Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Earned premiums	$ 1,910.1	$ 1,780.5	$ 1,729.7
Net investment income	187.6	112.7	151.1
Net realized investment and other gains (losses)	32.6	(3.6)	80.1
Total revenue	2,130.3	1,889.6	1,960.9
Foreign currency exchange losses	(1.6)	(15.4)	9.8
Impairment of intangibles	(43.2)	0.0	(15.6)
Non-operating expenses	(43.7)	(21.1)	(37.6)
Pre-Tax Income (Loss)	5.3	(46.4)	0.0
U.S. Operations
Segment Reporting Information [Line Items]
Earned premiums	1,283.7	1,207.6	1,119.9
Net investment income	119.4	80.3	100.0
International Operations
Segment Reporting Information [Line Items]
Earned premiums	625.8	572.5	609.6
Net investment income	50.6	26.7	44.2
Run-off Lines
Segment Reporting Information [Line Items]
Earned premiums	0.6	0.4	0.2
Net investment income	3.6	4.0	5.7
Operating Segments
Segment Reporting Information [Line Items]
Pre-Tax Income (Loss)	83.7	28.2	8.1
Operating Segments | U.S. Operations
Segment Reporting Information [Line Items]
Earned premiums	1,283.7	1,207.6	1,119.9
Net investment income	119.4	80.3	100.0
Pre-Tax Income (Loss)	61.1	113.1	139.3
Operating Segments | International Operations
Segment Reporting Information [Line Items]
Earned premiums	625.8	572.5	609.6
Net investment income	50.6	26.7	44.2
Pre-Tax Income (Loss)	64.1	(75.3)	(121.4)
Operating Segments | Run-off Lines
Segment Reporting Information [Line Items]
Earned premiums	0.6	0.4	0.2
Net investment income	3.6	4.0	5.7
Pre-Tax Income (Loss)	(41.5)	(9.6)	(9.8)
Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	14.0	1.7	1.2
Pre-Tax Income (Loss)	$ (22.5)	$ (34.5)	$ (44.8)