Significant Accounting Policies - Effect of CECL Adoption (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases	$ 1,734,559	$ 1,929,323	$ 441,912	$ 441,912	$ 414,406	$ 383,997	$ 341,121
Off-balance sheet exposures			2,481
Retained earnings	1,302,654		1,850,512	1,850,512
Deferred tax asset
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax asset			109,369
Private Education Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases	1,728,811		374,300	374,300
FFELP Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases	4,363		1,633	1,633
Personal Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases	0		65,877	65,877
Credit Cards
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases	$ 1,385		102	$ 102
Cumulative Effect, Period Of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases			1,584,965
Off-balance sheet exposures			118,239
Retained earnings			897,873
Cumulative Effect, Period Of Adoption, Adjusted Balance | Deferred tax asset
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax asset			415,540
Cumulative Effect, Period Of Adoption, Adjusted Balance | Private Education Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases			1,435,130
Cumulative Effect, Period Of Adoption, Adjusted Balance | FFELP Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases			4,485
Cumulative Effect, Period Of Adoption, Adjusted Balance | Personal Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases			145,060
Cumulative Effect, Period Of Adoption, Adjusted Balance | Credit Cards
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases			290
Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases			1,143,053
Off-balance sheet exposures			115,758
Retained earnings			(952,639)
Cumulative Effect, Period Of Adoption, Adjustment | Deferred tax asset
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax asset			306,171
Cumulative Effect, Period Of Adoption, Adjustment | Private Education Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases			1,060,830
Cumulative Effect, Period Of Adoption, Adjustment | FFELP Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases			2,852
Cumulative Effect, Period Of Adoption, Adjustment | Personal Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases			79,183
Cumulative Effect, Period Of Adoption, Adjustment | Credit Cards
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases			$ 188