Debt (Public Debt) (Detail)			1 Months Ended		2 Months Ended	3 Months Ended	12 Months Ended
	Oct. 02, 2017 shares	Jun. 19, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2014 shares	May 31, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Apr. 30, 2017 USD ($) cash_tender_offer
Debt Instrument [Line Items]
Cash paid to holders of debt instrument, net						$ 409,000,000
Common stock sold (in shares) | shares	24,022			25,000,000		12,958,767
Common stock sold per share (usd per share) | $ / shares						$ 35.91
Debt instrument, repurchase amount						$ 874,000,000
Interest expense, debt						58,000,000
Interest paid						27,000,000
Legal fees						2,000,000
Pre-tax gain on sale of common stock						167,000,000	$ 167,000,000
Interest expense (S)		$ 24,000,000					496,000,000	$ 499,000,000	$ 473,000,000
Principal amount of convertible notes			$ 8,048,000,000				6,807,000,000	$ 8,048,000,000
6.50% Bonds, due 2018
Debt Instrument [Line Items]
Convertible notes, interest rate			6.50%					6.50%
Principal amount of convertible notes			$ 250,000,000				0	$ 250,000,000
6.75% Notes, due 2018
Debt Instrument [Line Items]
Convertible notes, interest rate			6.75%					6.75%
Principal amount of convertible notes			$ 750,000,000				$ 0	$ 750,000,000
5.72% Notes, due 2019
Debt Instrument [Line Items]
Convertible notes, interest rate							5.72%
Principal amount of convertible notes			$ 750,000,000				$ 500,000,000	$ 750,000,000
6.5% Senior Notes Due 2018
Debt Instrument [Line Items]
Debt instrument, face amount		100,000,000
6.75% Senior Notes Due 2018
Debt Instrument [Line Items]
Debt instrument, face amount		345,000,000
Debt instrument redemption amount		$ 479,000,000
5.55% Notes Due February 2017
Debt Instrument [Line Items]
Convertible notes, interest rate			5.55%					5.55%
Cash paid to holders of debt instrument, net			$ 770,000,000
Principal amount of convertible notes			750,000,000					$ 750,000,000
Cash paid to holders of debt instrument, portion of interest			17,000,000
Cash paid to holders of debt instrument, portion of purchase premium			$ 3,000,000
Investment Banks
Debt Instrument [Line Items]
Number of cash tender offers | cash_tender_offer										3
Debt instrument, face amount										$ 805,000,000
Common stock sold (in shares) | shares					12,958,767
Common stock sold per share (usd per share) | $ / shares					$ 35.91
Pre-tax gain on sale of common stock					$ 167,000,000
Investment Banks | 6.50% Bonds, due 2018
Debt Instrument [Line Items]
Debt instrument, face amount										150,000,000
Investment Banks | 6.75% Notes, due 2018
Debt Instrument [Line Items]
Debt instrument, face amount										405,000,000
Investment Banks | 5.72% Notes, due 2019
Debt Instrument [Line Items]
Debt instrument, face amount										250,000,000
Interest Rate Swap
Debt Instrument [Line Items]
Proceeds from interest received						$ 8,000,000
Maximum | Investment Banks
Debt Instrument [Line Items]
Debt instrument, face amount										$ 1,000,000,000