Credit Facilities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Line of Credit Facility [Line Items]
Weighted average interest rate on outstanding borrowings	4.01%	4.01%
Unsecured revolving line of credit facility	$ 850,000,000
Line of credit facility, interest rate at period end	0.96%
Line of credit facility amount of option to increase additional borrowing	350,000,000
Outstanding line of credit	0	0
Unused letter of credit outstanding due to workers' compensation and insurance reserve	$ 61,119,000	$ 53,703,000
Line of credit facility, Maturity date	September 2017