Convertible Debt - Additional Information (Detail) (USD $)	3 Months Ended				0 Months Ended	1 Months Ended	3 Months Ended		0 Months Ended	1 Months Ended			3 Months Ended				1 Months Ended			12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Convertible Senior Notes 2013	Nov. 13, 2006 Convertible Senior Notes 2013	Jan. 09, 2012 Convertible Senior Notes 2011	Nov. 30, 2006 Convertible Senior Notes 2011 and 2013	Mar. 31, 2013 Convertible Senior Notes 2011 and 2013	Mar. 31, 2012 Convertible Senior Notes 2011 and 2013	Jan. 09, 2012 Purchased Options	Nov. 30, 2006 Purchased Options	Nov. 30, 2006 Sold Warrants	Nov. 13, 2006 Sold Warrants	Mar. 31, 2013 Exercised Warrants	Dec. 31, 2011 Group 1 Purchased Options	Mar. 31, 2013 Group 2 Purchased Options	Dec. 31, 2010 Interest Rate Swap	Jul. 31, 2012 Interest Rate Swap	Apr. 30, 2012 Interest Rate Swap	Nov. 30, 2011 Interest Rate Swap	Dec. 31, 2012 Interest Rate Swap	Mar. 31, 2013 Interest Rate Swap
Issuance date	Nov. 13, 2006
Senior Notes			$ 1,725,000,000	$ 1,725,000,000
Proceeds from issuance of senior unsecured obligations						3,450,000,000
Cash payment to noteholders for outstanding principal of converted Notes	0	1,699,816,000			1,699,800,000
Shares issued to noteholders for the excess of the conversion value over the principal amount of converted Notes					29,500,000
Value of the conversion value over the principal amount of converted Notes					661,400,000
Business day period			5 days
Trading-day period			5 days
Product of last reported sale price and the conversion rate			98.00%
Number of trading days			20 days
Trading day period during quarter			30 days
Ratio of stock price to conversion price			130.00%
Observation period			20 days
Debt Instrument, Convertible, Conversion Ratio, Principal Amount Basis			1,000
Conversion rate for the 2013 Notes will be 62.1978 shares of our common stock per one thousand dollars of principal amount of 2013 Notes			62.1978
Conversion premium			27.50%
Conversion price			$ 16.08
Amount of converted Notes			30,000,000
Convertible debt, carrying amount			1,695,000,000
Convertible debt, fair value			2,534,000,000
Carrying amount of the equity component			283,500,000
Debt Instrument, unamortized discount			41,600,000
Semi-annual interest rate			1.75%	1.75%
Debt Instrument, frequency of periodic payment			semi-annually
Effective interest rate							5.60%	5.60%
Incremental common shares attributable to call options and warrants										215,000,000	215,000,000
Aggregate amount paid for purchased options										669,100,000
Purchased options exercised														107,500,000
Purchased options shares received									29,500,000
Purchased Options Remaining															107,500,000
Common stock at an exercise price											$ 19.55	$ 19.55
Proceeds from issuance of warrants											391,100,000
Shares issues for exercised warrants													32,300,000
Conversion premium based on the closing price											55.00%
Closing price per share												$ 12.61
Aggregate notional amount of interest rate swap contracts designated as cash flow hedges																900,000,000
Realized loss on replaced interest rate swap contracts																	45,500,000	23,000,000	141,000,000
Accumulated realized losses in other comprehensive income related to previously-anticipated interest payments reclassified from other comprehensive income and recognized in the consolidated income statements																				39,500,000
Accumulated Other Comprehensive Income (Loss) related to realized losses on settled swaps																					$ 176,500,000