Parent Company Condensed Financial Statements (Condensed Statement of Cash Flows) (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
Statement of cash flows
Net income	$ 173,018	$ 171,302	$ 172,349	$ 187,032	$ 84,898	$ 132,660	$ 118,330	$ 169,736	$ 703,701	$ 505,624	$ 431,677
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income tax expense									(16,470)	33,856	26,966
Gains on sales of available-for-sale debt and equity securities										(8,037)	(10,362)
Net change in other assets									(60,791)	45,354	23,205
Net change in other liabilities									88,070	(1,965)	15,354
Net cash provided by operating activities									883,172	703,275	650,183
CASH FLOWS FROM INVESTING ACTIVITIES
Net increase in investments in tax credit and other investments									(132,605)	(173,630)	(100,514)
Proceeds from distributions received from equity method investees									5,185	8,387	7,964
Available-for-sale investment securities:
Proceeds from sales available-for sale debt securities									364,270
Proceeds from sales of available-for-sale debt and equity securities										832,844	1,275,645
Purchases of available-for-sale debt securities									(888,673)
Purchases of available-for-sale debt and equity securities										(828,604)	(2,396,199)
Net cash provided by (used in) investing activities									(3,832,412)	(2,506,824)	(1,800,086)
Common stock:
Proceeds from issuance pursuant to various stock compensation plans and agreements									2,846	2,280	2,081
Stocks tendered for payment of withholding taxes									(15,634)	(12,940)	(3,225)
Cash dividends paid									(125,988)	(116,820)	(115,828)
Other net financing activities									0	0	1,055
Net cash provided by (used in) by financing activities									3,800,808	2,068,460	1,679,459
Net (decrease) increase in cash and cash equivalents									826,785	296,089	517,616
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR				2,174,592				1,878,503	2,174,592	1,878,503	1,360,887
CASH AND CASH EQUIVALENTS, END OF YEAR	3,001,377				2,174,592				3,001,377	2,174,592	1,878,503
Parent Company
Statement of cash flows
Net income									703,701	505,624	431,677
Adjustments to reconcile net income to net cash provided by operating activities:
Undistributed earnings of subsidiaries, principally bank									(554,070)	(245,262)	(330,006)
Amortization expenses									671	6,158	14,094
Deferred income tax expense									3,517	940	6,349
Gains on sales of available-for-sale debt securities									0
Gains on sales of available-for-sale debt and equity securities										(326)	0
Net change in other assets									(595)	(3,341)	39,929
Net change in other liabilities									(45)	(560)	794
Net cash provided by operating activities									153,179	263,233	162,837
CASH FLOWS FROM INVESTING ACTIVITIES
Net increase in investments in tax credit and other investments									(1,049)	(11,591)	(8,229)
Proceeds from distributions received from equity method investees									1,491	1,814	1,675
Available-for-sale investment securities:
Proceeds from sales available-for sale debt securities									0
Proceeds from sales of available-for-sale debt and equity securities										18,326	0
Purchases of available-for-sale debt securities									0
Purchases of available-for-sale debt and equity securities										(9,000)	0
Net cash provided by (used in) investing activities									442	(451)	(6,554)
CASH FLOWS FROM FINANCING ACTIVITIES
Repayment of long-term debt									(25,000)	(15,000)	(20,000)
Common stock:
Proceeds from issuance pursuant to various stock compensation plans and agreements									2,846	2,280	2,081
Stocks tendered for payment of withholding taxes									(15,634)	(12,940)	(3,225)
Cash dividends paid									(125,988)	(116,820)	(115,828)
Other net financing activities									0	0	1,055
Net cash provided by (used in) by financing activities									(163,776)	(142,480)	(135,917)
Net (decrease) increase in cash and cash equivalents									(10,155)	120,302	20,366
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR				$ 159,566				$ 39,264	159,566	39,264	18,898
CASH AND CASH EQUIVALENTS, END OF YEAR	$ 149,411				$ 159,566				$ 149,411	$ 159,566	$ 39,264