Allowance for Credit Losses (Summary of Activity in the Allowance for Credit Losses by Loan Portfolio) (Details) - USD ($) $ in Thousands		12 Months Ended
	Nov. 15, 2021	Dec. 31, 2021	Dec. 31, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period		$ 379,910	$ 158,461
Other adjustments		3	179
Charge-offs		(35,019)	(53,205)
Recoveries		13,569	12,896
Provision for credit losses		(59,280)	214,235
Allowance for credit losses at period end		299,653	379,910
Individually evaluated for impairment		8,552	8,026
Collectively evaluated for impairment		291,101	371,884
Impaired loans—collateral based		93,270	139,662
Loans at period end, collectively evaluated for impairment		34,657,236	31,884,277
Loans held-for-investment		38,598	55,134
Allstate Corporation Loan Portfolio
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Initial allowance for credit losses recognized on PCD assets acquired during period		470
Allowance for credit losses	$ 2,806
Allowance for credit losses, amounts charged-off	$ 2,300
Loans acquired with deteriorated credit quality | Allstate Corporation Loan Portfolio
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Initial allowance for credit losses recognized on PCD assets acquired during period		470
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period		94,212	64,920
Other adjustments		0	0
Charge-offs		(20,801)	(18,293)
Recoveries		2,559	5,092
Provision for credit losses		42,867	33,454
Allowance for credit losses at period end		119,307	94,212
Individually evaluated for impairment		5,196	4,820
Collectively evaluated for impairment		114,111	89,392
Impaired loans—collateral based		24,530	29,442
Loans at period end, collectively evaluated for impairment		11,879,538	11,926,525
Loans held-for-investment		0	0
Commercial | Loans acquired with deteriorated credit quality | Allstate Corporation Loan Portfolio
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Initial allowance for credit losses recognized on PCD assets acquired during period		470
Commercial Real Estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period		243,603	68,511
Other adjustments		0	0
Charge-offs		(3,293)	(15,960)
Recoveries		1,304	1,835
Provision for credit losses		(97,031)	157,153
Allowance for credit losses at period end		144,583	243,603
Individually evaluated for impairment		2,237	2,237
Collectively evaluated for impairment		142,346	241,366
Impaired loans—collateral based		30,167	56,656
Loans at period end, collectively evaluated for impairment		8,960,119	8,437,476
Loans held-for-investment		0	0
Commercial Real Estate | Loans acquired with deteriorated credit quality | Allstate Corporation Loan Portfolio
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Initial allowance for credit losses recognized on PCD assets acquired during period		0
Home Equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period		11,437	3,878
Other adjustments		0	0
Charge-offs		(336)	(2,061)
Recoveries		1,203	528
Provision for credit losses		(1,605)	31
Allowance for credit losses at period end		10,699	11,437
Individually evaluated for impairment		192	197
Collectively evaluated for impairment		10,507	11,240
Impaired loans—collateral based		14,656	23,173
Loans at period end, collectively evaluated for impairment		320,499	402,090
Loans held-for-investment		0	0
Home Equity | Loans acquired with deteriorated credit quality | Allstate Corporation Loan Portfolio
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Initial allowance for credit losses recognized on PCD assets acquired during period		0
Residential Real Estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period		12,459	9,800
Other adjustments		0	0
Charge-offs		(1,082)	(891)
Recoveries		330	184
Provision for credit losses		(2,925)	364
Allowance for credit losses at period end		8,782	12,459
Individually evaluated for impairment		899	684
Collectively evaluated for impairment		7,883	11,775
Impaired loans—collateral based		23,306	29,886
Loans at period end, collectively evaluated for impairment		1,575,195	1,174,578
Loans held-for-investment		38,598	55,134
Residential Real Estate | Loans acquired with deteriorated credit quality | Allstate Corporation Loan Portfolio
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Initial allowance for credit losses recognized on PCD assets acquired during period		0
Premium Finance Receivable
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period		17,777	9,647
Other adjustments		3	179
Charge-offs		(9,020)	(15,472)
Recoveries		7,989	5,108
Provision for credit losses		(890)	23,274
Allowance for credit losses at period end		15,859	17,777
Individually evaluated for impairment		0	0
Collectively evaluated for impairment		15,859	17,777
Impaired loans—collateral based		0	0
Loans at period end, collectively evaluated for impairment		11,898,297	9,911,925
Loans held-for-investment		0	0
Premium Finance Receivable | Loans acquired with deteriorated credit quality | Allstate Corporation Loan Portfolio
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Initial allowance for credit losses recognized on PCD assets acquired during period		0
Consumer and other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period		422	1,705
Other adjustments		0	0
Charge-offs		(487)	(528)
Recoveries		184	149
Provision for credit losses		304	(41)
Allowance for credit losses at period end		423	422
Individually evaluated for impairment		28	88
Collectively evaluated for impairment		395	334
Impaired loans—collateral based		611	505
Loans at period end, collectively evaluated for impairment		23,588	31,683
Loans held-for-investment		0	0
Consumer and other | Loans acquired with deteriorated credit quality | Allstate Corporation Loan Portfolio
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Initial allowance for credit losses recognized on PCD assets acquired during period		$ 0
Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			47,344
Cumulative Effect, Period of Adoption, Adjustment | Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			9,039
Cumulative Effect, Period of Adoption, Adjustment | Commercial Real Estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			32,064
Cumulative Effect, Period of Adoption, Adjustment | Home Equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			9,061
Cumulative Effect, Period of Adoption, Adjustment | Residential Real Estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			3,002
Cumulative Effect, Period of Adoption, Adjustment | Premium Finance Receivable
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			(4,959)
Cumulative Effect, Period of Adoption, Adjustment | Consumer and other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			$ (863)