Schedule of Debt Outstanding (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended		9 Months Ended				9 Months Ended
	Sep. 30, 2012 Revolving Credit Facility	Dec. 31, 2011 Revolving Credit Facility	Sep. 30, 2012 Senior Notes 6.25% Due 2013	Sep. 30, 2012 Senior Notes 6.75% Due 2014	Jul. 31, 2012 Senior Notes 6.75% Due 2014	Jun. 30, 2012 Senior Notes 6.75% Due 2014	Sep. 30, 2012 Senior Notes 7.75% Due 2017	Dec. 31, 2011 Senior Notes 7.75% Due 2017
Debt Instrument [Line Items]
Interest rate stated percentage of Senior Notes			6.25%	6.75%			7.75%
Revolving Credit Facility maturity date	Dec. 31, 2016
Note maturity date			Mar. 15, 2013	Jan. 31, 2014			Jun. 01, 2017
Weighted average rate	1.70%	1.00%
Senior Notes interest payable dates	interest payable periodically at variable interest rates		interest payable semi-annually in March and September at 6.25%.	interest payable semi-annually in January and July at 6.75%.			interest payable semi-annually in June and December at 7.75%.
Notes issued with a discount							$ 13.4
Redeemable aggregate principal amount					50.0	60.0
Notes, unamortized							$ 8.8	$ 10.0