Investment Securities (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Accrued interest available-for-sale debt securities	$ 24	$ 20
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	19,757
Available-for-sale debt securities, Estimated Fair Value	19,869	8,328
Equity securities
Equity securities, Cost	53	55
Equity securities, Gross Unrealized Gains	22	25
Equity securities, Gross Unrealized Losses	(2)	(2)
Equity securities, Estimated Fair Value	73	78
Total Cost	19,810	8,339
Total Gross Unrealized Gains	134	71
Total Gross Unrealized Losses	(2)	(4)
Total Estimated Fair Value	19,942	8,406
State and municipal obligations [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	185	236
Available-for-sale debt securities, Gross Unrealized Gains	7	8
Available-for-sale debt securities, Gross Unrealized Losses	0	(1)
Available-for-sale debt securities, Estimated Fair Value	192	243
US Government agency obligations [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	8	9
Available-for-sale debt securities, Gross Unrealized Gains	0	0
Available-for-sale debt securities, Gross Unrealized Losses	0	0
Available-for-sale debt securities, Estimated Fair Value	8	9
U.S. Government treasury obligations [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	18,959	7,395
Available-for-sale debt securities, Gross Unrealized Gains	100	35
Available-for-sale debt securities, Gross Unrealized Losses	0	(1)
Available-for-sale debt securities, Estimated Fair Value	19,059	7,429
Corporate debt securities [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	24	27
Available-for-sale debt securities, Gross Unrealized Gains	0	0
Available-for-sale debt securities, Gross Unrealized Losses	0	0
Available-for-sale debt securities, Estimated Fair Value	24	27
Mortgage-backed securities [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	35	39
Available-for-sale debt securities, Gross Unrealized Gains	3	2
Available-for-sale debt securities, Gross Unrealized Losses	0	0
Available-for-sale debt securities, Estimated Fair Value	38	41
Foreign government bonds and obligations [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	546	578
Available-for-sale debt securities, Gross Unrealized Gains	2	1
Available-for-sale debt securities, Gross Unrealized Losses	0	0
Available-for-sale debt securities, Estimated Fair Value	$ 548	$ 579

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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