Revolving Bank Loan Facility - Additional Information (Detail) (Line of Credit, USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Line of Credit
Line of Credit Facility [Line Items]
Revolving loan facility, borrowing capacity	$ 45,000,000
Revolving loan facility, maturity date	Apr. 30, 2015
Alternative interest rate (applies if higher than Prime Rate) (percent)	3.25%
Interest rate (applies if higher than alternative interest rate)	U.S. Prime Rate
Weighted average daily borrowings outstanding under the loan facility	$ 33,900,000	$ 30,000,000
Weighted average interest rate (percent)	3.30%	3.30%	3.50%