Unitholders' Capital (Details) (USD $)	1 Months Ended		3 Months Ended	9 Months Ended
	Oct. 31, 2011	Oct. 31, 2008	Sep. 30, 2011	Sep. 30, 2011
Equity [Abstract]
Date of equity distribution agreement			Aug. 23, 2011
Equity distribution agreement maximum value			$ 500,000,000
Equity distribution agreement origination expense			400,000
Equity distribution agreement units sold			16,060
Equity distribution agreement price per unit sold			38.25
Equity distribution agreement net proceeds			602,000
Equity distribution agreement commission fees			12,000
Equity distribution agreement remaining balance			499,000,000	499,000,000
Public offering units sold				16,726,067
Public offering price per unit				38.80
Public offering price per unit, net of underwriting discount				37.248
Public offering net proceeds				623,000,000
Public offering underwriting discount and offering expenses				26,000,000
Authorized repurchase value of units		100,000,000
Units repurchased during the period	129,734		400,000
Average unit price of units repurchased	$ 32.08		$ 32.98
Total cost of units repurchased	4,000,000		13,000,000
Remaining balance of unit repurchase program			61,000,000	61,000,000
Per unit cash dividend declared (in dollars per share)			$ 0.69	$ 0.69
Estimated distribution payable in subsequent period			$ 122,000,000	$ 122,000,000