Business Segment Information - Amounts of Consolidated Revenue, Expense, Tax and Assets (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting, Measurement Disclosures [Abstract]
Number of operating segments | segment	4
Segment Reporting Information [Line Items]
Net interest income		$ 1,210,187	$ 1,220,317	$ 842,314
Provision/(provision credit) for loan losses		47,000	7,000	0
Noninterest income		654,080	722,788	490,219
Noninterest expense		1,231,603	1,221,996	1,023,661
Income/(loss) before income taxes		585,664	714,109	308,872
Provision/(benefit) for income taxes (Note 15)		133,291	157,602	131,892
Net income/(loss)		452,373	556,507	176,980
Average assets		41,744,264	40,225,459	29,924,813
Depreciation and amortization		65,239	59,125	70,924
Expenditures for long-lived assets		49,159	38,166	287,642
Expenses associated with rebranding initiatives		21,300
Charitable foundation contributions		11,000		8,800
Regional Banking
Segment Reporting Information [Line Items]
Net interest income		1,196,318	1,197,471	844,439
Provision/(provision credit) for loan losses		66,059	24,643	21,451
Noninterest income		329,834	311,763	259,546
Noninterest expense		789,033	826,262	628,050
Income/(loss) before income taxes		671,060	658,329	454,484
Provision/(benefit) for income taxes (Note 15)		158,148	154,659	162,348
Net income/(loss)		512,912	503,670	292,136
Average assets		30,785,775	28,366,987	19,469,287
Depreciation and amortization		31,719	31,316	45,734
Expenditures for long-lived assets		35,207	36,164	274,992
Fixed Income
Segment Reporting Information [Line Items]
Net interest income		26,044	35,753	18,122
Noninterest income		278,423	164,769	217,086
Noninterest expense		236,660	189,373	206,427
Income/(loss) before income taxes		67,807	11,149	28,781
Provision/(benefit) for income taxes (Note 15)		16,137	2,097	9,698
Net income/(loss)		51,670	9,052	19,083
Average assets		2,961,655	3,297,579	2,539,899
Depreciation and amortization		8,150	9,603	8,036
Expenditures for long-lived assets		909	646	1,877
Corporate
Segment Reporting Information [Line Items]
Net interest income		(40,774)	(64,191)	(59,261)
Noninterest income		41,357	239,263	8,887
Noninterest expense		195,683	177,923	144,333
Income/(loss) before income taxes		(195,100)	(2,851)	(194,707)
Provision/(benefit) for income taxes (Note 15)		(51,348)	(10,889)	(47,967)
Net income/(loss)		(143,752)	8,038	(146,740)
Average assets		6,951,611	7,090,069	6,370,951
Depreciation and amortization		27,649	23,285	16,764
Expenditures for long-lived assets		12,560	308	8,951
(Gain)/loss on securities				(14,300)
Non-Strategic
Segment Reporting Information [Line Items]
Net interest income		28,599	51,284	39,014
Provision/(provision credit) for loan losses		(19,059)	(17,643)	(21,451)
Noninterest income		4,466	6,993	4,700
Noninterest expense		10,227	28,438	44,851
Income/(loss) before income taxes		41,897	47,482	20,314
Provision/(benefit) for income taxes (Note 15)		10,354	11,735	7,813
Net income/(loss)		31,543	35,747	12,501
Average assets		1,045,223	1,470,824	1,544,676
Depreciation and amortization		(2,279)	(5,079)	390
Expenditures for long-lived assets		$ 483	1,048	$ 1,822
Visa Class B Shares
Segment Reporting Information [Line Items]
Gain on sale of investments			$ 212,900