Summary of Changes in the LP Unit Options Outstanding, All of which are Vested or Expected to Vest, Under the Option Plan (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Year		Dec. 31, 2010 Year
Number of LP Units
Beginning Balance	241,800		349,400
Exercised	(144,800)		(107,900)
Forfeited, cancelled or expired			300
Ending Balance	97,000		241,800
Exercisable at December 31, 2011	97,000
Weighted-Average Strike Price ($/LP Unit)
Beginning Balance	$ 47.04		$ 46.25
Exercised	$ 47.10		$ 44.41
Forfeited, cancelled or expired			$ 58.96
Ending Balance	$ 46.81		$ 47.04
Exercisable at December 31, 2011	$ 46.81
Weighted-Average Remaining Contractual Term (in years)
Outstanding at year end	4.2		5.8
Exercisable at December 31, 2011	4.2
Aggregate Intrinsic Value
Exercisable at December 31, 2011	$ 1,666	[1]
Outstanding at year end	$ 1,666	[1]	$ 4,785	[1]

[1] Aggregate intrinsic value reflects fully vested LP Unit options at the date indicated. Intrinsic value is determined by calculating the difference between our closing LP Unit price on the last trading day in 2011 and the exercise price, multiplied by the number of exercisable, in-the-money options.