Investments (Pledged Investments) (Details) - USD ($) $ in Billions	Mar. 31, 2018	Dec. 31, 2017
Investments [Abstract]
Cash and investments at fair value on deposit with, or in trust accounts for the benefit of various counterparties	$ 4.7	$ 3.3
Cash and investments at fair value on deposit with, or in trust accounts for the benefit of U.S. state regulatory authorities	$ 1.7	$ 1.4

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