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Available-for-Sale Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	$ 18,708	$ 23,126
Available-for-sale debt securities, accumulated gross unrealized gain, before tax	903	341
Available-for-sale debt securities, accumulated gross unrealized loss, before tax	(110)	(314)
Available-for-sale debt securities, fair value	19,501	23,153
Transfer of available-for-sale securities to held-to-maturity	744	1,161	$ 0
Transfer of held-to-maturity securities to available-for-sale securities	0	4,700	0
Other comprehensive income, transfers from held-to-maturity to available-for-sale securities, before tax	$ 0	7	$ 0
Credit card receivable [Member]
Debt Securities, Available-for-sale [Line Items]
Percentage of asset backed securities collateral	54.00%
Auto loan receivable [Member]
Debt Securities, Available-for-sale [Line Items]
Percentage of asset backed securities collateral	18.00%
Student loan receivable [Member]
Debt Securities, Available-for-sale [Line Items]
Percentage of asset backed securities collateral	28.00%
Agency mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	$ 16,267	22,140
Available-for-sale debt securities, accumulated gross unrealized gain, before tax	836	327
Available-for-sale debt securities, accumulated gross unrealized loss, before tax	(68)	(305)
Available-for-sale debt securities, fair value	17,035	22,162
Agency debentures [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	632	833
Available-for-sale debt securities, accumulated gross unrealized gain, before tax	27	13
Available-for-sale debt securities, accumulated gross unrealized loss, before tax	0	(7)
Available-for-sale debt securities, fair value	659	839
US Treasuries [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	1,233
Available-for-sale debt securities, accumulated gross unrealized gain, before tax	34
Available-for-sale debt securities, accumulated gross unrealized loss, before tax	(40)
Available-for-sale debt securities, fair value	1,227
Non-agency asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	417
Available-for-sale debt securities, accumulated gross unrealized gain, before tax	2
Available-for-sale debt securities, accumulated gross unrealized loss, before tax	(2)
Available-for-sale debt securities, fair value	417
Other agency debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis		140
Available-for-sale debt securities, accumulated gross unrealized gain, before tax		1
Available-for-sale debt securities, accumulated gross unrealized loss, before tax		(2)
Available-for-sale debt securities, fair value		139
Non-agency mortgage backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	159
Available-for-sale debt securities, accumulated gross unrealized gain, before tax	4
Available-for-sale debt securities, accumulated gross unrealized loss, before tax	0
Available-for-sale debt securities, fair value	$ 163
Municipal bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis		12
Available-for-sale debt securities, accumulated gross unrealized gain, before tax		0
Available-for-sale debt securities, accumulated gross unrealized loss, before tax		0
Available-for-sale debt securities, fair value		12
Other [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis		1
Available-for-sale debt securities, accumulated gross unrealized gain, before tax		0
Available-for-sale debt securities, accumulated gross unrealized loss, before tax		0
Available-for-sale debt securities, fair value		$ 1