Debt - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		1 Months Ended		12 Months Ended			60 Months Ended
	Jan. 07, 2014	Mar. 17, 2014	Nov. 30, 2006	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Feb. 27, 2020	Feb. 24, 2016	Mar. 23, 2015	Feb. 27, 2015	Jun. 30, 2013	Jan. 07, 2013	Jun. 30, 2012	Dec. 31, 2010	Nov. 13, 2006
Line of Credit Facility, Maximum Borrowing Capacity										$ 2,500
line of credit available increase to credit facility										$ 1,000
Line of Credit, Current				$ 600
Commercial Paper				699
Interest rate swap contract
Aggregate notional amount of interest rate swap contracts designated as cash flow hedges														$ 900
Unrealized gain on derivatives													$ 176
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net				$ 22	$ 11	$ 0
Convertible Senior Notes 2013
Senior Notes															$ 1,725
Cash payment to noteholders for outstanding principal of converted Notes	$ 1,700
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
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 and 2013
Effective interest rate						5.60%
Purchased Options
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
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
Long-term Debt
Debt Instrument, Fair Value Disclosure											$ 5,500
Scenario, Forecast [Member]
Line of Credit Facility, Expiration Date							Feb. 27, 2020
Line of Credit Facility, Interest Rate Description							At our option, subject to certain conditions, any loan under the Credit Agreement will bear interest at a rate equal to, either (i) the LIBOR Rate or (ii) the Base Rate (defined as the highest of (a) the Federal Funds rate plus 0.50%, (b) Citibank, N.A.’s “prime rate” as announced from time to time, or (c) one-month LIBOR plus 1.00%), plus, in each case the Applicable Margin, as defined in the Credit Agreement
Credit Facility
Short-term Debt, Weighted Average Interest Rate				1.08%
Commercial Paper
Commercial Paper Maximum Borrowing Amount									$ 2,500
Short-term Debt, Weighted Average Interest Rate				0.73%
Short-term Debt, Terms				The Commercial Paper will have maturities of up to 397 days from the date of issue.
Commercial Paper | Subsequent Event
Commercial Paper								$ 967
Minimum | Commercial Paper
Short-term Debt, Terms				26
Maximum | Commercial Paper
Short-term Debt, Terms				97