Investments (Leverage Leases - Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable, Other Information	The payment periods for leveraged leases generally range from one to 15 years but in certain circumstances can be over 25 years, while the payment periods for direct financing leases range from one to 20 years.
Percentage of rental receivables performing	100.00%	100.00%
Leveraged Leases [Abstract]
Deferred income tax liability related to leveraged leases	$ 934	$ 1,400
Performing Financing Receivable [Member] \| Minimum
Percentage of rental receivables performing	99.00%	99.00%

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