Debt - Summary of Debt (Parenthetical) (Detail)	Dec. 31, 2017
7.00% Senior Notes 2019 [Member]
Debt Instrument [Line Items]
Senior notes, issued at par percentage	98.238%
3.80% Senior Notes 2025 [Member]
Debt Instrument [Line Items]
Senior notes, issued at par percentage	99.975%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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