Long-Term Debt and Credit Facility (Details Textual) (USD $)	Sep. 30, 2012	Aug. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Fixed-rate notes, due	$ 1,700,000,000		$ 1,200,000,000
Fixed interest rate on senior note		2.50%
Effective intrest rate		2.585%
Percent discount		99.255%
Redemption rate	101.00%
Unsecured revolving facility loans aggregate available amount to be drawn	500,000,000
Amount outstanding on line of credit facility	0		0
2.500% Fixed Rate Notes Due 2022 [Member]
Debt Instrument [Line Items]
Fixed-rate notes, due	$ 500,000,000		$ 0
Fixed interest rate on senior note	2.50%