Senior Notes - Summary of Fixed-Rate Senior Notes (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Senior Notes [Line Items]
Senior Notes	$ 1,825,200,000		$ 1,658,600,000
Debt premium (discount)	4,000,000		(5,400,000)
Senior notes	2,147,097,000		2,139,718,000
Debt Instrument, Stated Percentage Rate, Minimum	4.625%
Debt Instrument, Stated Percentage Rate, Maximum	9.625%
Debt Instrument Effective Interest Rate Minimum	4.80%
Debt Instrument Effective Interest Rate Maximum	9.90%
Maturity Date Range, Start	May 31, 2015
Maturity Date Range, End	Jul. 31, 2022
Senior Convertible Notes Due 2040 [Member]
Senior Notes [Line Items]
Senior Convertible Notes	317,900,000	[1]	307,600,000	[1]
Debt Instrument, Stated Percentage	1.75%	[1]
Debt Instrument Effective Interest Rate	5.30%	[1]
Debt Instrument, Maturity Date	Nov. 30, 2040	[1]
Senior Convertible Notes Due 2012 [Member]
Senior Notes [Line Items]
Senior Convertible Notes	$ 0		$ 178,900,000

[1] The Company may redeem the senior convertible notes any time on or after November 15, 2015, in whole or in part, for cash equal to 100% of the principal amount of the notes plus accrued and unpaid interest through, but excluding, the redemption date. In addition, the holders of the senior convertible notes may require the Company to repurchase their notes in whole or in part on specified dates, the first of which is November 15, 2015.