Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of Significant Accounting Policies [Line Items]
Maturity period of short-term investments	13 weeks
Assets held for sale	$ 0	$ 0
Impairments of goodwill	0	0	0
Insurance coverage of general liability and auto liability risks	100,000,000
Insurance policy covering pollution liability	$ 50,000,000
Tax benefit recognized	Greater than 50%
Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Ownership interest in equity method investment	20.00%
Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Ownership interest in equity method investment	50.00%