Capital Adequacy (Schedule of Minimum and Well-Capitalized Requirements) (Details) $ in Millions		Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Discover Bank [Member]
Compliance with Regulatory Capital Requirements [Line Items]
Total capital to risk-weighted assets, actual amount		$ 13,838	$ 13,441
Total capital to risk-weighted assets, actual ratio (in percent)	[1]	0.152	0.138
Total capital to risk-weighted assets, minimum capital requirements, amount		$ 7,304	$ 7,776
Total capital to risk-weighted assets, minimum capital requirements, ratio (in percent)		0.080	0.080
Total capital to risk-weighted assets, capital requirements to be classified as well-capitalized, amount	[2]	$ 9,130	$ 9,720
Total capital to risk-weighted assets, capital requirements to be classified as well-capitalized, ratio (in percent)	[2]	0.100	0.100
Tier I capital to risk-weighted assets, actual amount		$ 11,729	$ 11,203
Tier I capital to risk-weighted assets, actual ratio (in percent)	[1]	0.128	0.115
Tier I capital to risk-weighted assets, minimum capital requirements, amount		$ 5,478	$ 5,832
Tier I capital to risk-weighted assets, minimum capital requirements, ratio (in percent)		0.060	0.060
Tier I capital to risk-weighted assets, capital requirements to be classified as well-capitalized, amount	[2]	$ 7,304	$ 7,776
Tier I capital to risk-weighted assets, capital requirements to be classified as well-capitalized, ratio (in percent)	[2]	0.080	0.080
Tier I capital to average assets, actual amount		$ 11,729	$ 11,203
Tier I capital to average assets, actual ratio (in percent)	[1]	0.102	0.101
Tier I capital to average assets, minimum capital requirements, amount		$ 4,614	$ 4,435
Tier I capital to average assets, minimum capital requirements, ratio (in percent)		0.040	0.040
Tier I capital to average assets, capital requirements to be classified as well-capitalized, amount	[2]	$ 5,767	$ 5,544
Tier I capital to average assets, capital requirements to be classified as well-capitalized, ratio (in percent)	[2]	0.050	0.050
CET1 capital to risk-weighted assets, actual amount		$ 11,729	$ 11,203
CET1 capital to risk-weighted assets, actual ratio (in percent)	[1]	0.128	0.115
CET1 capital to risk-weighted assets, minimum capital requirements, amount		$ 4,109	$ 4,374
CET1 capital to risk-weighted assets, minimum capital requirements, ratio (in percent)		4.50%	4.50%
CET1 capital to risk-weighted assets, capital requirements to be classified as well-capitalized, amount	[2]	$ 5,935	$ 6,318
CET1 capital to risk-weighted assets, capital requirements to be classified as well-capitalized, ratio (in percent)	[2]	6.50%	6.50%
Parent Company [Member]
Compliance with Regulatory Capital Requirements [Line Items]
Total capital to risk-weighted assets, actual amount		$ 14,046	$ 13,250
Total capital to risk-weighted assets, actual ratio (in percent)	[1]	0.152	0.135
Total capital to risk-weighted assets, minimum capital requirements, amount		$ 7,389	$ 7,860
Total capital to risk-weighted assets, minimum capital requirements, ratio (in percent)		0.080	0.080
Total capital to risk-weighted assets, capital requirements to be classified as well-capitalized, amount	[2]	$ 9,236	$ 9,825
Total capital to risk-weighted assets, capital requirements to be classified as well-capitalized, ratio (in percent)	[2]	0.100	0.100
Tier I capital to risk-weighted assets, actual amount		$ 12,325	$ 11,595
Tier I capital to risk-weighted assets, actual ratio (in percent)	[1]	0.133	0.118
Tier I capital to risk-weighted assets, minimum capital requirements, amount		$ 5,541	$ 5,895
Tier I capital to risk-weighted assets, minimum capital requirements, ratio (in percent)		0.060	0.060
Tier I capital to risk-weighted assets, capital requirements to be classified as well-capitalized, amount	[2]	$ 5,541	$ 5,895
Tier I capital to risk-weighted assets, capital requirements to be classified as well-capitalized, ratio (in percent)	[2]	0.060	0.060
Tier I capital to average assets, actual amount		$ 12,325	$ 11,595
Tier I capital to average assets, actual ratio (in percent)	[1]	0.106	0.103
Tier I capital to average assets, minimum capital requirements, amount		$ 4,662	$ 4,482
Tier I capital to average assets, minimum capital requirements, ratio (in percent)		0.040	0.040
CET1 capital to risk-weighted assets, actual amount		$ 11,269	$ 11,032
CET1 capital to risk-weighted assets, actual ratio (in percent)	[1]	0.122	0.112
CET1 capital to risk-weighted assets, minimum capital requirements, amount		$ 4,156	$ 4,421
CET1 capital to risk-weighted assets, minimum capital requirements, ratio (in percent)		4.50%	4.50%

[1] Capital ratios are calculated based on the Basel III Standardized Approach rules, subject to applicable transition provisions, including CECL transition provisions.

[2] The Basel III rules do not establish well-capitalized thresholds for these measures for bank holding companies. Existing well-capitalized thresholds established in the Federal Reserve's Regulation Y have been included where available.