Investments Investments - Valuation Changes Included in Net Income for Investments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2018
Debt Securities, Available-for-sale [Line Items]
Total valuation changes	$ 31	$ 63
Equity securities
Debt Securities, Available-for-sale [Line Items]
Total valuation changes	20	14
Limited partnership interests
Debt Securities, Available-for-sale [Line Items]
Total valuation changes	$ 11	$ 49

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

Name:	us-gaap_EquitySecuritiesFvNiRealizedGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration