Debt - Summary of Redemption Prices Expressed as Percentage (Detail) - Senior Notes [Member]	6 Months Ended
	Jun. 30, 2018
2021 [Member]
Debt Instrument Redemption [Line Items]
Optional redemption price expressed as percentage	105.313%
2022 [Member]
Debt Instrument Redemption [Line Items]
Optional redemption price expressed as percentage	102.656%
2023 and thereafter [Member]
Debt Instrument Redemption [Line Items]
Optional redemption price expressed as percentage	100.00%