Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	$ 4,838
Available-for-sale debt securities, Estimated Fair Value	4,957
Equity securities
Equity securities, Cost	55	$ 55
Equity securities, Gross Unrealized Gains	22	25
Equity securities, Gross Unrealized Losses	(2)	(2)
Equity securities, Estimated Fair Value	75	78
Total Cost	4,893	8,339
Total Gross Unrealized Gains	141	71
Total Gross Unrealized Losses	(2)	(4)
Total Estimated Fair Value	5,032	8,406
Transferred Equity Investment [Member]
Equity securities
Equity securities, Gross Unrealized Gains			$ 9
State and municipal obligations [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	194	236
Available-for-sale debt securities, Gross Unrealized Gains	10	8
Available-for-sale debt securities, Gross Unrealized Losses	0	(1)
Available-for-sale debt securities, Estimated Fair Value	204	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	4,001	7,395
Available-for-sale debt securities, Gross Unrealized Gains	104	35
Available-for-sale debt securities, Gross Unrealized Losses	0	(1)
Available-for-sale debt securities, Estimated Fair Value	4,105	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	37	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	40	41
Foreign government bonds and obligations [Member]
Available-for-sale debt securities:
Available-for-sale debt securities, Cost	574	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	576	579
Other Assets [Member]
Equity securities
Equity securities, Cost		3
Equity securities, Estimated Fair Value	$ 25	$ 28

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