Senior Note Due May 1, 2016 - Additional Information (Detail) (USD $)	1 Months Ended		12 Months Ended
	Apr. 30, 2009	Apr. 17, 2006	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Senior notes			$ 114,424,000	$ 114,276,000
Senior Notes due May 1, 2016
Debt Instrument [Line Items]
Principal amount of senior notes		125,000,000
Interest rate stated on senior notes		7.00%
Senior notes due date		May 01, 2016
Net proceeds from issuance of senior notes		123,500,000
Aggregate principal amount of senior notes repurchased from unaffiliated note holder	10,000,000
Repurchased senior notes	7,000,000
Pretax gain from repurchase of senior notes	3,000,000
Outstanding principal amount of senior notes			114,400,000
Unamortized discount			600,000
Senior notes			115,000,000
Fair value of senior notes			123,200,000	119,300,000
Interest expense on senior notes			$ 8,200,000	$ 8,200,000
Effective interest rate related to senior notes			7.17%
Debt instrument, covenant compliance			As of December 31, 2012, the Company was in compliance with all such covenants.
Senior Notes due May 1, 2016 | Semi Annual Payment, First Payment
Debt Instrument [Line Items]
Interest payment date on senior notes			--05-01
Senior Notes due May 1, 2016 | Semi Annual Payment, Second Payment
Debt Instrument [Line Items]
Interest payment date on senior notes			--11-01