CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Millions	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Beginning balance (in shares) at Dec. 31, 2020		56,774,294
Beginning balance at Dec. 31, 2020	$ 541.0	$ 0.0	$ 859.8	$ (320.6)	$ 1.8
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Issuance of common stock (in shares)		4,018,647
Issuance of common stock	640.3		640.3
Exercise of stock options (in shares)		868,055
Exercise of stock options and distribution of restricted stock units	9.3		9.3
Stock-based compensation	44.1		44.1
Net loss	(241.3)			(241.3)
Other comprehensive income (loss)	(5.2)				(5.2)
Ending balance (in shares) at Dec. 31, 2021		61,660,996
Ending balance at Dec. 31, 2021	988.2	$ 0.0	1,553.5	(561.9)	(3.4)
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Issuance of common stock from acquisition of Metromile (Note 5) (in shares)		6,901,934
Stock Issued During Period, Value, Acquisitions	137.7		137.7
Exercise of stock options (in shares)		712,100
Exercise of stock options and distribution of restricted stock units	3.6		3.6
Stock-based compensation	59.3		59.3
Net loss	(297.8)			(297.8)
Other comprehensive income (loss)	$ (24.2)				(24.2)
Ending balance (in shares) at Dec. 31, 2022	69,275,030	69,275,030
Ending balance at Dec. 31, 2022	$ 866.8	$ 0.0	1,754.1	(859.7)	(27.6)
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (in shares)	103,350	888,673
Exercise of stock options and distribution of restricted stock units	$ 0.5		0.5
Stock-based compensation	59.9		59.9
Net loss	(236.9)			(236.9)
Other comprehensive income (loss)	$ 18.6				18.6
Ending balance (in shares) at Dec. 31, 2023	70,163,703	70,163,703
Ending balance at Dec. 31, 2023	$ 708.9	$ 0.0	$ 1,814.5	$ (1,096.6)	$ (9.0)