Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) $ in Millions		3 Months Ended		9 Months Ended
	Jan. 01, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($) SecurityLoan	Sep. 30, 2020 USD ($) SecurityLoan	Sep. 30, 2019 USD ($) SecurityLoan	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Allowance for Loan Losses
Allowance as of December 31, 2019 (prior to CECL)		$ 1,400.0	$ 1,218.0		$ 1,286.0
Total provision		14.0	64.0	$ 153.0	208.0
Charge-offs:
Net adjustment resulting from the change in the charge-off rate		(23.0)	(21.0)	(23.0)	(21.0)
Net charge-offs remaining		(49.0)	(96.0)	(196.0)	(302.0)
Total charge-offs		(72.0)	(117.0)	(219.0)	(323.0)
Decrease in expected future recoveries on charged-off loans		46.0		85.0
Reclassification of interest reserve			2.0		5.0
Loan sales and other transactions					(9.0)
Allowance as of January 1, 2020 after CECL		1,388.0	1,166.0	1,388.0	1,166.0
Plus: expected future recoveries on charged-off loans		503.0		503.0
Allowance at end of period excluding expected future recoveries on charged-off loans		1,891.0		1,891.0
Allowance Ending Balance:
Ending balance: individually evaluated for impairment - TDR			998.0		998.0
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans			168.0		168.0
Allowance as of January 1, 2020 after CECL		1,388.0	1,166.0	1,388.0	1,166.0
Loans Ending Balance:
Ending balance: individually evaluated for impairment - TDR			9,810.0		9,810.0
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans			74,692.0		74,692.0
Purchased Non-Credit Impaired Loans acquired at a discount			4,477.0		4,477.0
Purchased Credit Impaired Loans			208.0		208.0
Ending total loans			89,187.0		89,187.0
ASU 2016-13 [Member]
Allowance for Loan Losses
Allowance as of December 31, 2019 (prior to CECL)	$ 1,112.0			1,112.0
Charge-offs:
Allowance as of January 1, 2020 after CECL	1,369.0
Collectively evaluated for impairment:
Allowance as of January 1, 2020 after CECL	1,369.0
FFELP Loans [Member]
Allowance for Loan Losses
Allowance as of December 31, 2019 (prior to CECL)	64.0	302.0	67.0	64.0	76.0
Total provision		4.0	8.0	13.0	23.0
Charge-offs:
Net charge-offs remaining		(9.0)	(9.0)	(40.0)	(33.0)
Total charge-offs		(9.0)	(9.0)	(40.0)	(33.0)
Allowance as of January 1, 2020 after CECL		297.0	65.0	297.0	65.0
Allowance at end of period excluding expected future recoveries on charged-off loans		297.0		297.0
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans			65.0		65.0
Allowance as of January 1, 2020 after CECL		297.0	65.0	297.0	65.0
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans			63,003.0		63,003.0
Purchased Non-Credit Impaired Loans acquired at a discount			2,585.0		2,585.0
Ending total loans		$ 59,856.0	$ 65,588.0	$ 59,856.0	$ 65,588.0
Net charge-offs as a percentage of average loans in repayment, excluding the net adjustment resulting from the change in the charge-off rate		0.07%	0.06%	0.11%	0.08%
Allowance coverage of charge-offs		8.8	1.9	5.6	1.5
Allowance as a percentage of the ending total loan balance		0.50%	0.10%	0.50%	0.10%
Allowance as a percentage of the ending loans in repayment		0.60%	0.12%	0.60%	0.12%
Average loans in repayment		$ 47,597.0	$ 54,973.0	$ 48,065.0	$ 56,605.0
Ending loans in repayment		48,716.0	54,395.0	48,716.0	54,395.0
FFELP Loans [Member] | ASU 2016-13 [Member]
Allowance for Loan Losses
Allowance as of December 31, 2019 (prior to CECL)	64.0			64.0
Charge-offs:
Allowance as of January 1, 2020 after CECL	324.0
Collectively evaluated for impairment:
Allowance as of January 1, 2020 after CECL	324.0
Private Education Loans [Member]
Allowance for Loan Losses
Allowance as of December 31, 2019 (prior to CECL)	1,048.0	1,098.0	1,151.0	1,048.0	1,201.0
Total provision		10.0	56.0	140.0	185.0
Charge-offs:
Net adjustment resulting from the change in the charge-off rate		(23.0)	(21.0)	(23.0)	(21.0)
Net charge-offs remaining		(40.0)	(87.0)	(156.0)	(267.0)
Total charge-offs		(63.0)	(108.0)	(179.0)	(288.0)
Decrease in expected future recoveries on charged-off loans		46.0	37.0	85.0	71.0
Reclassification of interest reserve			2.0		5.0
Loan sales and other transactions					(1.0)
Allowance as of January 1, 2020 after CECL		1,091.0	1,101.0	1,091.0	1,101.0
Plus: expected future recoveries on charged-off loans		503.0	603.0	503.0	603.0	$ 549.0	$ 588.0	$ 640.0	$ 674.0
Allowance at end of period excluding expected future recoveries on charged-off loans		1,594.0		1,594.0
Allowance Ending Balance:
Ending balance: individually evaluated for impairment - TDR			998.0		998.0
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans			103.0		103.0
Allowance as of January 1, 2020 after CECL		1,091.0	1,101.0	1,091.0	1,101.0
Loans Ending Balance:
Ending balance: individually evaluated for impairment - TDR			9,810.0		9,810.0
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans			11,683.0		11,683.0
Purchased Non-Credit Impaired Loans acquired at a discount			1,892.0		1,892.0
Purchased Credit Impaired Loans			208.0		208.0
Ending total loans		$ 22,380.0	$ 23,593.0	$ 22,380.0	$ 23,593.0
Net charge-offs as a percentage of average loans in repayment, excluding the net adjustment resulting from the change in the charge-off rate		0.75%	1.60%	1.00%	1.64%
Net adjustment resulting from the change in charge-off rate as a percentage of average loans in repayment		0.44%	0.39%	0.15%	0.13%
Allowance coverage of charge-offs		6.4	2.6	6.7	2.9
Allowance as a percentage of the ending total loan balance		7.10%	4.67%	7.10%	4.67%
Allowance as a percentage of the ending loans in repayment		7.60%	5.08%	7.60%	5.08%
Average loans in repayment		$ 20,884.0	$ 21,549.0	$ 20,739.0	$ 21,819.0
Ending loans in repayment		$ 21,006.0	21,655.0	21,006.0	21,655.0
Private Education Loans [Member] | ASU 2016-13 [Member]
Allowance for Loan Losses
Allowance as of December 31, 2019 (prior to CECL)	1,048.0			$ 1,048.0
Charge-offs:
Allowance as of January 1, 2020 after CECL	1,045.0
Collectively evaluated for impairment:
Allowance as of January 1, 2020 after CECL	$ 1,045.0
Other Loans [Member]
Allowance for Loan Losses
Allowance as of December 31, 2019 (prior to CECL)					9.0
Total provision					1.0
Charge-offs:
Net charge-offs remaining					(2.0)
Total charge-offs					(2.0)
Loan sales and other transactions					(8.0)
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans			6.0		6.0
Ending total loans			6.0		6.0
Average loans in repayment			5.0		36.0
Ending loans in repayment			$ 6.0		$ 6.0