Debt (Details) (USD $)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2014 Credit Facility [Member]	Sep. 30, 2014 Unsecured Senior Notes Due 2019 [Member]	Sep. 30, 2014 Unsecured Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Credit Facility, Term Loan Amount Outstanding		$ 350,000,000
Credit facility, total borrowing capacity		1,000,000,000
Line of credit facility, revolving borrowing capacity		600,000,000
Aggregate principal amount of term loan		400,000,000
Line of Credit Facility, Periodic Payment		5,000,000
Letters of Credit Outstanding, Amount		48,200,000
Senior Notes			500,000,000	800,000,000
Stated interest rate on unsecured senior notes			6.375%
Gains (Losses) on Extinguishment of Debt	(884,000)
Line of Credit Facility, Amount Outstanding		$ 0