External Debt and Financing Arrangements - Components of External Long-Term Debt (Parenthetical) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2015
Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount			$ 900,000,000
Notes Due June Twenty Twenty [Member] | Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount	$ 400,000,000	$ 400,000,000
Debt instrument, maturity date	2020-06	2020-06
Notes Due June Twenty Twenty Five [Member] | Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount	$ 500,000,000	$ 500,000,000
Debt instrument, maturity date	2025-06	2025-06
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,250,000,000	$ 1,250,000,000
Revolving credit facility, expiration date	2021-06	2021-06