Summary of Accounting Policies - Notes and Accounts Receivable and Allowance for Doubtful Accounts (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Restricted cash	$ 5,000,000	$ 3,000,000
Allowance for doubtful accounts on short-term and long-term accounts receivable	17,000,000	16,000,000
Allowance for doubtful accounts on short-term and long-term notes receivable	$ 0	$ 0