Debt Obligations and Collateral Requirements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Debt Obligations And Collateral Requirements [Line Items]
Debt instrument, covenant compliance	The Company was in compliance with each of the covenants listed above at September 30, 2013
Unsecured Credit Agreement | Covenant Requirement
Debt Obligations And Collateral Requirements [Line Items]
Consolidated leverage ratio	1.5
Consolidated minimum annual EBITDA required computed quarterly on a rolling 12-month basis	$ 12
Consolidated tangible net worth required	36
Unsecured Credit Agreement | Revolving Credit Facility
Debt Obligations And Collateral Requirements [Line Items]
Credit facility, maximum borrowing capacity	10
Credit facility, expiration date	May 01, 2015
Credit facility, additional interest above Federal Fund Rate	0.50%
Credit facility, interest rate description	Interest on amounts borrowed is payable at a base rate or a Eurodollar rate, in each case as defined in the agreement. The base rate is equal to the higher of the Federal Funds Rate plus 0.5% or the bank's prime rate. Fees on outstanding letters of credit are either 1.50% (if the Company's consolidated leverage ratio is less than or equal to 1.251) or 1.75% (if the Company's consolidated leverage ratio is greater than 1.251) of face value per annum.
Credit facility, outstanding letters of credit	1.6	1.7
Credit facility, available capacity to issue additional letters of credit	3.4
Credit facility, available borrowing capacity	8.4
Unsecured Credit Agreement | Revolving Credit Facility | Letter of Credit
Debt Obligations And Collateral Requirements [Line Items]
Credit facility, maximum borrowing capacity	$ 5
Unsecured Credit Agreement | Revolving Credit Facility | Letter of Credit | If the Company's consolidated leverage ratio is less than or equal to 1.25:1
Debt Obligations And Collateral Requirements [Line Items]
Fees on outstanding letters of credit	1.50%
Consolidated leverage ratio	1.25
Unsecured Credit Agreement | Revolving Credit Facility | Letter of Credit | If the Company's consolidated leverage ratio is greater than 1.25:1
Debt Obligations And Collateral Requirements [Line Items]
Fees on outstanding letters of credit	1.75%
Consolidated leverage ratio	1.25