Debt and Credit Facility (Details) (USD $)	1 Months Ended	3 Months Ended
	Nov. 30, 2011	Mar. 31, 2014	Dec. 31, 2013	Nov. 01, 2010
Summary of debt
Principle Balance		$ 1,400,000,000
Unaccreted discount		(5,000,000)
Total		1,395,000,000	1,394,000,000
Unsecured revolving facility loans aggregate available amount to be drawn				500,000,000
Credit Facitlity Extension	2 years
Amount outstanding on line of credit facility		0
Line of Credit Facility, Covenant Terms		The credit facility contains customary representations, warranties and covenants. Financial covenants require us to maintain a consolidated leverage ratio of no more than 3.25 to 1.00 and a consolidated interest coverage ratio of no less than 3.00 to 1.00.
Leverage ratio		3.25
Interest cover ratio		300.00%
2.375% fixed-rate notes, due 2015
Summary of debt
Fixed interest rate on senior note		2.375%
2.375% fixed-rate notes, due 2015 | Senior Notes [Member]
Summary of debt
Principle Balance		400,000,000
2.700% fixed-rate notes, due 2018
Summary of debt
Fixed interest rate on senior note		2.70%
2.700% fixed-rate notes, due 2018 | Senior Notes [Member]
Summary of debt
Principle Balance		500,000,000
2.500% fixed-rate notes, due 2022
Summary of debt
Fixed interest rate on senior note		2.50%
2.500% fixed-rate notes, due 2022 | Senior Notes [Member]
Summary of debt
Principle Balance		$ 500,000,000