Investments (Details) - Investment Information, Equity Securities $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($)
Investment disclosure details
Equity securities, cost	$ 408
Gross gains	29
Gross losses	11
Equity securities, at fair value	426
Net recognized gains on equity securities still held	5
Public common stock [Member]
Investment disclosure details
Equity securities, cost	336
Gross gains	19
Gross losses	8
Equity securities, at fair value	347
Non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	72
Gross gains	10
Gross losses	3
Equity securities, at fair value	$ 79

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
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-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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