Consolidating Condensed Financial Information - Additional Information (Details)	Sep. 30, 2018
7% Senior Notes Due 2023
Condensed Financial Statements Captions [Line Items]
Interest rate (as a percent)	7.00%
6% Senior Notes Due 2025
Condensed Financial Statements Captions [Line Items]
Interest rate (as a percent)	6.00%
6% Senior Notes Due 2026
Condensed Financial Statements Captions [Line Items]
Interest rate (as a percent)	6.00%