Other Income (Expense) - Investment Income, Net and Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Other income (expense)
Capitalized interest cost	$ 0.1	$ 0.1	$ 0.5	$ 0.1
Senior Notes held by special purpose entity
Other income (expense)
Effective interest rate (as a percent)	4.90%		4.90%