Unit Option Activity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Unit options, outstanding	224,500	341,500
Granted	280,672	0
Unit options, exercised	(206,150)	(6,300)
Unit options, forfeited	(5,000)	(110,700)
Unit options, outstanding	294,022	224,500	341,500
Unit options exercisable	83,518	224,500
Unit options outstanding, weighted average exercise price	$ 24.4	$ 23.1
Unit options granted, weighted average exercise price	$ 29.53	$ 0.00
Unit options exercised, weighted average exercise price	$ 24.19	$ 20.29
Unit options forfeited, weighted average exercise price	$ 23.81	$ 20.6
Unit options outstanding, weighted average exercise price	$ 29.45	$ 24.4	$ 23.1
Unit options exercisable, weighted average exercise price	$ 29.26	$ 24.4
Cash received from unit option exercises	$ 76	$ 5	$ 7