Investments (Details) - Investment Information, Equity Securities $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)
Investment disclosure details
Equity securities, cost	$ 409
Gross gains	23
Gross losses	8
Equity securities, at fair value	424
Net recognized gains on equity securities still held	3
Public common stock [Member]
Investment disclosure details
Equity securities, cost	335
Gross gains	12
Gross losses	5
Equity securities, at fair value	342
Non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	74
Gross gains	11
Gross losses	3
Equity securities, at fair value	$ 82

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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