Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Earned premiums	$ 431.7	$ 417.7	$ 852.2	$ 832.4	$ 832.4
Net investment income	42.8	33.2	76.7	69.2	69.2
Fee and other income	2.1	1.9	4.4	3.9	3.9
Net realized investment gains (losses) before income taxes	12.0	10.5	64.5	(5.2)	(5.2)
Foreign currency exchange gains	5.3	5.5	4.6	0.6	0.6
Other corporate expenses	(7.5)	0.0	(8.0)		0.0
Total revenue	488.6	463.3	997.8	900.3	900.3
Income (loss) before income taxes	29.6	57.1	129.7	$ 82.1	82.1
Operating Segments [Member]
Segment Reporting Information [Line Items]
Income (loss) before income taxes	31.1	55.4	90.8		111.6
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income	0.4	2.1	0.7		4.5
Income (loss) before income taxes	(11.3)	(14.3)	(22.2)		(24.9)
U.S. Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	284.0	267.0	557.8		529.3
Net investment income	29.6	20.7	52.7		43.3
Income (loss) before income taxes	53.2	42.5	96.1		76.5
International Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	147.6	150.5	294.3		302.9
Net investment income	11.5	8.4	20.6		17.2
Income (loss) before income taxes	(21.0)	13.2	(4.8)		36.1
Run Off Lines [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	0.1	0.2	0.1		0.2
Net investment income	1.3	2.0	2.7		4.2
Income (loss) before income taxes	$ (1.1)	$ (0.3)	$ (0.5)		$ (1.0)