Investments (Details) - Investment Information, Equity Securities - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Investment disclosure details
Equity securities, cost	$ 373		$ 382
Gross gains	38		10
Gross losses	5		24
Equity securities, at fair value	406		368
Net recognized gains on equity securities still held	45	$ 3
Public common stock [Member]
Investment disclosure details
Equity securities, cost	337		338
Gross gains	31		2
Gross losses	5		24
Equity securities, at fair value	363		316
Non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	36		44
Gross gains	7		8
Gross losses	0		0
Equity securities, at fair value	$ 43		$ 52

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=84241550&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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84241550&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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