Debt - Additional Information (Detail) (USD $)	6 Months Ended			1 Months Ended		6 Months Ended
	Jun. 30, 2012 Senior Notes Three Point Nine Percentage Senior Notes	Jun. 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	Jun. 30, 2012 5.75% Senior Notes Repayment of Debt
Debt Instrument [Line Items]
Senior notes issued	$ 399,645,000	$ 399,657,000	$ 399,486,000
Senior notes, stated interest percentage	3.90%	5.75%	5.75%		5.75%	5.75%
Senior notes, due date	Jun. 15, 2022	Aug. 01, 2013	Aug. 01, 2013		Aug. 01, 2013
Redemption of senior notes					400,000,000	400,000,000
Senior notes, cost to redemption				432,500,000
Senior notes, redemption premium				21,300,000
Senior notes, accrued and unpaid interest				$ 11,200,000