Debt - Additional Information (Detail) (USD $)	12 Months Ended				12 Months Ended	1 Months Ended	12 Months Ended								1 Months Ended	12 Months Ended		12 Months Ended		1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 25, 2008	Dec. 31, 2012 New Senior Credit Facility	Oct. 31, 2011 Old Senior Credit Facility	Dec. 31, 2012 Receivables Credit Facility	Dec. 31, 2011 Receivables Credit Facility	Oct. 11, 2011 Receivables Credit Facility	Oct. 11, 2011 Term Loan Facility New Senior Credit Facility	Oct. 11, 2011 Line of Credit New Senior Credit Facility	Dec. 31, 2012 Letter of Credit New Senior Credit Facility	Oct. 11, 2011 Letter of Credit New Senior Credit Facility	Dec. 31, 2012 Line of Credit Receivables Credit Facility	Jun. 30, 2012 3.90% Senior Notes	Dec. 31, 2012 3.90% Senior Notes	Jun. 26, 2012 3.90% Senior Notes	Dec. 31, 2012 5.75% Senior Notes	Dec. 31, 2011 5.75% Senior Notes	Jul. 31, 2012 5.75% Senior Notes Repayment of Debt	Jun. 30, 2012 5.75% Senior Notes Repayment of Debt	Jul. 26, 2012 5.75% Senior Notes Repayment of Debt	Jun. 26, 2012 5.75% Senior Notes Repayment of Debt
Debt Instrument [Line Items]
Offerings to qualified institutional investors				$ 150,000,000
Senior notes, stated interest percentage				6.50%												3.90%	3.90%		5.75%				5.75%
Debt Instrument Maturity Year				2018
Credit facility borrowing capacity	450,000,000						200,000,000		150,000,000	150,000,000	250,000,000		50,000,000	200,000,000
Credit facility, expiration date					Oct. 11, 2016		Oct. 11, 2014	Oct. 11, 2014
Credit facility, expiration Month and Year						2013-04
Outstanding letters of credit	13,700,000											13,700,000
Unused borrowing capacity					236,300,000
Credit facility Additional borrowing capacity							91,000,000
Senior notes issued																399,662,000	400,000,000		399,486,000
Senior notes, due date															Jun. 15, 2022	Jun. 15, 2022		Aug. 01, 2013			Aug. 01, 2013
Redemption of senior notes																					400,000,000
Senior notes, cost to redemption																				432,500,000
Senior notes, redemption premium	(21,296,000)																			21,300,000
Senior notes, accrued and unpaid interest																						11,200,000
Annual principal maturities for debt, 2013	15,000,000
Annual principal maturities for debt, year two	124,000,000
Annual principal maturities for debt, year three	15,000,000
Annual principal maturities for debt, year four	90,000,000
Annual principal maturities for debt, after year five	550,000,000
Interest payments for debt obligations	66,300,000	35,200,000	35,300,000
Amortization of financing costs	1,116,000	712,000	648,000
Amortization of net loss (gain) on treasury lock	(2,988,000)	1,846,000	1,846,000
Credit facility outstanding	122,709,000						109,000,000
Outstanding principal balance, maximum							$ 109,000,000