Financial Instruments (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Fair Value of Debt Instruments [Abstract]
Fair value of total debt	$ 14,100,000,000us-gaap_DebtInstrumentFairValue	$ 13,000,000,000us-gaap_DebtInstrumentFairValue
Fair value of total debt in excess of carrying value	1,800,000,000	1,500,000,000
Fixed rate debt securities containing call provisions	$ 163,000,000	$ 163,000,000

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13476-108611

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