Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Earned premiums	$ 417.7	$ 399.1	$ 832.4	$ 778.5
Net investment income	33.2	43.6	69.2	74.1
Fee and other income	1.9	3.8	3.9	7.4
Net realized investment gains (losses) before income taxes	10.5	4.5	(5.2)	19.1
Total revenue	463.3	451.0	900.3	879.1
Income (loss) before income taxes	57.1	50.8	82.1	93.5
Operating Segments [Member]
Segment Reporting Information [Line Items]
Income (loss) before income taxes	55.4	60.9	111.6	103.9
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income	2.1	3.3	4.5	5.2
Income (loss) before income taxes	(8.8)	(14.6)	(24.3)	(29.5)
U.S. Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	267.0	229.1	529.3	450.3
Net investment income	20.7	27.3	43.3	47.2
Income (loss) before income taxes	42.5	53.0	76.5	90.2
International Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	150.5	169.9	302.9	328.1
Net investment income	8.4	10.1	17.2	16.7
Income (loss) before income taxes	13.2	9.1	36.1	17.4
Run Off Lines [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	0.2	0.1	0.2	0.1
Net investment income	2.0	2.9	4.2	5.0
Income (loss) before income taxes	$ (0.3)	$ (1.2)	$ (1.0)	$ (3.7)