Allowance for Loan Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - Consumer Portfolio Segment - Student Loan - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 23,120,169	$ 20,504,465
Total in percent	100.00%	100.00%
School FICO, Refreshed Amounts | Less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,819,600	$ 2,416,979
Private Education Loans At Origination	12.00%	12.00%
School FICO, Refreshed Amounts | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,843,463	$ 2,504,467
Private Education Loans At Origination	12.00%	12.00%
School FICO, Refreshed Amounts | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 6,830,464	$ 6,144,489
Private Education Loans At Origination	30.00%	30.00%
School FICO, Refreshed Amounts | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 10,626,642	$ 9,438,530
Private Education Loans At Origination	46.00%	46.00%
Private Education Loans | With Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 20,663,133	$ 18,378,398
Private Education Loans with cosigner	89.00%	90.00%
Private Education Loans | Without Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,457,036	$ 2,126,067
Private Education Loans without cosigner	11.00%	10.00%
Private Education Loans | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 23,120,169	$ 20,504,465
Total in percent	100.00%	100.00%
Private Education Loans | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 23,120,169	$ 20,504,465
Total in percent	100.00%	100.00%
Private Education Loans | FICO at Original Approval | Less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,635,201	$ 1,409,789
Private Education Loans At Origination	7.00%	7.00%
Private Education Loans | FICO at Original Approval | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 3,528,152	$ 3,106,983
Private Education Loans At Origination	15.00%	15.00%
Private Education Loans | FICO at Original Approval | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 7,632,035	$ 6,759,721
Private Education Loans At Origination	33.00%	33.00%
Private Education Loans | FICO at Original Approval | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 10,324,781	$ 9,227,972
Private Education Loans At Origination	45.00%	45.00%
Private Education Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 23,120,169	$ 20,504,465
Total in percent	100.00%	100.00%
Private Education Loans | Seasoning | 1-12 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 5,970,773	$ 4,969,334
Seasoning based on monthly scheduled payments due from 1-12 payments	26.00%	24.00%
Private Education Loans | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 3,652,078	$ 3,481,235
Seasoning based on monthly scheduled payments due from 13 - 24 payments	16.00%	17.00%
Private Education Loans | Seasoning | 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,684,175	$ 2,741,954
Seasoning based on monthly scheduled payments due from 25 - 36 payments	11.00%	13.00%
Private Education Loans | Seasoning | 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,031,488	$ 1,990,049
Seasoning based on monthly scheduled payments due from 37 - 48 payments	9.00%	10.00%
Private Education Loans | Seasoning | More than 48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,337,745	$ 2,061,448
Seasoning based on monthly scheduled payments due from more than 48 payments	10.00%	10.00%
Private Education Loans | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 6,443,910	$ 5,260,445
Seasoning based on monthly scheduled payments due from not yet in repayment	28.00%	26.00%
Personal Loans | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,131,833	$ 1,190,091
Total in percent	100.00%	100.00%
Personal Loans | FICO at Original Approval | Less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 55,467	$ 77,702
Private Education Loans At Origination	5.00%	7.00%
Personal Loans | FICO at Original Approval | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 286,460	$ 339,053
Private Education Loans At Origination	25.00%	28.00%
Personal Loans | FICO at Original Approval | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 557,828	$ 554,700
Private Education Loans At Origination	49.00%	47.00%
Personal Loans | FICO at Original Approval | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 232,078	$ 218,636
Private Education Loans At Origination	21.00%	18.00%
Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,131,833	$ 1,190,091
Seasoning based on monthly scheduled payments due from 1-12 payments	50.00%	85.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments	49.00%	15.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments	1.00%	0.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments	0.00%	0.00%
Seasoning based on monthly scheduled payments due from more than 48 payments	0.00%	0.00%
Total in percent	100.00%	100.00%
Personal Loans | Seasoning | 1-12 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 561,102	$ 1,008,758
Personal Loans | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	552,313	181,333
Personal Loans | Seasoning | 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	18,418	0
Personal Loans | Seasoning | 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
Personal Loans | Seasoning | More than 48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 0	$ 0