Debt (Narrative) (Details) (USD $)	1 Months Ended	9 Months Ended		1 Months Ended	9 Months Ended	1 Months Ended	9 Months Ended	1 Months Ended	9 Months Ended
	Oct. 31, 2011	Sep. 30, 2011	Sep. 30, 2011 Line of Credit [Member]	Aug. 31, 2011 4.25% Notes Due 2021 [Member]	Sep. 30, 2011 4.25% Notes Due 2021 [Member]	Oct. 31, 2011 Maximum [Member]	Sep. 30, 2011 Maximum [Member]	Oct. 31, 2011 Minimum [Member]	Sep. 30, 2011 Minimum [Member]
Debt Instrument [Line Items]
Long-term debt		$ 2,100,000,000		$ 250,000,000	$ 550,000,000
Line of credit facility maturity date	October 2016	September 2012
Borrowing capacity	800,000,000	550,000,000
Interest on LIBOR rate						1.75%	0.80%	0.875%	0.30%
Commitment fee						0.30%	0.125%	0.125%	0.05%
Line of credit facility amount outstanding		0
Repayments through debt proceeds				193,000,000
Obligation for letters of credit			4,600,000
Debt instrument interest rate				4.25%	4.25%
Debt instrument, maturity date				2021	2021
Percentage of notes price				104.10%
Notes price				260,200,000
Accrued interest				700,000
Debt after underwriting discounts				258,700,000
Underwriting discounts				1,600,000
Other offering costs				$ 600,000