Carrying Value and Fair Values of Outstanding Debt (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Outstanding debt	$ 643.0		$ 635.9
Estimate of Fair Value, Fair Value Disclosure | 8.375% Senior Notes Due In 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Outstanding debt	233.8		231.4
Estimate of Fair Value, Fair Value Disclosure | 8.125% Senior Notes Due In 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Outstanding debt	409.2		404.5
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Outstanding debt	590.0	[1]	590.0	[1]
Carrying (Reported) Amount, Fair Value Disclosure | 8.375% Senior Notes Due In 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Outstanding debt	215.0	[1]	215.0	[1]
Carrying (Reported) Amount, Fair Value Disclosure | 8.125% Senior Notes Due In 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Outstanding debt	$ 375.0	[1]	$ 375.0	[1]

[1] The fair values are based on the trading levels and bid/offer prices observed by a market participant and are considered Level 1 inputs.