Partnership Organization (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Limited Partners' Capital Account [Line Items]
General partner ownership interest (in percent)	0.001%
Limited Partners' Capital Account, Units Outstanding	55,617,901	55,346,287
Partners' capital account, units held in treasury	1,444,082	215,696
Treasury Stock, Shares, Acquired	1,500,000
Units issued for options and compensation	271,614
Per-unit distribution made to limited partners	$ 1.6
Aggregate distribution made to limited partners	$ 88.8