Debt - Additional Information (Detail) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	12 Months Ended			1 Months Ended	0 Months Ended	1 Months Ended	0 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Jan. 07, 2014	Nov. 30, 2006	Jan. 09, 2012	Mar. 17, 2014	Dec. 31, 2010	Jan. 07, 2013	Nov. 13, 2006	Jun. 30, 2013
Aggregate amount paid for purchased options	$ 0	$ 0	$ 70,000,000
Interest rate swap contract
Aggregate notional amount of interest rate swap contracts designated as cash flow hedges									900,000,000
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				40,000,000
Unrealized gain on derivatives				176,000,000
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(11,000,000)
Convertible Senior Notes 2013
Senior Notes											1,725,000,000
Cash payment to noteholders for outstanding principal of converted Notes					1,700,000,000
Shares issued to noteholders for the excess of the conversion value over the principal amount of converted Notes					35
Value of the conversion value over the principal amount of converted Notes					858,000,000
Observation period					20 days
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					62.6978
Conversion premium					26.50%
Conversion price										$ 15.95
Semi-annual interest rate											1.75%
Debt Instrument, frequency of periodic payment						semi-annually
Convertible Senior Notes 2011
Cash payment to noteholders for outstanding principal of converted Notes							1,700,000,000
Shares issued to noteholders for the excess of the conversion value over the principal amount of converted Notes							30
Value of the conversion value over the principal amount of converted Notes							661,000,000
Convertible Senior Notes 2011 and 2013
Proceeds from issuance of senior unsecured obligations						3,500,000,000
Effective interest rate		5.60%	5.60%
Purchased Options
Incremental common shares attributable to call options and warrants						215
Aggregate amount paid for purchased options						669,000,000
Purchased options shares received					35
Purchased options remaining		108
Sold Warrants
Incremental common shares attributable to call options and warrants						215
Common stock at an exercise price						19.55					$ 19.31
Proceeds from issuance of warrants						391,000,000
Conversion premium based on the closing price						53.20%
Closing price per share											$ 12.61
Exercised Warrants
Shares issues for exercised warrants								29
Warrant
Warrants and Rights Outstanding								109,000,000
Long-term Debt
Debt Instrument, Fair Value Disclosure												$ 5,500,000,000