Fair Value (Carrying amount and estimated fair value of financial instrument assets and liabilities which are not measured at fair value) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Financial Assets [Abstract]
Mortgage loans	$ 675	$ 678
Financial Liabilities [Abstract]
Short term debt	0	350
Long term debt	2,708	2,210
Carrying Amount [Member]
Financial Assets [Abstract]
Mortgage loans	675	678
Financial Liabilities [Abstract]
Short term debt		350
Long term debt	2,708	2,210
Estimated Fair Value [Member]
Financial Assets [Abstract]
Mortgage loans	697	688
Financial Liabilities [Abstract]
Short term debt		360
Long term debt	2,943	2,433
Level 1 [Member] \| Estimated Fair Value [Member]
Financial Assets [Abstract]
Mortgage loans	0	0
Financial Liabilities [Abstract]
Short term debt		0
Long term debt	0	0
Level 2 [Member] \| Estimated Fair Value [Member]
Financial Assets [Abstract]
Mortgage loans	0	0
Financial Liabilities [Abstract]
Short term debt		360
Long term debt	2,943	2,433
Level 3 [Member] \| Estimated Fair Value [Member]
Financial Assets [Abstract]
Mortgage loans	697	688
Financial Liabilities [Abstract]
Short term debt		0
Long term debt	$ 0	$ 0

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- References

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- Definition

Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(c))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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