Investments (Leverage Leases - Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Percentage of rental receivables performing	100.00%	100.00%
Deferred income tax liability related to leveraged leases	$ 43	$ 74
Minimum
Loans and Leases Receivable Disclosure [Line Items]
Lessor, Operating Lease, Term of Contract	1 year
Maximum
Loans and Leases Receivable Disclosure [Line Items]
Lessor, Operating Lease, Term of Contract	15 years

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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