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Senior Notes due May 1, 2016 - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended			3 Months Ended		9 Months Ended
	Mar. 31, 2011	Apr. 30, 2009 Senior Notes due May 1, 2016	Apr. 17, 2006 Senior Notes due May 1, 2016	Sep. 30, 2011 Senior Notes due May 1, 2016	Sep. 30, 2010 Senior Notes due May 1, 2016	Sep. 30, 2011 Senior Notes due May 1, 2016	Sep. 30, 2010 Senior Notes due May 1, 2016
Debt Instrument [Line Items]
Principal amount of senior notes			$ 125
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			123.5
Interest payment dates on senior notes			The interest payment dates on the Senior Notes are each May 1 and November 1.
Effective interest rate related to senior notes			7.17%
Interest expense on senior notes				2.0	2.0	6.1	6.1
Debt instrument, covenant compliance						As of September 30, 2011, the Company was in compliance with all such covenants.
Aggregate principal amount of senior notes repurchased from unaffiliated note holder		10.0
Repurchased senior notes		7.0
Pretax gain from repurchased of senior notes		2.9
Outstanding principal amount of senior notes				$ 114		$ 114