Allowance for Loan Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 17,132,907	$ 14,251,675
Private Education Loans | Consumer Portfolio Segment | Student Loan | With Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 15,395,579	$ 12,816,512
Private Education Loans with cosigner (in percentage)	90.00%	90.00%
Private Education Loans | Consumer Portfolio Segment | Student Loan | Without Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,737,328	$ 1,435,163
Private Education Loans without cosigner (in percentage)	10.00%	10.00%
Private Education Loans | Consumer Portfolio Segment | Student Loan | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 17,132,907	$ 14,251,675
Total in percent	100.00%	100.00%
Private Education Loans | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 17,132,907	$ 14,251,675
Total in percent	100.00%	100.00%
Private Education Loans | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 17,132,907	$ 14,251,675
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments (in percentage)	28.00%	26.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments (in percentage)	18.00%	20.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments (in percentage)	12.00%	13.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments (in percentage)	6.00%	7.00%
Seasoning based on monthly scheduled payments due from more than 48 payments (in percentage)	5.00%	5.00%
Seasoning based on monthly scheduled payments due from not yet in repayment (in percentage)	31.00%	29.00%
Private Education Loans | FICO score less than 670 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,119,376	$ 920,132
Private Education Loans At Origination	6.00%	6.00%
Private Education Loans | FICO score 670-699 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,532,389	$ 2,092,722
Private Education Loans At Origination	15.00%	15.00%
Private Education Loans | FICO score 700-749 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 5,605,519	$ 4,639,958
Private Education Loans At Origination	33.00%	33.00%
Private Education Loans | FICO score greater than or equal to 750 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 7,875,623	$ 6,598,863
Private Education Loans At Origination	46.00%	46.00%
Private Education Loans | 1-12 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 4,758,249	$ 3,737,110
Private Education Loans | 13-24 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	3,017,316	2,841,107
Private Education Loans | 25-36 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	2,033,608	1,839,764
Private Education Loans | 37-48 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	1,105,979	917,633
Private Education Loans | More than 48 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	865,084	726,106
Private Education Loans | Not yet in repayment | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	5,352,671	4,189,955
Personal Loans | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 132,100	$ 12,894
Total in percent	100.00%	100.00%
Personal Loans | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 132,100	$ 12,894
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments (in percentage)	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments (in percentage)	0.00%	0.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments (in percentage)	0.00%	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 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 10,910	$ 1,189
Private Education Loans At Origination	8.00%	9.00%
Personal Loans | FICO score 670-699 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 38,330	$ 3,139
Private Education Loans At Origination	29.00%	24.00%
Personal Loans | FICO score 700-749 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 60,601	$ 5,678
Private Education Loans At Origination	46.00%	44.00%
Personal Loans | FICO score greater than or equal to 750 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 22,259	$ 2,888
Private Education Loans At Origination	17.00%	23.00%
Personal Loans | 1-12 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 132,100	$ 12,894
Personal Loans | 13-24 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
Personal Loans | 25-36 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
Personal Loans | 37-48 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
Personal Loans | More than 48 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 0	$ 0