Parent Company Condensed Financial Statements (Condensed Statement of Income) (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 income
Net gains on sales of available-for-sale debt investment securities									$ 2,535
Net gains on sales of available-for-sale debt and equity securities										$ 8,037	$ 10,362
Interest expense on long-term debt	$ 87,918	$ 73,465	$ 58,632	$ 45,180	$ 40,064	$ 36,755	$ 32,684	$ 30,547	265,195	140,050	104,843
Compensation and employee benefits									379,622	335,291	300,115
Amortization of tax credit and other investments									89,628	87,950	83,446
Other expense									88,042	82,974	86,382
Income tax benefit	(32,037)	(33,563)	(24,643)	(24,752)	(89,229)	(42,624)	(39,355)	(58,268)	(114,995)	(229,476)	(140,511)
NET INCOME	$ 173,018	$ 171,302	$ 172,349	$ 187,032	$ 84,898	$ 132,660	$ 118,330	$ 169,736	703,701	505,624	431,677
Parent Company
Statement of income
Net gains on sales of available-for-sale debt investment securities									0
Net gains on sales of available-for-sale debt and equity securities										326	0
Other income									2	395	610
Total income									160,177	259,839	100,717
Interest expense on long-term debt									6,488	5,429	5,017
Compensation and employee benefits									5,559	5,065	5,001
Amortization of tax credit and other investments									413	5,908	13,851
Other expense									1,490	1,257	1,218
Total expense									13,950	17,659	25,087
Income before income tax benefit and equity in undistributed income of subsidiaries									146,227	242,180	75,630
Income tax benefit									3,404	18,182	26,041
Undistributed earnings of subsidiaries, primarily bank									554,070	245,262	330,006
NET INCOME									703,701	505,624	431,677
Parent Company | Bank
Statement of income
Dividends from subsidiaries									160,000	255,000	100,000
Parent Company | Nonbank
Statement of income
Dividends from subsidiaries									$ 175	$ 4,118	$ 107