Income Taxes (Details 3) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Taxes
Unrecognized tax benefits	$ 543,000								$ 543,000
Decrease in unrecognized tax benefits necessary on or before December 31, 2013 due to the settlement of audits	500,000								500,000
Net income (loss), as reported	14,674,000	10,380,000	(19,050,000)	4,505,000	4,515,000	3,253,000	(1,938,000)	223,000	10,509,000	6,053,000	(23,333,000)
Book/tax depreciation and amortization adjustment									44,329,000	6,201,000	7,524,000
Book/tax adjustment for unit-based compensation expense									797,000	532,000	1,209,000
Book/tax adjustment for interest rate swap terminations									898,000	10,252,000	(12,386,000)
Other temporary differences									(4,104,000)	(2,496,000)	2,212,000
Other permanent differences									9,000	93,000	4,000
U.S. federal partnership taxable income (loss)									52,438,000	20,635,000	(24,770,000)
Net tax basis in the entity's assets and liabilities which is less than the reported amounts on the financial statements	$ 352,400,000								$ 352,400,000