Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Earned premiums	$ 445.5	$ 451.5	$ 1,313.9	$ 1,303.7
Net investment income	42.0	40.2	79.0	116.9
Fee and other income	1.7	1.9	5.8	6.3
Net realized investment (losses) gains	(5.7)	(6.2)	(21.7)	58.3
Foreign currency exchange (losses) gains	(11.6)	1.6	(15.0)	6.2
Other corporate expenses	(0.4)	(3.7)	(6.2)	(11.7)
Total revenue	483.5	487.4	1,377.0	1,485.2
Income (loss) before income taxes	(29.4)	(26.2)	(45.3)	103.5
Operating Segments
Segment Reporting Information [Line Items]
Income (loss) before income taxes	(4.8)	(10.0)	22.7	80.8
Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	0.5	0.4	1.1	1.1
Income (loss) before income taxes	(6.9)	(7.9)	(25.1)	(30.1)
U.S. Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	298.7	290.8	902.8	848.6
Net investment income	30.1	27.4	56.1	80.1
Income (loss) before income taxes	20.1	42.2	88.2	138.3
International Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	146.7	160.7	410.8	455.0
Net investment income	9.9	10.7	19.0	31.3
Income (loss) before income taxes	(14.7)	(52.9)	(54.5)	(57.7)
Run-off Lines | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	0.1	0.0	0.3	0.1
Net investment income	1.5	1.7	2.8	4.4
Income (loss) before income taxes	$ (10.2)	$ 0.7	$ (11.0)	$ 0.2