Parent Company Condensed Financial Statements (Condensed Statement of Income) (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 income
Net gains on sales of AFS debt investment securities									$ 3,930	$ 2,535
Net gains on sales of AFS debt and equity investment securities											$ 8,037
Interest expense on long-term debt	$ 99,014	$ 107,105	$ 107,518	$ 100,850	$ 87,918	$ 73,465	$ 58,632	$ 45,180	414,487	265,195	140,050
Compensation and employee benefits									401,700	379,622	335,291
Amortization of tax credit and other investments									85,515	89,628	87,950
Other expense									92,249	88,042	82,974
Income tax benefit	(31,067)	(34,951)	(72,797)	(31,067)	(32,037)	(33,563)	(24,643)	(24,752)	(169,882)	(114,995)	(229,476)
NET INCOME	$ 188,215	$ 171,416	$ 150,380	$ 164,024	$ 173,018	$ 171,302	$ 172,349	$ 187,032	674,035	703,701	505,624
Parent Company
Statement of income
Net gains on sales of AFS debt investment securities									0	0
Net gains on sales of AFS debt and equity investment securities											326
Other income									425	2	395
Total income									190,614	160,177	259,839
Interest expense on long-term debt									6,482	6,488	5,429
Compensation and employee benefits									5,479	5,559	5,065
Amortization of tax credit and other investments									8,437	413	5,908
Other expense									1,487	1,490	1,257
Total expense									21,885	13,950	17,659
Income before income tax benefit and equity in undistributed income of subsidiaries									168,729	146,227	242,180
Income tax benefit									6,737	3,404	18,182
Undistributed earnings of subsidiaries, primarily bank									498,569	554,070	245,262
NET INCOME									674,035	703,701	505,624
Parent Company | Bank
Statement of income
Dividends from subsidiaries									190,000	160,000	255,000
Parent Company | Nonbank
Statement of income
Dividends from subsidiaries									$ 189	$ 175	$ 4,118