Equity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	1 Months Ended			3 Months Ended			6 Months Ended		8 Months Ended	12 Months Ended
	Nov. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Sep. 30, 2013 USD ($)	Jun. 30, 2015 USD ($) $ / shares shares	Mar. 31, 2015 $ / shares	Jun. 30, 2014 USD ($) shares	Jun. 30, 2015 USD ($) item installment $ / shares shares	Jun. 30, 2014 USD ($) shares	Jun. 30, 2015 USD ($) $ / shares shares	Dec. 31, 2014 $ / shares
Equity Offerings
Proceeds from public equity offerings, net							$ 39,630	$ 711,837
Common Units
Equity Offerings
Aggregate common units sold (in units) | shares							636	11,284
Distributions of Available Cash
Distribution per common unit (in dollars per unit) | $ / shares					$ 0.91		$ 0.92			$ 0.90
Common Units | ATM
Equity Offerings
Aggregate common units sold (in units) | shares				600		7,000	600	11,200
Proceeds from public equity offerings, net				$ 40,000		$ 440,000	$ 40,000	$ 712,000
Common Units | September 2013 ATM
Equity Offerings
Aggregate common units sold (in units) | shares								4,200
Proceeds from public equity offerings, net								$ 272,000
Offering under program			$ 1,000,000
Manager fees								$ 4,000
Common Units | March 2014 ATM
Equity Offerings
Aggregate common units sold (in units) | shares						7,000		7,000
Proceeds from public equity offerings, net						$ 440,000		$ 440,000
Offering under program		$ 1,200,000
Manager fees								$ 3,500
Common Units | November 2014 ATM
Equity Offerings
Aggregate common units sold (in units) | shares				600			600		3,100
Proceeds from public equity offerings, net				$ 40,000			$ 40,000		$ 214,000
Offering under program	$ 1,500,000
Class B Units | M & R
Equity Offerings
Conversion ratio | item							1
Number of installments | installment							2
Maximum | Common Units
Distributions of Available Cash
Common Unit Price (in dollars per share) | $ / shares				$ 69.50	69.16		$ 69.50		$ 69.50	77.31
Minimum | Common Units
Distributions of Available Cash
Common Unit Price (in dollars per share) | $ / shares				$ 56.20	$ 54.04		$ 56.20		$ 56.20	$ 58.67