Transfers of Financial Assets - Unpaid Principal Balances of Transferred Loans and Cash Flows Received (Details) - Variable interest entity, not primary beneficiary - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	$ 19,457,183		$ 19,886,137
Servicing fees collected	20,441	$ 20,128
Charge-offs, net of recoveries	25,358	40,870
Loans in repayment
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	19,184,036		19,566,257
Loans in-school/grace/deferment
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	31,450		37,949
Loans in forbearance
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	59,403		86,306
Loans in delinquency
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	182,294		195,625
Student Loans
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	9,472,323		10,048,624
Servicing fees collected	9,168	9,025
Charge-offs, net of recoveries	8,220	3,053
Student Loans | Loans in repayment
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	9,302,518		9,852,957
Student Loans | Loans in-school/grace/deferment
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	31,450		37,949
Student Loans | Loans in forbearance
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	45,096		44,833
Student Loans | Loans in delinquency
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	93,259		112,885
Home Loans
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	4,828,944		4,622,819
Servicing fees collected	2,636	1,613
Charge-offs, net of recoveries	0	0
Home Loans | Loans in repayment
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	4,805,945		4,575,001
Home Loans | Loans in-school/grace/deferment
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	0		0
Home Loans | Loans in forbearance
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	13,554		40,353
Home Loans | Loans in delinquency
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	9,445		7,465
Personal Loans
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	5,155,916		5,214,694
Servicing fees collected	8,637	9,490
Charge-offs, net of recoveries	17,138	$ 37,817
Personal Loans | Loans in repayment
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	5,075,573		5,138,299
Personal Loans | Loans in-school/grace/deferment
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	0		0
Personal Loans | Loans in forbearance
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	753		1,120
Personal Loans | Loans in delinquency
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Total loans serviced	$ 79,590		$ 75,275