Debt and Financing Arrangements - Additional Information (Detail) (USD $)	9 Months Ended		9 Months Ended						1 Months Ended	9 Months Ended					9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Senior Notes under a Master Shelf Agreement [Member]	Dec. 31, 2012 Senior Notes under a Master Shelf Agreement [Member]	Jan. 31, 2008 Senior Notes under a Master Shelf Agreement [Member]	Nov. 30, 2007 Senior Notes under a Master Shelf Agreement [Member]	Apr. 30, 2005 Senior Notes under a Master Shelf Agreement [Member]	Sep. 20, 2002 Senior Notes under a Master Shelf Agreement [Member]	Jun. 28, 2013 Revolving credit facility [Member]	Sep. 30, 2013 Revolving credit facility [Member]	Jun. 28, 2013 Revolving credit facility [Member] Credit Agreement Before Amended [Member]	Sep. 30, 2013 Credit Agreement with Banks [Member]	Dec. 31, 2012 Credit Agreement with Banks [Member]	Sep. 30, 2012 Credit Agreement with Banks [Member]	Sep. 30, 2013 Credit Agreement with Banks [Member] Minimum [Member]	Sep. 30, 2013 Credit Agreement with Banks [Member] Maximum [Member]	Sep. 30, 2013 Credit Agreement with Banks [Member] LIBOR Rate Margin [Member] Minimum [Member]	Sep. 30, 2013 Credit Agreement with Banks [Member] LIBOR Rate Margin [Member] Maximum [Member]	Sep. 30, 2013 Credit Agreement with Banks [Member] Base rate margin [Member] Minimum [Member]	Sep. 30, 2013 Credit Agreement with Banks [Member] Base rate margin [Member] Maximum [Member]
Debt Disclosure [Line Items]
Borrowing capacity under credit agreement									$ 200,000,000	$ 200,000,000	$ 150,000,000
Amendment line of credit facility expiration year and month									2018-06
Line of credit facility expiration year and month										2018-06
Additional borrowing capacity under revolving credit facility										40,000,000
Rate margin																	1.25%	2.50%	0.125%	0.50%
Letter of credit fee															1.375%	2.625%
Letter of credit unused portion fee															0.20%	0.325%
Company's leverage ratio	3.00%
Letter of credit facility outstanding amount												59,100,000		58,600,000
Total debt	91,506,000	60,705,000	39,643,000	50,715,000								51,863,000	9,990,000	19,400,000
Senior notes issued, value					25,000,000	25,000,000		100,000,000
Senior notes authorized amount							$ 150,000,000	$ 125,000,000
Interest rate on notes					6.17%	6.14%		7.38%
Senior credit facility, Interest rate terms description			Payments due under the $100 million Senior Notes were interest only until June 30, 2006 and at that time semi-annual principal payments began with the final payment due December 2013. The November 2007 issuance of $25 million Senior Notes has a fixed interest rate of 6.14 percent. The January 2008 issuance of $25 million Senior Notes has a fixed interest rate of 6.17 percent. Payments due for both $25 million issuances were interest only until June 30, 2011 and at that time semi-annual principal payments began with the final payments due January 1, 2018.