The average effective interest rate during the reporting period.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

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

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-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

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-Subparagraph (SX 210.5-02.22(a)(1))

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Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

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-Article 5

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

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Debt Obligations (Changes in Debt Obligations Additional Information) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Jan. 31, 2010	Dec. 31, 2010	Sep. 30, 2011
Senior Notes [Member] \| 4.00% senior unsecured notes due January 18, 2014 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	4.00%
Debt instrument, maturity date	Jun. 18, 2014
Senior Notes [Member] \| 5.55% senior unsecured notes due January 15, 2020 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	5.55%	5.55%
Debt instrument, maturity date	Jan. 15, 2020
Senior Notes [Member] \| 5.25% senior unsecured notes due January 16, 2018 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	5.25%		5.25%
Debt instrument, maturity date	Jan. 16, 2018
Senior Notes [Member] \| 3.875% senior unsecured notes due June 7, 2021 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	3.875%
Debt instrument, maturity date	Jun. 07, 2021
Senior Notes [Member] \| 4.25% Senior Unsecured Notes Due June 1 2024 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	4.25%
Debt instrument, maturity date	Jun. 01, 2024
Senior Notes [Member] \| 2011 Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility, borrowing capacity	1,200			$ 1,200
Senior Notes [Member] \| $450 million senior unsecured term loan facility credit agreement due September 19, 2016 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	2.50%
Debt instrument, maturity date	Nov. 24, 2014
Debt Instrument, interest rate during period	1.53%
Credit facility, borrowing capacity	450			450
Revolving Credit Facility [Member] \| $750 million revolving credit commitment due November 25, 2019 [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	Nov. 25, 2019
Debt Instrument, interest rate during period	1.41%
Credit facility, borrowing capacity	750
Revolving Credit Facility [Member] \| $750 million revolving credit commitment due September 19, 2016 [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	Nov. 24, 2014
Debt Instrument, interest rate during period	1.33%
Credit facility, borrowing capacity	750			$ 750