Segment Reporting - Asset Information for Loans Segment (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Other	$ 3,383	$ 3,404
Total assets	98,320	104,176
Federal Education Loans [Member]
Segment Reporting Information [Line Items]
Loans, net	67,956	72,253
Cash and investments	2,219	3,368
Other	2,174	2,100
Total assets	72,349	77,721
Consumer Lending [Member]
Segment Reporting Information [Line Items]
Loans, net	21,564	22,245
Cash and investments	931	732
Other	1,012	1,076
Total assets	$ 23,507	$ 24,053