Allowance for Loan Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - Consumer Portfolio Segment - Student Loan - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 21,628,847	$ 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	$ 2,697,197	$ 2,416,979
Private Education Loans At Origination	13.00%	12.00%
School FICO, Refreshed Amounts | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,616,109	$ 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	$ 6,339,380	$ 6,144,489
Private Education Loans At Origination	29.00%	30.00%
School FICO, Refreshed Amounts | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 9,976,161	$ 9,438,530
Private Education Loans At Origination	46.00%	46.00%
Personal Loans | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,134,637	$ 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	$ 63,155	$ 77,702
Private Education Loans At Origination	6.00%	7.00%
Personal Loans | FICO at Original Approval | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 303,193	$ 339,053
Private Education Loans At Origination	27.00%	28.00%
Personal Loans | FICO at Original Approval | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 547,280	$ 554,700
Private Education Loans At Origination	48.00%	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	$ 221,009	$ 218,636
Private Education Loans At Origination	19.00%	18.00%
Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,134,637	$ 1,190,091
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments	59.00%	85.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments	40.00%	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%
Personal Loans | Seasoning | 1-12 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 669,391	$ 1,008,758
Personal Loans | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	452,428	181,333
Personal Loans | Seasoning | 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	12,818	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
Private Education Loans | With Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 19,335,960	$ 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	$ 2,292,887	$ 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	$ 21,628,847	$ 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	$ 21,628,847	$ 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	$ 1,517,608	$ 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	$ 3,298,903	$ 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	$ 7,146,638	$ 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	$ 9,665,698	$ 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	$ 21,628,847	$ 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	$ 5,508,604	$ 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	$ 3,527,814	$ 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	$ 2,674,655	$ 2,741,954
Seasoning based on monthly scheduled payments due from 25 - 36 payments	12.00%	13.00%
Private Education Loans | Seasoning | 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,993,995	$ 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	$ 2,201,198	$ 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	$ 5,722,581	$ 5,260,445
Seasoning based on monthly scheduled payments due from not yet in repayment	27.00%	26.00%