Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Earned premiums	$ 451.5	$ 446.9	$ 1,303.7	$ 1,279.3
Net investment income	40.2	34.5	116.9	103.7
Fee and other income	1.9	3.3	6.3	7.2
Net realized investment (losses) gains before income taxes	(6.2)	14.2	58.3	9.0
Foreign currency exchange gains	1.6	1.7	6.2	2.3
Other corporate expenses	(3.7)	0.0	(11.7)	0.0
Total revenue	487.4	498.9	1,485.2	1,399.2
Income (loss) before income taxes	(26.2)	45.3	103.5	127.4
Operating Segments [Member]
Segment Reporting Information [Line Items]
Income (loss) before income taxes	(10.0)	41.5	80.8	153.1
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income	0.4	2.6	1.1	7.1
Income (loss) before income taxes	(7.9)	(12.1)	(30.1)	(37.0)
U.S. Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	290.8	278.3	848.6	807.6
Net investment income	27.4	21.4	80.1	64.7
Income (loss) before income taxes	42.2	50.3	138.3	126.8
International Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	160.7	168.5	455.0	471.4
Net investment income	10.7	8.4	31.3	25.6
Income (loss) before income taxes	(52.9)	(1.2)	(57.7)	34.9
Run Off Lines [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	0.0	0.1	0.1	0.3
Net investment income	1.7	2.1	4.4	6.3
Income (loss) before income taxes	$ 0.7	$ (7.6)	$ 0.2	$ (8.6)