Investment Securities (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	$ 8,319
Available-for-sale debt securities, Estimated Fair Value	8,370
Equity securities
Equity securities, Cost	62		$ 51
Equity securities, Gross Unrealized Gains	15	$ 29	0
Equity securities, Gross Unrealized Losses	(2)		(3)
Equity securities, Estimated Fair Value	75		48
Total Cost	8,381		4,659
Total Gross Unrealized Gains	68		10
Total Gross Unrealized Losses	(4)		(22)
Total Estimated Fair Value	8,445		4,647
State and municipal obligations [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	230		594
Available-for-sale debt securities, Gross Unrealized Gains	10		4
Available-for-sale debt securities, Gross Unrealized Losses	0		(2)
Available-for-sale debt securities, Estimated Fair Value	240		596
US Government agency obligations [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	10		10
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	10		10
U.S. Government treasury obligations [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	7,401		3,452
Available-for-sale debt securities, Gross Unrealized Gains	40		5
Available-for-sale debt securities, Gross Unrealized Losses	(2)		(17)
Available-for-sale debt securities, Estimated Fair Value	7,439		3,440
Corporate debt securities [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	27		28
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	27		28
Mortgage-backed securities [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	41		50
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	43		51
Foreign government bonds and obligations [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	610		474
Available-for-sale debt securities, Gross Unrealized Gains	1		0
Available-for-sale debt securities, Gross Unrealized Losses	0		0
Available-for-sale debt securities, Estimated Fair Value	611		$ 474
Other Assets [Member]
Equity securities
Equity securities, Cost	10
Equity securities, Estimated Fair Value	$ 25

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