Debt Credit Agreement (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2012 Letter of Credit [Member]	Dec. 31, 2011 Letter of Credit [Member]	Jun. 30, 2012 Previous Contract [Member] Letter of Credit [Member]	Mar. 31, 2012 Previous Contract [Member] Letter of Credit [Member]	Dec. 31, 2012 New Contract [Member] Letter of Credit [Member]	Dec. 31, 2012 Letter of Credit [Member]	Dec. 31, 2012 Unitholder Distributions [Member] Letter of Credit [Member]
Debt Instrument [Line Items]
Loss on early extinguishment of debt	$ (2,979,000)	$ 0	[1]	$ 0	[1]
Credit agreement, maximum borrowing capacity								375,000,000	275,000,000	550,000,000	50,000,000	60,000,000
Proceeds from lines of credit	587,000,000	118,000,000	[1]	66,000,000	[1]	587,000,000
Repayments of lines of credit	366,000,000	77,000,000	[1]	113,000,000	[1]	(366,000,000)
Proceeds from (repayments of) lines of credit						221,000,000
Long-term line of credit						$ 421,000,000	$ 200,000,000

[1] (1) Restated as described in Note 2.