Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2017
Available-for-sale debt securities
Available-for-sale debt securities, Cost		$ 8,284
Available-for-sale debt securities, Estimated Fair Value	$ 4,599	8,328
Equity securities
Equity securities, Cost	51	55	$ 51
Equity securities, Unrealized Gains	0	25	0
Equity securities, Unrealized Losses	(3)	(2)	(3)
Equity securities	48	78	48
Total Cost	4,659	8,339	3,158
Total Unrealized Gains	10	71	16
Total Unrealized Losses	(22)	(4)	(15)
Total Estimated Fair Value	4,647	8,406	3,159
State and municipal obligations [Member]
Available-for-sale debt securities
Available-for-sale debt securities, Cost	594	236	1,369
Available-for-sale debt securities, Unrealized Gains	4	8	11
Available-for-sale debt securities, Unrealized Losses	(2)	(1)	(3)
Available-for-sale debt securities, Estimated Fair Value	596	243	1,377
US Government Agencies Obligations [Member]
Available-for-sale debt securities
Available-for-sale debt securities, Cost	10	9	11
Available-for-sale debt securities, Unrealized Gains	0	0	0
Available-for-sale debt securities, Unrealized Losses	0	0	0
Available-for-sale debt securities, Estimated Fair Value	10	9	11
US government treasury obligations [Member]
Available-for-sale debt securities
Available-for-sale debt securities, Cost	3,452	7,395	1,051
Available-for-sale debt securities, Unrealized Gains	5	35	3
Available-for-sale debt securities, Unrealized Losses	(17)	(1)	(9)
Available-for-sale debt securities, Estimated Fair Value	3,440	7,429	1,045
Corporate Debt Securities [Member]
Available-for-sale debt securities
Available-for-sale debt securities, Cost	28	27	28
Available-for-sale debt securities, Unrealized Gains	0	0	0
Available-for-sale debt securities, Unrealized Losses	0	0	0
Available-for-sale debt securities, Estimated Fair Value	28	27	28
Mortgage-backed securities [Member]
Available-for-sale debt securities
Available-for-sale debt securities, Cost	50	39	67
Available-for-sale debt securities, Unrealized Gains	1	2	2
Available-for-sale debt securities, Unrealized Losses	0	0	0
Available-for-sale debt securities, Estimated Fair Value	51	41	69
Foreign Government Bonds and Obligations [Member]
Available-for-sale debt securities
Available-for-sale debt securities, Cost	474	578	581
Available-for-sale debt securities, Unrealized Gains	0	1	0
Available-for-sale debt securities, Unrealized Losses	0	0	0
Available-for-sale debt securities, Estimated Fair Value	474	579	$ 581
Transferred Equity Investment [Member]
Equity securities
Equity securities, Cost		3
Equity securities		$ 28
Equity securities, unrealized gains	$ 9

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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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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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Amount 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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Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

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