Allowance for Credit Losses and Credit Quality - Rollfowards of the Allowance for Loan and Lease Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Allowance for loan and lease losses [Roll Forward]
Balance, beginning of period	$ 461,114	$ 146,503	$ 113,052	$ 157,446
Charge-offs	(32,235)	(35,547)	(56,922)	(81,044)
Recoveries	7,659	6,969	23,474	19,730
Net (charge-offs) recoveries	(24,576)	(28,578)	(33,448)	(61,314)
Provision for credit losses	78,323	27,188	229,439	50,879
Other	368	(23,895)	194	(25,793)
Balance, end of period	515,229	121,218	515,229	121,218
Unfunded commitments [Roll Forward]
Balance, beginning of period	42,788	1,936	3,528	1,428
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Net (charge-offs) recoveries	0	0	0	0
Provision for credit losses	(8,659)	(342)	15,894	166
Other	0	0	0	0
Balance, end of period	34,129	3,461	34,129	3,461
Allowance for loan, leases and unfunded commitments [Roll Forward]
Balance, beginning of period	503,902	148,439	116,580	158,874
Charge-offs	(32,235)	(35,547)	(56,922)	(81,044)
Recoveries	7,659	6,969	23,474	19,730
Net (charge-offs) recoveries	(24,576)	(28,578)	(33,448)	(61,314)
Provision for credit losses	69,664	26,846	245,333	51,045
Other	368	(23,895)	194	(25,793)
Balance, end of period	549,358	124,679	549,358	124,679
PCD Loans
Allowance for loan and lease losses [Roll Forward]
Addition due to merger		0		0
Unfunded commitments [Roll Forward]
Addition due to merger		1,867		1,867
Allowance for loan, leases and unfunded commitments [Roll Forward]
Addition due to merger		1,867		1,867
Consumer
Allowance for loan and lease losses [Roll Forward]
Balance, beginning of period	157,685	70,711	28,572	80,017
Charge-offs	(5,768)	(14,098)	(16,613)	(43,922)
Recoveries	3,698	5,330	12,035	15,840
Net (charge-offs) recoveries	(2,070)	(8,768)	(4,578)	(28,082)
Provision for credit losses	(20,835)	4,693	3,449	16,644
Other	357	(23,849)	357	(25,792)
Balance, end of period	135,137	42,787	135,137	42,787
Consumer | PCD Loans
Allowance for loan and lease losses [Roll Forward]
Addition due to merger		0		0
Commercial
Allowance for loan and lease losses [Roll Forward]
Balance, beginning of period	303,429	75,792	84,480	77,429
Charge-offs	(26,467)	(21,449)	(40,309)	(37,122)
Recoveries	3,961	1,639	11,439	3,890
Net (charge-offs) recoveries	(22,506)	(19,810)	(28,870)	(33,232)
Provision for credit losses	99,158	22,495	225,990	34,235
Other	11	(46)	(163)	(1)
Balance, end of period	$ 380,092	78,431	380,092	78,431
Commercial | PCD Loans
Allowance for loan and lease losses [Roll Forward]
Addition due to merger		$ 0		$ 0
Cumulative Effect, Period of Adoption, Adjustment
Allowance for loan and lease losses [Roll Forward]
Balance, beginning of period			205,992
Unfunded commitments [Roll Forward]
Balance, beginning of period			14,707
Allowance for loan, leases and unfunded commitments [Roll Forward]
Balance, beginning of period			220,699
Cumulative Effect, Period of Adoption, Adjustment | Consumer
Allowance for loan and lease losses [Roll Forward]
Balance, beginning of period			107,337
Cumulative Effect, Period of Adoption, Adjustment | Commercial
Allowance for loan and lease losses [Roll Forward]
Balance, beginning of period			98,655
Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for loan and lease losses [Roll Forward]
Balance, beginning of period			319,044
Unfunded commitments [Roll Forward]
Balance, beginning of period			18,235
Allowance for loan, leases and unfunded commitments [Roll Forward]
Balance, beginning of period			337,279
Cumulative Effect, Period of Adoption, Adjusted Balance | Consumer
Allowance for loan and lease losses [Roll Forward]
Balance, beginning of period			135,909
Cumulative Effect, Period of Adoption, Adjusted Balance | Commercial
Allowance for loan and lease losses [Roll Forward]
Balance, beginning of period			$ 183,135