Long-Term Debt - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Debt Instrument [Line Items]
Total long-term debt	$ 159,900		$ 161,500
Repayments of Long-term Debt	3,934	$ 1,921
5.68% note, due June 30, 2026 [Member]
Debt Instrument [Line Items]
Total long-term debt	$ 29,000		29,000
Long-term debt, interest percentage	5.68%
Uncollateralized Senior Notes Due On Two Thousand Twenty Nine [Member]
Debt Instrument [Line Items]
Total long-term debt	$ 50,000		50,000
Long-term debt, interest percentage	3.88%
Uncollateralized Senior Note Two Due on December Two Thousand Twenty Eight [Member]
Debt Instrument [Line Items]
Total long-term debt	$ 20,000		$ 20,000
Long-term debt, interest percentage	3.73%