Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Earned premiums	$ 435.0	$ 420.5
Net investment income	35.5	33.9
Fee and other income	2.1	2.3
Net realized investment (losses) gains	(36.1)	52.5
Foreign currency exchange gains (loss)	3.0	(0.7)
Other corporate expenses	(3.3)	(0.5)	$ (3.3)
Total revenue	436.5	509.2
Income (loss) before income taxes	(21.6)	100.1
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income	0.5	0.3
Income (loss) before income taxes	(10.3)	(10.9)
U.S. Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	302.5	273.8
Net investment income	25.2	23.1
Income (loss) before income taxes	36.9	42.9
International Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	132.3	146.7
Net investment income	8.6	9.1
Income (loss) before income taxes	(12.3)	16.2
Run Off Lines [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned premiums	0.2	0.0
Net investment income	1.2	1.4
Income (loss) before income taxes	$ 0.5	$ 0.6