Investments (Other Invested Assets - Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Leveraged Leases, Net Investment in Leveraged Leases Disclosure, Deferred Taxes Arising from Leveraged Leases	$ 934	$ 1,400
Carrying value of Tax Credits	1,800	1,700
Losses From Tax Credits	$ 259	$ 167	$ 163

Taxes arising from arrangements meeting the criteria for classification as leveraged leases due to timing differences between accounting and tax income recognition.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 35
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=77915205&loc=d3e43930-112732

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77907235&loc=d3e45152-112736

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