Estimated Fair Values of Other Financial Instruments (Parenthetical) (Detail)	Sep. 30, 2019	Mar. 14, 2019	Dec. 31, 2018	Sep. 30, 2018	Feb. 22, 2018	Sep. 30, 2017
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Percentage of conversion of convertible notes				5.75%
5.75% Senior Subordinated Notes Due August 2023
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Percentage of conversion of convertible notes	5.75%	5.75%		5.75%
Convertible Notes
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Percentage of conversion of convertible notes						1.50%
Convertible Notes | 5.75% Senior Subordinated Notes Due August 2023
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Percentage of conversion of convertible notes	5.75%		5.75%	5.75%	5.75%