Interest Expense - Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Debt Disclosure [Abstract]
Contractual coupon interest	$ 2.1	$ 2.6	$ 6.4	$ 7.5
Accretion of debt discount	10.6	8.2	31.5	22.2
Amortization of debt issuance costs	0.7	0.7	2.1	2.1
Capitalized interest	(1.5)	(2.1)	(4.7)	(8.1)
Interest expense, net of portion capitalized	11.9	9.4	35.3	23.7
Interest income	(0.5)	(1.7)	(2.7)	(5.4)
Interest expense, net	$ 11.4	$ 7.7	$ 32.6	$ 18.3