Investments (Securities Lending Remaining Tenor - Narrative) (Details) $ in Billions	Dec. 31, 2018 USD ($)
Estimated fair value
Securities Financing Transaction [Line Items]
Securities Loaned, Not Subject to Master Netting Arrangement	$ 2.7

Amount of securities loaned to entities in exchange for collateral, not subject to a master netting arrangement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313

Name:	us-gaap_SecuritiesLoanedNotSubjectToMasterNettingArrangement
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant