Guarantor Financial Statement Information - Narrative (Details) $ in Millions	Sep. 30, 2018 USD ($) subsidiary
Condensed Financial Statements, Captions [Line Items]
Ownership percentage	100.00%
Number of subsidiaries as guarantors of unsecured debt | subsidiary	3
Unsecured Senior Notes
Condensed Financial Statements, Captions [Line Items]
Unsecured debt	$ 2,498
6.500% interest rate payable semi-annually, due July 2021 | Unsecured Senior Notes
Condensed Financial Statements, Captions [Line Items]
Unsecured debt	$ 592
Interest rate	6.50%
6.500% interest rate payable semi-annually, due June 2022 | Unsecured Senior Notes
Condensed Financial Statements, Captions [Line Items]
Unsecured debt	$ 206
Interest rate	6.50%