CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue:
Net premiums earned	$ 44.1	$ 44.9	$ 115.7	$ 117.8	$ 100.9	$ 75.8	$ 59.5	$ 39.1	$ 322.5	$ 275.3	$ 40.2
Net investment income	1.1	1.1	1.3	1.9	2.4	1.1	1.0	0.7	5.4	5.2	1.2
Net realized gains on investments	0.1	0.1	0.1	0.0					0.3	0.0	0.0
Fee and other income	5.6	4.4	4.3	4.3	3.2	2.7	2.2	1.6	18.6	9.7	1.9
Total revenue	50.9	50.5	121.4	124.0	106.5	79.6	62.7	41.4	346.8	290.2	43.3
Operating expenses:
Loss and loss adjustment expenses	59.5	76.1	97.3	129.9	110.9	100.9	59.6	50.0	362.8	321.4	43.5
Sales and marketing	49.6	36.9	17.4	35.8	36.0	34.4	23.2	16.0	139.7	109.6	40.3
Other insurance (benefit) expense	(2.1)	(26.3)	15.3	11.3	18.2	15.2	10.7	8.2	(1.8)	52.3	10.2
Technology and development	12.7	12.9	11.3	16.0	8.6	7.0	5.6	2.8	52.9	24.0	8.2
General and administrative	19.7	16.6	11.3	30.9	11.6	9.0	8.5	13.9	78.5	43.0	9.3
Total operating expenses	139.4	116.2	152.6	223.9	185.3	166.5	107.6	90.9	632.1	550.3	111.5
Interest expense	44.8	19.5	7.7	5.7	6.4	13.3	2.4	0.2	77.7	22.3	0.9
Loss before income tax expense	(133.3)	(85.2)	(38.9)	(105.6)	(85.2)	(100.2)	(47.3)	(49.7)	(363.0)	(282.4)	(69.1)
Income tax expense	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Net loss	(133.3)	(85.2)	(38.9)	(105.6)	(85.2)	(100.2)	(47.3)	(49.7)	(363.0)	(282.4)	(69.1)
Other comprehensive income:
Changes in net unrealized gains on investments					(0.2)	0.1	0.5	0.2	5.0	0.6	0.0
Comprehensive loss	$ (133.3)	$ (85.1)	$ (32.2)	$ (107.4)	$ (85.4)	$ (100.1)	$ (46.8)	$ (49.5)	$ (358.0)	$ (281.8)	$ (69.1)
Loss per common share: basic and diluted (in dollars per share)	$ (0.72)	$ (2.20)	$ (1.03)	$ (2.69)	$ (2.30)	$ (2.88)	$ (1.44)	$ (1.61)	$ (4.81)	$ (8.33)	$ (2.73)
Weighted-average common shares outstanding: basic and diluted (in shares)	185.4	38.8	37.9	39.3	37.1	34.8	32.8	30.9	75.5	33.9	25.3