Nature of Business and Accounting Policies Nature of Business and Accounting Policies - Leases (Details) - Accounting Standards Update 2016-02 $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Expected future increase (decrease) in operating expenses	$ 26
Expected future increase (decrease) in interest expense	(13)
Expected right-of-use assets	60
Expected right-of-use liabilities	70
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Expected cumulative effect of new accounting principle in period of adoption	$ 50