Investments (Details) - Investment Information, Equity Securities - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Investment disclosure details
Equity securities, cost	$ 372		$ 376
Gross gains	15		52
Gross losses	45		3
Equity securities, at fair value	342		425
Net recognized gains (losses) on equity securities still held	(79)	$ 39
Public common stock [Member]
Investment disclosure details
Equity securities, cost	339		341
Gross gains	9		45
Gross losses	43		3
Equity securities, at fair value	305		383
Non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	33		35
Gross gains	6		7
Gross losses	2		0
Equity securities, at fair value	$ 37		$ 42

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=116654751&loc=SL75117546-209714

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Amount of unrealized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
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-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=116654751&loc=SL75117546-209714

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Amount of unrealized loss on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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