Subsequent Events - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		1 Months Ended			6 Months Ended	1 Months Ended	3 Months Ended		3 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jul. 31, 2012 Treasury Locks Agreements	Jul. 31, 2012 Three Point Six Zero Percentage Senior Notes Due Twenty Twenty Two Refinancing of Debt	Jul. 17, 2012 Three Point Six Zero Percentage Senior Notes Due Twenty Twenty Two Refinancing of Debt	Jun. 30, 2012 6 5/8% Senior Notes Due 2016	Jul. 31, 2012 6 5/8% Senior Notes Due 2016 Redemptions by Issuers	Sep. 30, 2012 6 5/8% Senior Notes Due 2016 Redemptions by Issuers	Jul. 30, 2012 6 5/8% Senior Notes Due 2016 Redemptions by Issuers	Sep. 30, 2012 6 5/8% Senior Notes Due 2016 Redemptions by Issuers Early Debt Redemption Premium	Sep. 30, 2012 6 5/8% Senior Notes Due 2016 Redemptions by Issuers Write Off Of Debt Issue Costs
Subsequent Event [Line Items]
Senior notes, principal amount issued							$ 250,000
Maturity						2022		2016
Senior notes, redeemed								250,000	250,000
Redemption price, percentage of principal amount											102.208%
Non-operating expense	1,107	1,632	2,454	2,839						(6,797)		(5,520)	(1,277)
Payment to counterparties					$ 1,940