,253)	(14,635)	(15,634)
Repurchase of common stocks pursuant to the Stock Repurchase Program	(145,966)	0	0
Cash dividends paid	(158,222)	(155,107)	(125,988)
Net cash provided by (used in) financing activities	6,908,793	2,124,962	3,800,808
Net increase (decrease) in cash and cash equivalents	756,822	259,772	826,785
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR	3,261,149	3,001,377	2,174,592
CASH AND CASH EQUIVALENTS, END OF YEAR	4,017,971	3,261,149	3,001,377
Parent Company
Statement of cash flows
Net income	567,797	674,035	703,701
Adjustments to reconcile net income to net cash provided by operating activities:
Undistributed earnings of subsidiaries, principally bank	(65,046)	(498,569)	(554,070)
Amortization expenses	1,523	8,703	671
Deferred income tax expense (benefit)	491	(10,132)	3,517
Net change in other assets	40	10,246	(595)
Net change in other liabilities	77,052	(18)	(45)
Net cash provided by operating activities	581,857	184,265	153,179
CASH FLOWS FROM INVESTING ACTIVITIES
Net increase in investments in tax credit investments	(172)	(292)	(1,049)
Distributions received from equity method investees	4,096	2,577	1,491
Net increase in investments in and advances to nonbank subsidiaries	(2,732)	(3,314)	0
Other investing activities	0	(157)	0
Net cash provided by (used in) investing activities	1,192	(1,186)	442
CASH FLOWS FROM FINANCING ACTIVITIES
Repayment of long-term debt	0	0	(25,000)
Common stock:
Proceeds from issuance pursuant to various stock compensation plans and agreements	2,326	3,383	2,846
Stocks tendered for payment of withholding taxes	(8,253)	(14,635)	(15,634)
Repurchase of common stocks pursuant to the Stock Repurchase Program	(145,966)	0	0
Cash dividends paid	(158,222)	(155,107)	(125,988)
Net cash provided by (used in) financing activities	(310,115)	(166,359)	(163,776)
Net increase (decrease) in cash and cash equivalents	272,934	16,720	(10,155)
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR	166,131	149,411	159,566
CASH AND CASH EQUIVALENTS, END OF YEAR	$ 439,065	$ 166,131	$ 149,411