Assumptions used for grants of Over Ride Units (Detail) (Override Units, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Year		Dec. 31, 2009 Year
Override Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Current equity value	$ 410,700	[1]	$ 439,060	[2]
Expected life in years	1.0	[1]	3.4	[2]
Risk-free interest rate	0.30%	[1]	1.80%	[2]
Volatility	0.25%	[1]	0.45%	[2]

[1] The assumptions were used for grants of the Value N-1 and N-2 Units during the period.

[2] The assumptions were used for grants of the Value A, Value B, and Operating Units during the period.