Investments (details) - Impairment Charges - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Impairment charges
Impairment charges	$ 14	$ 29	$ 52
Fixed maturities [Member]
Impairment charges
Impairment charges	4	15	13
All other corporate bonds [Member]
Impairment charges
Impairment charges	4	15	13
Equity securities [Member]
Impairment charges
Impairment charges	9	12	37
Public common stock [Member]
Impairment charges
Impairment charges	9	9	37
Non-redeemable preferred stock [Member]
Impairment charges
Impairment charges		3
Other investments [Member]
Impairment charges
Impairment charges	$ 1	$ 2	$ 2

The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563

Name:	us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration