Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Earned premiums	$ 487.5	$ 445.5	$ 1,423.9	$ 1,313.9
Net investment income	46.1	42.0	143.2	79.0
Net realized investment gains (losses)	(5.3)	(5.7)	32.5	(24.0)
Total revenue	528.3	481.8	1,599.6	1,368.9
(Loss) income before income taxes	28.2	(26.5)	138.1	(46.3)
Foreign currency exchange losses	1.3	(8.4)	(4.4)	(13.6)
Operating Segments
Segment Reporting Information [Line Items]
(Loss) income before income taxes	43.2	(4.1)	140.2	23.4
Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	3.7	0.5	10.5	1.1
(Loss) income before income taxes	(11.0)	(8.3)	(30.2)	(32.1)
U.S. Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	323.5	298.7	952.4	902.8
Net investment income	29.2	30.1	91.7	56.1
(Loss) income before income taxes	39.9	20.1	130.1	88.2
International Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	163.9	146.7	471.1	410.8
Net investment income	12.3	9.9	38.2	19.0
(Loss) income before income taxes	6.9	(14.7)	14.6	(54.5)
Run-off Lines | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	0.1	0.1	0.4	0.3
Net investment income	0.9	1.5	2.8	2.8
(Loss) income before income taxes	$ (3.6)	$ (9.5)	$ (4.5)	$ (10.3)