Income Taxes (Details 3) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes
Net income, as reported	$ 18,925,000	$ 18,103,000	$ 17,752,000	$ 6,939,000	$ 11,359,000	$ 10,035,000	$ 27,896,000	$ 14,733,000	$ 61,719,000	$ 64,023,000	$ 10,509,000
Book/tax depreciation and amortization adjustment									18,644,000	15,979,000	44,329,000
Book/tax adjustment for unit-based compensation expense									941,000	1,073,000	797,000
Book/tax adjustment for interest rate swap terminations									2,025,000	(4,983,000)	898,000
Other temporary differences									(9,435,000)	(4,147,000)	(4,104,000)
Other permanent differences									1,413,000	5,000	9,000
U.S. federal partnership taxable income									75,307,000	71,950,000	52,438,000
Accruals for taxes									4,100,000	6,500,000
Net tax basis in the entity's assets and liabilities which is less than the reported amounts on the financial statements	$ 381,600,000								$ 381,600,000