Debt - Details of Senior Notes (Detail) - Unsecured Debt [Member]	Dec. 31, 2017 USD ($)
Senior Notes 6.125% Due in 2018 [Member]
Debt Instrument [Line Items]
Principal amount	$ 500,000,000
Interest rate	6.125%
Senior Notes 5.50% Due in 2019 [Member]
Debt Instrument [Line Items]
Principal amount	$ 750,000,000
Interest rate	5.50%
Senior Notes 4.625% Due in 2020 [Member]
Debt Instrument [Line Items]
Principal amount	$ 500,000,000
Interest rate	4.625%