Senior Notes due May 1, 2016 - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended			1 Months Ended		12 Months Ended
	Mar. 31, 2011	Dec. 31, 2009	Dec. 31, 2011	Dec. 31, 2010	Apr. 30, 2009 Senior Notes due May 1, 2016	Apr. 17, 2006 Senior Notes due May 1, 2016	Dec. 31, 2011 Senior Notes due May 1, 2016	Dec. 31, 2010 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	Mar. 31, 2011					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			7,000,000
Pretax gain from repurchase of senior notes					3,000,000
Outstanding principal amount of senior notes							115,000,000
Unamortized discount							700,000
Senior Notes			114,276,000	114,138,000			114,300,000
Interest expense on senior notes							$ 8,200,000	$ 8,200,000
Effective interest rate related to senior notes							7.17%
Interest payment dates on senior notes						Interest is payable on the Senior Notes each May 1 and November 1.
Debt instrument, covenant compliance							As of December 31, 2011, the Company was in compliance with all such covenants.