Fair Value of the Stock-Settled Performance Units (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Performance Unit Awards 2012
Schedule of Share based Compensation Arrangements by Share based Payment Award, Performance Options [Line Items]
Fair value at date of grant	$ 3,065
Performance Unit Awards 2011
Schedule of Share based Compensation Arrangements by Share based Payment Award, Performance Options [Line Items]
Fair value at date of grant	5,569
Performance Unit Awards 2010
Schedule of Share based Compensation Arrangements by Share based Payment Award, Performance Options [Line Items]
Fair value at date of grant	$ 3,117