Debt - Additional Information (Detail) (USD $)	9 Months Ended	1 Months Ended	9 Months Ended		9 Months Ended		1 Months Ended
	Sep. 30, 2012	Jun. 30, 2012 3.90% Senior Notes	Sep. 30, 2012 3.90% Senior Notes	Jun. 26, 2012 3.90% Senior Notes	Sep. 30, 2012 5.75% Senior Notes	Dec. 31, 2011 5.75% Senior Notes	Jul. 31, 2012 5.75% Senior Notes Repayment of Debt	Jun. 30, 2012 5.75% Senior Notes Repayment of Debt	Jul. 26, 2012 5.75% Senior Notes Repayment of Debt	Jun. 26, 2012 5.75% Senior Notes Repayment of Debt
Debt Instrument [Line Items]
Senior notes issued			$ 399,654,000	$ 400,000,000		$ 399,486,000
Senior notes, stated interest percentage			3.90%	3.90%		5.75%				5.75%
Senior notes, due date		Jun. 15, 2022	Jun. 15, 2022		Aug. 01, 2013			Aug. 01, 2013
Redemption of senior notes								400,000,000
Senior notes, cost to redemption							432,500,000
Senior notes, redemption premium	(21,296,000)						21,300,000
Senior notes, accrued and unpaid interest									$ 11,200,000