Income Taxes - Net Deferred Tax Assets (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Net operating loss carryforwards	$ 299.5	$ 262.2
Deferred ceding commission	8.5	8.6
Net unearned premium	6.6	5.3
Stock-based compensation	5.3	3.7
Lease liabilities	4.7	5.6
Charitable contributions	2.0	1.6
Depreciation and amortization	1.4	0.0
Unrealized loss on investments	0.9	5.5
Startup costs	0.5	0.5
Other	0.0	0.0
Total gross deferred tax assets	329.4	293.0
Deferred tax liabilities:
Right-of-use assets	(2.4)	(3.8)
Deferred acquisition costs	(1.8)	(1.4)
Depreciation and amortization	0.0	(1.0)
Other	(2.8)	(3.8)
Total gross deferred tax liabilities	(7.0)	(10.0)
Valuation allowance	(322.4)	(283.0)
Total deferred tax assets, net	$ 0.0	$ 0.0