Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Earned premiums	$ 446.9	$ 389.3	$ 1,279.3	$ 1,167.8
Net investment income	34.5	30.9	103.7	105.0
Fee and other income	3.3	13.0	7.2	20.4
Net realized investment gains before income taxes	14.2	6.0	9.0	25.1
Total revenue	498.9	439.2	1,399.2	1,318.3
Income (loss) before income taxes	45.3	(65.9)	127.4	27.6
Operating Segments [Member]
Segment Reporting Information [Line Items]
Income (loss) before income taxes	41.5	(61.5)	153.1	42.4
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income	2.6	2.4	7.1	7.6
Income (loss) before income taxes	(10.4)	(10.4)	(34.7)	(39.9)
U.S. Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	278.3	242.6	807.6	692.9
Net investment income	21.4	18.8	64.7	66.0
Income (loss) before income taxes	50.3	28.7	126.8	118.9
International Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	168.5	146.8	471.4	474.9
Net investment income	8.4	7.7	25.6	24.4
Income (loss) before income taxes	(1.2)	(77.5)	34.9	(60.1)
Run Off Lines [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	0.1	(0.1)	0.3	0.0
Net investment income	2.1	2.0	6.3	7.0
Income (loss) before income taxes	$ (7.6)	$ (12.7)	$ (8.6)	$ (16.4)