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Significant Accounting Policies - Effect of CECL Adoption (Details) - USD ($) $ in Thousands	Jan. 01, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases	$ 441,912	$ 1,673,324	$ 441,912	$ 358,325	$ 341,121
Off-balance sheet exposures	2,481
Retained Earnings	1,850,512
Other Assets
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	374,300	1,515,781	374,300
FFELP Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases	1,633	4,296	1,633
Personal Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases	65,877	152,673	65,877
Credit Cards
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans and leases	102	$ 574	$ 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 \| Other Assets
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 \| Other Assets
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