Long-Term Obligations - Senior Notes - due 2026 - Additional Information (Detail) - Senior Notes - Due 2026 [Member] - Senior Notes [Member]	9 Months Ended
	Sep. 30, 2018
Debt Instrument [Line Items]
Repurchase of senior notes of principal amount	101.00%
Debt Instrument Redemption By Equity Offering Before August 1, 2021 [Member]
Debt Instrument [Line Items]
Equity offering for senior notes description	Cash proceeds from certain equity offerings
Percentage price of principal amount redeemed	106.625%
Debt Instrument Redemption By Equity Offering Before August 1, 2021 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Percentage of principal amount redeemed	40.00%
Debt Instrument Redemption Period With Premium Before August 1, 2021 [Member]
Debt Instrument [Line Items]
Percentage of principal amount redeemed	100.00%