Unit Option Activity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Unit options, outstanding	294,022	224,500
Granted, net of forfeitures	409,837
Unit options, exercised	(16,278)	(206,150)
Unit options, forfeited	(6,170)	(5,000)
Unit options, outstanding	684,822	294,022	224,500
Unit options exercisable	274,252	83,518
Unit options outstanding, weighted average exercise price	$ 29.45	$ 24.4
Unit options granted, weighted average exercise price	$ 36.95	$ 29.53
Unit options exercised, weighted average exercise price	$ 28.36	$ 24.19
Unit options forfeited, weighted average exercise price	$ 32.93	$ 23.81
Unit options outstanding, weighted average exercise price	$ 33.97	$ 29.45	$ 24.4
Unit options exercisable, weighted average exercise price	$ 32.61	$ 29.26
Cash received from unit option exercises	$ 52	$ 76	$ 5