Senior Notes - Additional Information (Detail) (USD $)	12 Months Ended					1 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Senior Convertible Notes Due 2040 [Member]	Dec. 31, 2011 Senior Convertible Notes Due 2040 [Member]	Jun. 30, 2012 4.625% senior unsecured notes [Member]	Dec. 31, 2012 4.625% senior unsecured notes [Member]	Nov. 30, 2012 4.625% senior unsecured notes [Member]	Dec. 31, 2012 Senior Unsecured Notes [Member]	Dec. 31, 2011 Senior Unsecured Notes [Member]	Dec. 31, 2010 Senior Unsecured Notes [Member]	Dec. 31, 2011 Senior Convertible Notes [Member]	Dec. 31, 2010 Senior Convertible Notes [Member]	Dec. 31, 2012 Senior Convertible Notes [Member]
Debt Instrument [Line Items]
Senior note principal amount	$ 450,000,000					$ 300,000,000		$ 150,000,000				$ 529,509,000		$ 350,000,000
Interest rate of senior notes							4.625%
Notes issued						300,000,000
Discount rate of Senior notes						98.104%
Premium rate of Senior notes								109.223%
Conversion price				$ 15.76	$ 16.19
Conversion price per share greater than applicable trading day				125.00%
Trading days, minimum				20 days
Trading days, maximum				30 days
Remaining amortization periods for debt discount				34 months
Aggregate Extinguishment of Debt, Amount	60,000,000	36,100,000	259,100,000
(Loss) gain on debt retirement, net	(13,495,000)	(89,000)	485,000						(13,500,000)	(100,000)	100,000
Senior convertible notes repurchased												19,400,000	140,600,000
Debt instrument - impact of convertible debt accretion												100,000	4,900,000
Average interest rate	5.80%	5.90%
Notes issued which may not be redeemed prior to maturity	$ 82,200,000
Covenant compliance	The Company was in compliance with all of the financial and other covenants.	The Company was in compliance with all of the financial and other covenants.