Long-term Obligations - Summary of Debt Instrument Redemption (Detail) - Senior Notes - Due 2026 [Member] - Senior Notes [Member]	9 Months Ended
	Sep. 30, 2018
Twelve-Month Period Beginning on August 1, 2021 [Member]
Debt Instrument, Redemption [Line Items]
Percentage of principal amount to be redeemed	103.313%
Twelve-Month Period Beginning on August 1, 2022 [Member]
Debt Instrument, Redemption [Line Items]
Percentage of principal amount to be redeemed	101.656%
Twelve-Month Period Beginning on August 1, 2023 and Thereafter [Member]
Debt Instrument, Redemption [Line Items]
Percentage of principal amount to be redeemed	100.00%