Allowance for Credit Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
% of Balance
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff	$ 431,244	$ 230,268	$ 245,043
Recoveries	41,742	29,506	29,007
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff (Recovery)	389,502	200,762	216,036
Consumer Portfolio Segment | Student Loan
% of Balance
Current period gross charge-offs, Year 1	(2,224)	(1,183)
Current period gross charge-offs, Year 2	(25,698)	(8,604)
Current period gross charge-offs, Year 3	(48,271)	(23,866)
Current period gross charge-offs, Year 4	(62,071)	(32,741)
Current period gross charge-offs, Year 5	(57,505)	(37,186)
Current period gross charge-offs, Year 6	(231,647)	(126,011)
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff	427,416	229,591
Current period recoveries, Year 1	124	35
Current period recoveries, Year 2	1,841	540
Current period recoveries, Year 3	4,170	2,092
Current period recoveries, Year 4	5,556	3,693
Current period recoveries, Year 5	5,407	4,450
Current period recoveries, Year 6	24,639	18,684
Recoveries	41,737	29,494
Current period net charge-offs, Year 1	(2,100)	(1,148)
Current period net charge-offs, Year 2	(23,857)	(8,064)
Current period net charge-offs, Year 3	(44,101)	(21,774)
Current period net charge-offs, Year 4	(56,515)	(29,048)
Current period net charge-offs, Year 5	(52,098)	(32,736)
Current period net charge-offs, Year 6	(207,008)	(107,327)
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff (Recovery)	385,679	200,097
Total accrued interest by origination vintage, Year 1	142,915	109,233
Total accrued interest by origination vintage, Year 2	315,308	247,418
Total accrued interest by origination vintage, Year 3	207,858	270,242
Total accrued interest by origination vintage, Year 4	184,832	198,816
Total accrued interest by origination vintage, Year 5	116,211	131,685
Total accrued interest by origination vintage, Year 6	210,438	229,729
Total accrued interest by origination vintage	1,177,562	1,187,123
Consumer Portfolio Segment | Student Loan | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	2,411,441	2,234,876	2,043,033
Year 2	2,991,839	2,786,646	2,573,228
Year 3	1,907,574	2,321,728	2,045,012
Year 4	1,683,986	1,772,651	1,850,539
Year 5	1,301,809	1,570,815	1,685,572
After Year 5	4,262,698	4,319,057	3,701,564
Loan type	14,559,347	15,005,773	13,898,948
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	14,164	15,148	6,400
Year 2	30,740	29,146	16,983
Year 3	30,877	46,616	26,934
Year 4	35,213	43,197	30,771
Year 5	31,366	41,695	33,040
After Year 5	144,948	132,757	91,400
Loan type	287,308	308,559	205,528
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	5,523	3,194	2,628
Year 2	15,056	7,441	9,143
Year 3	14,433	14,044	15,026
Year 4	18,201	14,310	18,121
Year 5	16,697	16,425	19,064
After Year 5	77,595	61,533	55,661
Loan type	147,505	116,947	119,643
Consumer Portfolio Segment | Student Loan | Loan delinquent 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,710	642	460
Year 2	11,842	3,683	4,642
Year 3	14,872	8,453	9,396
Year 4	16,819	10,632	12,939
Year 5	16,107	11,935	14,710
After Year 5	74,040	44,588	38,555
Loan type	135,390	79,933	$ 80,702
Consumer Portfolio Segment | Student Loan | With and without cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	4,276,533	3,822,361
Year 2	4,547,159	4,166,283
Year 3	2,584,622	3,216,525
Year 4	2,172,660	2,320,060
Year 5	1,615,842	1,978,233
After Year 5	5,106,872	5,213,401
Loan type	$ 20,303,688	$ 20,716,863
% of Balance
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | With cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 3,656,111	$ 3,263,892
Year 2	3,941,921	3,604,553
Year 3	2,208,033	2,778,262
Year 4	1,853,619	2,025,463
Year 5	1,402,828	1,765,719
After Year 5	4,626,491	4,753,775
Loan type	$ 17,689,003	$ 18,191,664
% of Balance
Private education loans	87.00%	88.00%
Consumer Portfolio Segment | Student Loan | Without cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 620,422	$ 558,469
Year 2	605,238	561,730
Year 3	376,589	438,263
Year 4	319,041	294,597
Year 5	213,014	212,514
After Year 5	480,381	459,626
Loan type	$ 2,614,685	$ 2,525,199
% of Balance
Private education loans	13.00%	12.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 4,276,533	$ 3,822,361
Year 2	4,547,159	4,166,283
Year 3	2,584,622	3,216,525
Year 4	2,172,660	2,320,060
Year 5	1,615,842	1,978,233
After Year 5	5,106,872	5,213,401
Loan type	$ 20,303,688	$ 20,716,863
% of Balance
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | Less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 326,991	$ 248,368
Year 2	307,646	238,005
Year 3	158,606	251,157
Year 4	177,098	193,123
Year 5	143,674	166,048
After Year 5	439,587	428,416
Loan type	$ 1,553,602	$ 1,525,117
% of Balance
Private Education Loans at origination	8.00%	7.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 593,216	$ 508,264
Year 2	611,649	564,497
Year 3	356,541	493,237
Year 4	339,685	363,313
Year 5	259,142	329,807
After Year 5	878,426	884,981
Loan type	$ 3,038,659	$ 3,144,099
% of Balance
Private Education Loans at origination	15.00%	15.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,336,765	$ 1,210,833
Year 2	1,440,510	1,348,269
Year 3	834,819	1,057,001
Year 4	719,777	770,452
Year 5	537,680	660,270
After Year 5	1,722,068	1,753,709
Loan type	$ 6,591,619	$ 6,800,534
% of Balance
Private Education Loans at origination	32.00%	33.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 2,019,561	$ 1,854,896
Year 2	2,187,354	2,015,512
Year 3	1,234,656	1,415,130
Year 4	936,100	993,172
Year 5	675,346	822,108
After Year 5	2,066,791	2,146,295
Loan type	$ 9,119,808	$ 9,247,113
% of Balance
Private Education Loans at origination	45.00%	45.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 4,276,533	$ 3,822,361
Year 2	4,547,159	4,166,283
Year 3	2,584,622	3,216,525
Year 4	2,172,660	2,320,060
Year 5	1,615,842	1,978,233
After Year 5	5,106,872	5,213,401
Loan type	$ 20,303,688	$ 20,716,863
% of Balance
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | Less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 443,868	$ 326,613
Year 2	461,589	279,578
Year 3	242,310	273,652
Year 4	237,105	235,684
Year 5	204,894	233,022
After Year 5	773,324	739,268
Loan type	$ 2,363,090	$ 2,087,817
% of Balance
Private Education Loans at origination	12.00%	10.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 594,118	$ 506,021
Year 2	579,784	475,674
Year 3	284,244	365,133
Year 4	240,999	256,400
Year 5	173,754	209,536
After Year 5	564,344	570,605
Loan type	$ 2,437,243	$ 2,383,369
% of Balance
Private Education Loans at origination	12.00%	12.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,322,558	$ 1,209,493
Year 2	1,378,910	1,285,015
Year 3	748,368	978,763
Year 4	628,060	682,024
Year 5	449,701	568,766
After Year 5	1,388,090	1,448,692
Loan type	$ 5,915,687	$ 6,172,753
% of Balance
Private Education Loans at origination	29.00%	30.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,915,989	$ 1,780,234
Year 2	2,126,876	2,126,016
Year 3	1,309,700	1,598,977
Year 4	1,066,496	1,145,952
Year 5	787,493	966,909
After Year 5	2,381,114	2,454,836
Loan type	$ 9,587,668	$ 10,072,924
% of Balance
Private Education Loans at origination	47.00%	48.00%
Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 4,276,533	$ 3,822,361
Year 2	4,547,159	4,166,283
Year 3	2,584,622	3,216,525
Year 4	2,172,660	2,320,060
Year 5	1,615,842	1,978,233
After Year 5	5,106,872	5,213,401
Loan type	$ 20,303,688	$ 20,716,863
% of Balance
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 1-12 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 2,448,884	$ 2,265,811
Year 2	636,073	594,850
Year 3	384,334	515,328
Year 4	330,316	385,246
Year 5	235,878	340,242
After Year 5	424,636	501,269
Loan type	$ 4,460,121	$ 4,602,746
% of Balance
Seasoning based on monthly scheduled payments due from 1-12 payments	22.00%	22.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 0	$ 0
Year 2	2,477,764	2,287,737
Year 3	255,510	362,674
Year 4	195,753	203,674
Year 5	166,045	211,064
After Year 5	455,782	479,540
Loan type	$ 3,550,854	$ 3,544,689
% of Balance
Seasoning based on monthly scheduled payments due from 13 - 24 payments	18.00%	17.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 0	$ 0
Year 2	0	173
Year 3	1,366,398	1,565,203
Year 4	257,534	312,049
Year 5	126,223	164,575
After Year 5	489,157	482,369
Loan type	$ 2,239,312	$ 2,524,369
% of Balance
Seasoning based on monthly scheduled payments due from 25 - 36 payments	11.00%	12.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 0	$ 0
Year 2	0	0
Year 3	127	0
Year 4	1,008,418	983,434
Year 5	224,805	295,206
After Year 5	451,102	464,563
Loan type	$ 1,684,452	$ 1,743,203
% of Balance
Seasoning based on monthly scheduled payments due from 37 - 48 payments	8.00%	8.00%
Consumer Portfolio Segment | Student Loan | Seasoning | More than 48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 0	$ 0
Year 2	0	0
Year 3	0	0
Year 4	0	0
Year 5	643,611	671,138
After Year 5	2,830,285	2,726,304
Loan type	$ 3,473,896	$ 3,397,442
% of Balance
Seasoning based on monthly scheduled payments due from more than 48 payments	17.00%	16.00%
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,827,649	$ 1,556,550
Year 2	1,433,322	1,283,523
Year 3	578,253	773,320
Year 4	380,639	435,657
Year 5	219,280	296,008
After Year 5	455,910	559,356
Loan type	$ 4,895,053	$ 4,904,414
% of Balance
Seasoning based on monthly scheduled payments due from not yet in repayment	24.00%	25.00%