Fair Value Measurements - (Details) (Fair Value, Inputs, Level 2 [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fair Value of Assets and Liabilities Measured on a Recurring Basis [Roll Forward]
Fair Value, Beginning Balance	$ 53,804	$ 51,177
Investment accretion	3,827	3,451
Unrealized losses included in interest expense	(48)	(48)
Losses on interest rate swaps reclassified into interest expense from accumulated other comprehensive loss	1,685	1,678
Unrealized gains (losses) included in equity and partners’ capital	(3,498)	(2,454)
Fair Value, Ending Balance	55,770	53,804
Available-for-sale Securities [Member]
Fair Value of Assets and Liabilities Measured on a Recurring Basis [Roll Forward]
Fair Value, Beginning Balance	58,408	59,145
Investment accretion	3,827	3,451
Unrealized gains (losses) included in equity and partners’ capital	(1,192)	(4,188)
Fair Value, Ending Balance	61,043	58,408
Interest Rate Swap [Member]
Fair Value of Assets and Liabilities Measured on a Recurring Basis [Roll Forward]
Fair Value, Beginning Balance	(4,604)	(7,968)
Unrealized losses included in interest expense	(48)	(48)
Losses on interest rate swaps reclassified into interest expense from accumulated other comprehensive loss	1,685	1,678
Unrealized gains (losses) included in equity and partners’ capital	(2,306)	1,734
Fair Value, Ending Balance	$ (5,273)	$ (4,604)