CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Cash flows from operating activities:
Net loss	$ (194.5)	$ (234.1)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	15.2	6.8
Stock-based compensation	45.6	43.6
Amortization of discount on bonds	(1.3)	5.9
Provision for bad debt	6.0	6.4
Asset impairment charge	3.7	0.0
Changes in operating assets and liabilities:
Premium receivable	(49.6)	(49.8)
Reinsurance recoverable	12.2	(32.2)
Prepaid reinsurance premium	(33.1)	(32.1)
Deferred acquisition costs	(1.2)	(1.4)
Other assets	5.0	(5.1)
Unpaid loss and loss adjustment expense	(1.1)	46.5
Unearned premium	67.6	71.5
Trade payables	1.6	(0.3)
Funds held for reinsurance treaties	(4.9)	28.6
Deferred ceding commissions	(0.6)	6.5
Ceded premium payable	17.9	(5.8)
Other liabilities and accrued expenses	8.5	10.6
Net cash used in operating activities	(103.0)	(134.4)
Cash flows from investing activities:
Acquisition of business, net of cash acquired	0.0	98.8
Proceeds from short-term investments sold or matured	115.2	150.0
Proceeds from bonds sold or matured	300.0	90.0
Cost of short-term investments acquired	(71.5)	(118.3)
Cost of bonds acquired	(288.8)	(119.4)
Purchases of property and equipment	(6.8)	(7.5)
Net cash provided by investing activities	48.1	93.6
Cash flows from financing activities:
Proceeds from borrowings under financing agreement	9.3	0.0
Payments on borrowings under financing agreement	(1.6)	0.0
Proceeds from stock exercises	0.5	3.3
Net cash provided by financing activities	8.2	3.3
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(1.7)	(8.1)
Net decrease in cash, cash equivalents and restricted cash	(48.4)	(45.6)
Cash, cash equivalents and restricted cash at beginning of period	286.5	270.6
Cash, cash equivalents and restricted cash at end of period	238.1	225.0
Supplemental disclosure of cash flow information:
Cash paid for income taxes	0.6	3.4
Cash paid for interest expense on borrowings under financing agreement	0.1	0.0
Additional Cash Flow Elements and Supplemental Cash Flow Information [Abstract]
Warrants assumed from acquisition of Metromile	$ 0.0	$ 0.8