Investments (Invested Assets on Deposit, Held in Trust and Pledged as Collateral-Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deposit Assets	$ 8,259	$ 7,644
Assets Held-in-trust	2,634	9,054
Policyholder Account Balances [Member]
Deposit Assets		34
Fixed maturity securities \| Policyholder Account Balances [Member]
Deposit Assets	34
Fixed maturity securities \| Reinsurance [Member]
Assets Held-in-trust	$ 42	$ 15

The total amount of cash and securities held by third party trustees pursuant to terms of debt instruments or other agreements as of the date of each statement of financial position presented, which can be used by the trustee only to pay the noncurrent portion of specified obligations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(b))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Name:	us-gaap_AssetsHeldInTrust
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant