Members' Capital and Distributions (Details 2) (USD $)	0 Months Ended		3 Months Ended					9 Months Ended
	Oct. 31, 2012	Jan. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Oct. 10, 2012	Jul. 11, 2012	Apr. 11, 2012	Jan. 11, 2012
Members' Capital and Distributions
Issuance of common units (in shares)	6,526,078	5,750,000
Per unit price of underwritten offering of common units	$ 32.13	$ 34.03
Proceeds from public offering of common units	$ 201,449,000	$ 187,763,000						$ 188,083,000
Distribution Per Unit (in dollars per share)			$ 0.575	$ 0.575	$ 0.575	$ 0.575	$ 0.575	$ 1.725	$ 1.725
Cash and amount distributions to common unitholders										$ 46,087,000	$ 42,336,000	$ 42,113,000	$ 42,064,000