Allowance for Credit Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transfer of allowance for funded loans	$ (230,268)	$ (245,043)	$ (284,114)
Current period recoveries	29,506	29,007	30,971
Net charge-offs	(200,762)	(216,036)	(253,143)
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current period gross charge-offs, Year 1	(1,183)	(1,087)
Current period gross charge-offs, Year 2	(8,604)	(10,940)
Current period gross charge-offs, Year 3	(23,866)	(27,000)
Current period gross charge-offs, Year 4	(32,741)	(35,851)
Current period gross charge-offs, Year 5	(37,186)	(36,416)
Current period gross charge-offs, Year 6	(126,011)	(94,032)
Transfer of allowance for funded loans	(229,591)	(205,326)
Current period recoveries, Year 1	35	42
Current period recoveries, Year 2	540	636
Current period recoveries, Year 3	2,092	2,274
Current period recoveries, Year 4	3,693	3,585
Current period recoveries, Year 5	4,450	4,284
Current period recoveries, Year 6	18,684	13,200
Current period recoveries	29,494	24,021
Current period net charge-offs, Year 1	(1,148)	(1,045)
Current period net charge-offs, Year 2	(8,064)	(10,304)
Current period net charge-offs, Year 3	(21,774)	(24,726)
Current period net charge-offs, Year 4	(29,048)	(32,266)
Current period net charge-offs, Year 5	(32,736)	(32,132)
Current period net charge-offs, Year 6	(107,327)	(80,832)
Net charge-offs	(200,097)	(181,305)
Total accrued interest by origination vintage, Year 1	109,233	90,438
Total accrued interest by origination vintage, Year 2	247,418	265,688
Total accrued interest by origination vintage, Year 3	270,242	252,251
Total accrued interest by origination vintage, Year 4	198,816	209,178
Total accrued interest by origination vintage, Year 5	131,685	141,094
Total accrued interest by origination vintage, Year 6	229,729	210,247
Total accrued interest by origination vintage	1,187,123	1,168,896
Consumer Portfolio Segment | Student Loan | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	2,234,876	2,043,033	2,340,221
Year 2	2,786,646	2,573,228	3,159,878
Year 3	2,321,728	2,045,012	2,781,132
Year 4	1,772,651	1,850,539	2,566,815
Year 5	1,570,815	1,685,572	2,225,721
Year 6	4,319,057	3,701,564	3,241,884
Total	15,005,773	13,898,948	16,315,651
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	15,148	6,400	11,152
Year 2	29,146	16,983	26,096
Year 3	46,616	26,934	44,382
Year 4	43,197	30,771	51,656
Year 5	41,695	33,040	54,559
Year 6	132,757	91,400	100,206
Total	308,559	205,528	288,051
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	3,194	2,628	3,087
Year 2	7,441	9,143	9,527
Year 3	14,044	15,026	17,048
Year 4	14,310	18,121	21,161
Year 5	16,425	19,064	24,562
Year 6	61,533	55,661	45,917
Total	116,947	119,643	121,302
Consumer Portfolio Segment | Student Loan | Loan delinquent 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	642	460	935
Year 2	3,683	4,642	3,850
Year 3	8,453	9,396	7,818
Year 4	10,632	12,939	12,314
Year 5	11,935	14,710	12,946
Year 6	44,588	38,555	24,803
Total	79,933	80,702	62,666
Consumer Portfolio Segment | Student Loan | With cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	3,263,892	2,915,328
Year 2	3,604,553	3,467,219
Year 3	2,778,262	2,556,400
Year 4	2,025,463	2,262,635
Year 5	1,765,719	1,977,952
Year 6	4,753,775	4,198,748
Total	$ 18,191,664	$ 17,378,282
Private education loans	88.00%	88.00%
Consumer Portfolio Segment | Student Loan | Without cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 558,469	$ 527,437
Year 2	561,730	559,629
Year 3	438,263	384,111
Year 4	294,597	277,159
Year 5	212,514	211,270
Year 6	459,626	391,449
Total	$ 2,525,199	$ 2,351,055
Private education loans	12.00%	12.00%
Consumer Portfolio Segment | Student Loan | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 3,822,361	$ 3,442,765
Year 2	4,166,283	4,026,848
Year 3	3,216,525	2,940,511
Year 4	2,320,060	2,539,794
Year 5	1,978,233	2,189,222
Year 6	5,213,401	4,590,197
Total	$ 20,716,863	$ 19,729,337
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 3,822,361	$ 3,442,765
Year 2	4,166,283	4,026,848
Year 3	3,216,525	2,940,511
Year 4	2,320,060	2,539,794
Year 5	1,978,233	2,189,222
Year 6	5,213,401	4,590,197
Total	$ 20,716,863	$ 19,729,337
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | School Type/FICO Scores | FICO score less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 248,368	$ 195,214
Year 2	238,005	290,711
Year 3	251,157	225,276
Year 4	193,123	197,948
Year 5	166,048	162,413
Year 6	428,416	369,609
Total	$ 1,525,117	$ 1,441,171
Private Education Loans at origination	7.00%	7.00%
Consumer Portfolio Segment | Student Loan | School Type/FICO Scores | FICO score 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 508,264	$ 464,785
Year 2	564,497	594,950
Year 3	493,237	441,357
Year 4	363,313	407,394
Year 5	329,807	351,303
Year 6	884,981	771,477
Total	$ 3,144,099	$ 3,031,266
Private Education Loans at origination	15.00%	16.00%
Consumer Portfolio Segment | Student Loan | School Type/FICO Scores | FICO score 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,210,833	$ 1,111,373
Year 2	1,348,269	1,310,390
Year 3	1,057,001	967,802
Year 4	770,452	846,983
Year 5	660,270	740,028
Year 6	1,753,709	1,533,517
Total	$ 6,800,534	$ 6,510,093
Private Education Loans at origination	33.00%	33.00%
Consumer Portfolio Segment | Student Loan | School Type/FICO Scores | FICO score greater than 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,854,896	$ 1,671,393
Year 2	2,015,512	1,830,797
Year 3	1,415,130	1,306,076
Year 4	993,172	1,087,469
Year 5	822,108	935,478
Year 6	2,146,295	1,915,594
Total	$ 9,247,113	$ 8,746,807
Private Education Loans at origination	45.00%	44.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 3,822,361	$ 3,442,765
Year 2	4,166,283	4,026,848
Year 3	3,216,525	2,940,511
Year 4	2,320,060	2,539,794
Year 5	1,978,233	2,189,222
Year 6	5,213,401	4,590,197
Total	$ 20,716,863	$ 19,729,337
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts | FICO score less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 326,613	$ 240,154
Year 2	279,578	331,229
Year 3	273,652	301,784
Year 4	235,684	298,195
Year 5	233,022	293,077
Year 6	739,268	734,599
Total	$ 2,087,817	$ 2,199,038
Private Education Loans at origination	10.00%	11.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts | FICO score 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 506,021	$ 438,665
Year 2	475,674	493,135
Year 3	365,133	336,966
Year 4	256,400	283,906
Year 5	209,536	231,759
Year 6	570,605	504,779
Total	$ 2,383,369	$ 2,289,210
Private Education Loans at origination	12.00%	12.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts | FICO score 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,209,493	$ 1,102,666
Year 2	1,285,015	1,248,806
Year 3	978,763	871,677
Year 4	682,024	734,222
Year 5	568,766	603,160
Year 6	1,448,692	1,220,468
Total	$ 6,172,753	$ 5,780,999
Private Education Loans at origination	30.00%	29.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts | FICO score greater than 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,780,234	$ 1,661,280
Year 2	2,126,016	1,953,678
Year 3	1,598,977	1,430,084
Year 4	1,145,952	1,223,471
Year 5	966,909	1,061,226
Year 6	2,454,836	2,130,351
Total	$ 10,072,924	$ 9,460,090
Private Education Loans at origination	48.00%	48.00%
Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 3,822,361	$ 3,442,765
Year 2	4,166,283	4,026,848
Year 3	3,216,525	2,940,511
Year 4	2,320,060	2,539,794
Year 5	1,978,233	2,189,222
Year 6	5,213,401	4,590,197
Total	$ 20,716,863	$ 19,729,337
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	22.00%	23.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	17.00%	17.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments, in percent	12.00%	12.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments, in percent	8.00%	9.00%
Seasoning based on monthly scheduled payments due from more than 48 payments, in percent	16.00%	15.00%
Seasoning - based on monthly scheduled payments due from not yet in repayment, in percent	25.00%	24.00%
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	$ 2,265,811	$ 2,068,517
Year 2	594,850	600,038
Year 3	515,328	469,143
Year 4	385,246	472,258
Year 5	340,242	381,197
Year 6	501,269	507,343
Total	4,602,746	4,498,496
Consumer Portfolio Segment | Student Loan | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	0	163
Year 2	2,287,737	2,096,635
Year 3	362,674	383,977
Year 4	203,674	223,332
Year 5	211,064	217,379
Year 6	479,540	425,345
Total	3,544,689	3,346,831
Consumer Portfolio Segment | Student Loan | Seasoning | 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	0	0
Year 2	173	0
Year 3	1,565,203	1,353,567
Year 4	312,049	370,250
Year 5	164,575	181,940
Year 6	482,369	439,337
Total	2,524,369	2,345,094
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	0	0
Year 4	983,434	965,476
Year 5	295,206	351,433
Year 6	464,563	402,552
Total	1,743,203	1,719,461
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	671,138	729,510
Year 6	2,726,304	2,310,905
Total	3,397,442	3,040,415
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,556,550	1,374,085
Year 2	1,283,523	1,330,175
Year 3	773,320	733,824
Year 4	435,657	508,478
Year 5	296,008	327,763
Year 6	559,356	504,715
Total	4,904,414	4,779,040
Consumer Portfolio Segment | Student Loan | Loans In-School/Grace/Deferment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,556,550	1,374,085	1,670,644
Year 2	1,283,523	1,330,175	1,580,513
Year 3	773,320	733,824	1,010,764
Year 4	435,657	508,478	635,798
Year 5	296,008	327,763	375,585
Year 6	559,356	504,715	414,101
Total	$ 4,904,414	4,779,040	$ 5,687,405
Consumer Portfolio Segment | Student Loan, Borrowers Who Did Not Return To School And Received Extenstion | Loans In-School/Grace/Deferment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1		$ 401,000