Condensed Consolidated Statements of Changes in Equity (USD $) In Thousands, unless otherwise specified	Total	Common Units	Class B Units	Non-controlling Interest	Redeemable Non-controlling Interest (Temporary Equity)
Balance at Dec. 31, 2011	$ 1,502,067	$ 679,309	$ 752,531	$ 70,227
Balance (in units) at Dec. 31, 2011		94,940	19,954
Increase (Decrease) in Equity
Issuance of units in public equity offerings, net of offering costs	425,629	425,629
Issuance of units in public equity offerings, net of offering costs (in units)		7,508
Distributions paid	(75,372)	(73,410)		(1,962)
Contributions from non-controlling interest	755			755
Share-based compensation activity	(2,343)	(2,343)
Share-based compensation activity (in units)		246
Excess tax benefits related to share-based compensation	2,207	2,207
Deferred income tax impact from changes in equity	(16,915)	(16,915)
Net (loss) income	16,273	16,020		253
Balance at Mar. 31, 2012	1,852,301	1,030,497	752,531	69,273
Balance (in units) at Mar. 31, 2012		102,694	19,954
Balance at Dec. 31, 2012	3,215,591	2,134,714	752,531	328,346
Balance (in units) at Dec. 31, 2012		127,494	19,954
Increase (Decrease) in Equity
Issuance of units in public equity offerings, net of offering costs	103,937	103,937
Issuance of units in public equity offerings, net of offering costs (in units)	1,900	1,921
Distributions paid	(106,793)	(105,945)		(848)
Contributions from non-controlling interest	385,219			385,219
Redeemable non-controlling interest classified as temporary equity	(326,249)			(326,249)	326,249
Share-based compensation activity	(1,204)	(1,204)
Share-based compensation activity (in units)		161
Excess tax benefits related to share-based compensation	651	651
Deferred income tax impact from changes in equity	(10,088)	(10,088)
Net (loss) income	(20,764)	(15,460)		(5,304)
Balance at Mar. 31, 2013					326,249
Balance at Mar. 31, 2013	$ 3,240,300	$ 2,106,605	$ 752,531	$ 381,164
Balance (in units) at Mar. 31, 2013		129,576	19,954