Investment Securities (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	$ 8,410
Available-for-sale debt securities, Estimated Fair Value	8,457
Equity securities
Equity securities, Cost	61		$ 51
Equity securities, Gross Unrealized Gains	27	$ 29	0
Equity securities, Gross Unrealized Losses	(3)		(3)
Equity securities, Estimated Fair Value	85		48
Total Cost	8,471		4,659
Total Gross Unrealized Gains	77		10
Total Gross Unrealized Losses	(6)		(22)
Total Estimated Fair Value	8,542		4,647
US States and Political Subdivisions Debt Securities [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	324		594
Available-for-sale debt securities, Gross Unrealized Gains	9		4
Available-for-sale debt securities, Gross Unrealized Losses	0		(2)
Available-for-sale debt securities, Estimated Fair Value	333		596
US Government Agencies Debt Securities [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
US Treasury Securities [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	7,383		3,452
Available-for-sale debt securities, Gross Unrealized Gains	38		5
Available-for-sale debt securities, Gross Unrealized Losses	(3)		(17)
Available-for-sale debt securities, Estimated Fair Value	7,418		3,440
Corporate Debt Securities [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	28		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	28		28
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	45		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	47		51
Debt Security, Government, Non-US [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	620		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	621		$ 474
Other Assets [Member]
Equity securities
Equity securities, Cost	10
Equity securities, Estimated Fair Value	$ 37

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