Fair Value Measurements - Additional Information (Detail)	Dec. 31, 2018	Dec. 01, 2018	Dec. 31, 2017	Feb. 16, 2016	Sep. 21, 2015	Feb. 11, 2015	Jul. 01, 2014	Mar. 12, 2013
6.125% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.125%		6.125%					6.125%
5.125% Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.125%		5.125%				5.125%
5.625% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.625%		5.625%		5.625%	5.625%
6.500% Senior Notes Due 2024 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.50%		6.50%	6.50%
9.0% Revenue Bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	9.00%	9.00%	9.00%
9.5% Revenue Bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	9.50%	9.50%	9.50%