Allowance for Loan Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - Consumer Portfolio Segment - Student Loan - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 21,791,622	$ 20,504,465
Total in percent	100.00%	100.00%
FICO score less than 670 | School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,720,777	$ 2,416,979
Private Education Loans At Origination	12.00%	12.00%
FICO score 670-699 | School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,721,243	$ 2,504,467
Private Education Loans At Origination	13.00%	12.00%
FICO score 700-749 | School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 6,462,874	$ 6,144,489
Private Education Loans At Origination	30.00%	30.00%
FICO score greater than or equal to 750 | School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 9,886,728	$ 9,438,530
Private Education Loans At Origination	45.00%	46.00%
Private Education Loans | With Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 19,531,175	$ 18,378,398
Private Education Loans with cosigner (in percentage)	90.00%	90.00%
Private Education Loans | Without Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,260,447	$ 2,126,067
Private Education Loans without cosigner (in percentage)	10.00%	10.00%
Private Education Loans | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 21,791,622	$ 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	$ 21,791,622	$ 20,504,465
Total in percent	100.00%	100.00%
Private Education Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 21,791,622	$ 20,504,465
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments (in percentage)	25.00%	24.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments (in percentage)	16.00%	17.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments (in percentage)	13.00%	13.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments (in percentage)	9.00%	10.00%
Seasoning based on monthly scheduled payments due from more than 48 payments (in percentage)	10.00%	10.00%
Seasoning based on monthly scheduled payments due from not yet in repayment (in percentage)	27.00%	26.00%
Private Education Loans | FICO score less than 670 | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,517,014	$ 1,409,789
Private Education Loans At Origination	7.00%	7.00%
Private Education Loans | FICO score 670-699 | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 3,306,017	$ 3,106,983
Private Education Loans At Origination	15.00%	15.00%
Private Education Loans | FICO score 700-749 | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 7,186,454	$ 6,759,721
Private Education Loans At Origination	33.00%	33.00%
Private Education Loans | FICO score greater than or equal to 750 | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 9,782,137	$ 9,227,972
Private Education Loans At Origination	45.00%	45.00%
Private Education Loans | 1-12 payments | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 5,451,167	$ 4,969,334
Private Education Loans | 13-24 payments | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	3,543,836	3,481,235
Private Education Loans | 25-36 payments | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	2,729,369	2,741,954
Private Education Loans | 37-48 payments | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	2,017,498	1,990,049
Private Education Loans | More than 48 payments | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	2,178,899	2,061,448
Private Education Loans | Not yet in repayment | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	5,870,853	5,260,445
Personal Loans | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,162,874	$ 1,190,091
Total in percent	100.00%	100.00%
Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,162,874	$ 1,190,091
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments (in percentage)	72.00%	85.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments (in percentage)	27.00%	15.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments (in percentage)	1.00%	0.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments (in percentage)	0.00%	0.00%
Seasoning based on monthly scheduled payments due from more than 48 payments (in percentage)	0.00%	0.00%
Personal Loans | FICO score less than 670 | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 71,340	$ 77,702
Private Education Loans At Origination	6.00%	7.00%
Personal Loans | FICO score 670-699 | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 324,934	$ 339,053
Private Education Loans At Origination	28.00%	28.00%
Personal Loans | FICO score 700-749 | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 551,904	$ 554,700
Private Education Loans At Origination	47.00%	47.00%
Personal Loans | FICO score greater than or equal to 750 | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 214,696	$ 218,636
Private Education Loans At Origination	19.00%	18.00%
Personal Loans | 1-12 payments | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 832,583	$ 1,008,758
Personal Loans | 13-24 payments | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	320,058	181,333
Personal Loans | 25-36 payments | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	10,233	0
Personal Loans | 37-48 payments | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
Personal Loans | More than 48 payments | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 0	$ 0