Credit Card and Student Loan Securitization Activities (Schedule of Restricted Credit Card Securitized Assets) (Details) - USD ($) $ in Millions		Mar. 31, 2020		Jan. 01, 2020	Dec. 31, 2019		Mar. 31, 2019		Dec. 31, 2018	[3]
Variable Interest Entity [Line Items]
Restricted cash		$ 1,183			$ 40		$ 44
Loan receivables		92,963			95,894
Allowance for credit losses allocated to securitized loan receivables		(6,913)	[1],[2]	$ (5,844)	(3,383)	[1],[2],[3]	$ (3,134)	[3]	$ (3,041)
Net loan receivables		86,050			92,511
Other		3,134			2,459
Total assets		112,657			113,996
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Restricted cash		1,183			40
Loan receivables		29,436			31,840
Allowance for credit losses allocated to securitized loan receivables	[4]	(1,620)			(1,179)
Other		6			5
Variable Interest Entity, Primary Beneficiary [Member] | Credit Card Securitization Trusts [Member]
Variable Interest Entity [Line Items]
Restricted cash		1,172			28
Investors' interests held by third-party investors		13,600			14,100
Investors' interests held by wholly-owned subsidiaries of Discover Bank		4,750			4,796
Seller's interest		10,799			12,652
Loan receivables	[5]	29,149			31,548
Allowance for credit losses allocated to securitized loan receivables	[5],[6]	(1,620)			(1,179)
Net loan receivables		27,529			30,369
Other		6			5
Total assets		$ 28,707			$ 30,402

[1] Prior to adoption of Accounting Standards Update ("ASU") No. 2016-13 on January 1, 2020, credit losses were estimated using the incurred loss approach.

[2] Prior to adoption of ASU No. 2016-13 on January 1, 2020, credit losses were estimated using the incurred loss approach.

[3] Prior to adoption of ASU No. 2016-13 on January 1, 2020, credit losses were estimated using the incurred loss approach.

[4] Prior to adoption of ASU No. 2016-13 on January 1, 2020, credit losses were estimated using the incurred loss approach.

[5] The Company maintains its allowance for credit losses at an amount sufficient to absorb expected losses inherent in all loan receivables, which includes all loan receivables in the trusts. Therefore, credit risk associated with the transferred receivables is fully reflected on the Company's balance sheet in accordance with GAAP.

[6] Prior to adoption of ASU No. 2016-13 on January 1, 2020, credit losses were estimated using the incurred loss approach.