Investments Investments - Valuation Changes Included in Net Income for Investments (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total	$ 99	$ 29
Equity securities
Debt Securities, Available-for-sale [Line Items]
Total	118	(9)
Limited partnership interests
Debt Securities, Available-for-sale [Line Items]
Total	$ (19)	$ 38

Amount of realized gain (loss) from sale 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/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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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration