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% |