Tangible Equity Units - Additional Information (Detail) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 shares / unit $ / unit	Dec. 31, 2015 USD ($) $ / unit shares	Sep. 07, 2015 $ / unit
TEUs - senior amortizing notes due 2018 [Member]
Tangible Equity Units [Line Items]
Tangible equity unit rate	7.50%
Principal amount per amortizing note	10.95	10.95
Debt instrument, stated interest rate	1.50%
Quarterly cash installment payment per amortizing note	0.9375
First quarterly cash installment payment per amortizing note			0.9063
Debt instrument, maturity date	Jun. 07, 2018
7.50% Tangible Equity Units [Member]
Tangible Equity Units [Line Items]
Number of tangible equity units issued | shares		9.2
Tangible equity unit rate		7.50%
Stated amount per tangible equity unit		50.00
Total net proceeds | $		$ 445
Equity Component of 7.5% Tangible Equity Units [Member]
Tangible Equity Units [Line Items]
Purchase contract, mandatory settlement date	Jun. 07, 2018
Equity Component of 7.5% Tangible Equity Units [Member] | Settled In APC Shares [Member]
Tangible Equity Units [Line Items]
APC share cap | shares / unit	4.0792