Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Earned premiums	$ 433.4	$ 431.7	$ 868.4	$ 852.2
Net investment income	1.5	42.8	37.0	76.7
Fee and other income	2.0	2.1	4.1	4.4
Net realized investment gains (losses)	20.1	12.0	(16.0)	64.5
Foreign currency exchange (losses) gains	(6.4)	5.3	(3.4)	4.6
Other corporate expenses	(2.5)	(7.5)	(5.8)	(8.0)
Total revenue	457.0	488.6	893.5	997.8
Income (loss) before income taxes	5.7	29.6	(15.9)	129.7
Operating Segments
Segment Reporting Information [Line Items]
Income (loss) before income taxes	2.4	31.1	27.5	90.8
Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	0.1	0.4	0.6	0.7
Income (loss) before income taxes	(7.9)	(11.3)	(18.2)	(22.2)
U.S. Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	301.6	284.0	604.1	557.8
Net investment income	0.8	29.6	26.0	52.7
Income (loss) before income taxes	31.2	53.2	68.1	96.1
International Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	131.8	147.6	264.1	294.3
Net investment income	0.5	11.5	9.1	20.6
Income (loss) before income taxes	(27.5)	(21.0)	(39.8)	(4.8)
Run-off Lines | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	0.0	0.1	0.2	0.1
Net investment income	0.1	1.3	1.3	2.7
Income (loss) before income taxes	$ (1.3)	$ (1.1)	$ (0.8)	$ (0.5)