Debt (Convertible Debt) (Details)		6 Months Ended
		Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Interest Rate		3.61%
Convertible Senior Debt [Member]
Debt Instrument [Line Items]
Outstanding Balance		$ 1,000,000,000	$ 1,000,000,000
Interest Rate		3.30%
Amount of General Partner OP Units per principal amount		$ 1,000
Convertible Senior Debt [Member] | 2018 Convertible Notes [Member]
Debt Instrument [Line Items]
Outstanding Balance		$ 597,500,000
Interest Rate		3.00%
Conversion Rate	[1]	60.5997
Convertible Senior Debt [Member] | 2020 Convertible Notes [Member]
Debt Instrument [Line Items]
Outstanding Balance		$ 402,500,000
Interest Rate		3.75%
Conversion Rate	[1]	66.7249

[1] Conversion rate represents the amount of the General Partner OP Units per $1,000 principal amount.