Debt - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended			3 Months Ended		6 Months Ended
	Jun. 19, 2017	May 31, 2017	Apr. 30, 2017	Feb. 28, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Debt Instrument [Line Items]
Repayment of debt							$ 409
Common stock sold				23,353,000			12,958,767
Common stock sold per share				$ 38.03	$ 35.91		$ 35.91
Pre-tax gain on sale of common stock				$ 351			$ 167
Consideration amount including accrued and unpaid interest					$ 874		874
Interest expense							58
Interest paid							27
Legal fees							2
Interest expense for premium paid for early redemption of notes in excess of carrying value	$ 24				$ 183	$ 124	$ 298	$ 245
6.50% Bonds, Due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate					6.50%		6.50%
Debt instrument, maturity date							2018
6.50% Senior Notes due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	100
6.75% Notes, Due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate					6.75%		6.75%
Debt instrument, maturity date							2018
6.75% Senior Notes due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	345
Redemption amount	$ 479
5.72% Notes, Due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate					5.72%		5.72%
Debt instrument, maturity date							2019
Investment Banks [Member]
Debt Instrument [Line Items]
Long-term debt					$ 805		$ 805
Common stock sold		12,958,767	12,958,767
Common stock sold per share		$ 35.91	$ 35.91
Pre-tax gain on sale of common stock		$ 167	$ 167
Investment Banks [Member] | Maximum [Member]
Debt Instrument [Line Items]
Long-term debt			$ 1,000
Investment Banks [Member] | 6.50% Bonds, Due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate			6.50%		6.50%		6.50%
Debt instrument, maturity date			2018
Long-term debt					$ 150		$ 150
Investment Banks [Member] | 6.75% Notes, Due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate			6.75%		6.75%		6.75%
Debt instrument, maturity date			2018
Long-term debt					$ 405		$ 405
Investment Banks [Member] | 5.72% Notes, Due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate			5.72%		5.72%		5.72%
Debt instrument, maturity date			2019
Long-term debt					$ 250		$ 250
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Proceeds from interest rate swap agreement							$ 8