Debt - Additional Information 6 (Detail) (USD $)	1 Months Ended					1 Months Ended	3 Months Ended	1 Months Ended				1 Months Ended		1 Months Ended
	Apr. 18, 2013	Aug. 29, 2012	Mar. 31, 2013	Dec. 31, 2012	Apr. 24, 2009	Apr. 13, 2012 Senior Notes [Member]	Mar. 31, 2013 Senior Notes [Member]	Jul. 24, 2009 Senior Notes Due Two Thousand Sixteen [Member]	Jul. 24, 2009 Senior Notes Due Two Thousand Nineteen [Member]	Mar. 31, 2013 8.0% Senior Notes [Member]	Mar. 31, 2013 8.25% Senior Notes [Member]	Apr. 13, 2012 8.0% Senior Notes due 2016 [Member]	Mar. 31, 2013 8.0% Senior Notes due 2016 [Member]	Apr. 13, 2012 8.25% Senior Notes [Member]	Mar. 31, 2013 8.25% Senior Notes [Member]	Aug. 29, 2012 8.0% Senior Notes [Member]
Line of Credit Facility [Line Items]
Unsecured senior notes							$ 750,000,000	$ 375,000,000	$ 375,000,000
Debt instrument, maturity date	May 01, 2013							Aug. 15, 2016	Aug. 15, 2019
Equity component related to repurchases, convertible, effective interest rate										8.00%	8.25%		8.00%		8.25%	8.00%
Percentage of face value price for issuance of senior notes								99.33%	99.152%
Interest payable dates								February 15 and August 15	February 15 and August 15
Deferred financing fees					7,700,000		16,500,000
Net proceeds of offering after deducting discounts							727,800,000
Debt instrument, aggregate principal amount redeemed												131,300,000		131,300,000
Premium on redemption						21,300,000
Debt discount						1,500,000
Deferred financing fees						4,300,000
Debt instrument, aggregate principal amount redeemed																243,800,000
Premium on redemption		14,600,000
Write off of debt discount		1,000,000
Write off of deferred financing fees		3,400,000
Carrying value of senior note											243,800,000
Debt instrument, principal balance			$ 5,430,247,000	$ 5,440,073,000							$ 242,200,000