Special Items (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2011 Mr. J. Mike Walker [Member]	Dec. 31, 2011 Mr. J. Mike Walker [Member]	Dec. 31, 2010 Mr. Reimert [Member]	Dec. 31, 2010 Mr. Reimert [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Pre-tax expense	$ 4.7		$ 6.9
Salary and bonus including payroll taxes		3.3		4.9
Pre-tax non-cash expenses		$ 1.4		$ 2.0