Indebtedness (Commercial Paper Narrative) (Details) (USD $)	1 Months Ended
	Oct. 31, 2011 days	Dec. 31, 2011	Mar. 31, 2011
Debt Instrument [Line Items]
Commercial Paper		$ 406,861,000	$ 0
Commercial paper program, commencement date	2011-10
Short term debt maturity period (in days)	90
Credit Facility [Member]
Debt Instrument [Line Items]
Revolving line of credit, maximum borrowing capacity		750,000,000
Commercial Paper [Member]
Debt Instrument [Line Items]
Revolving line of credit, maximum borrowing capacity		$ 750,000,000
Debt Instrument, Interest Rate, Effective Percentage		0.55%