Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions		12 Months Ended
	Oct. 01, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Earned premiums		$ 1,740.4	$ 1,910.1	$ 1,780.5
Net investment income		129.8	187.6	112.7
Net investment and other gains (losses)		(115.3)	32.6	(3.6)
Total revenue		1,754.9	2,130.3	1,889.6
Foreign currency exchange gains (losses)		5.0	(1.6)	(15.4)
Impairment of goodwill and intangibles	$ (28.5)	(28.5)	(43.2)	0.0
Non-operating expenses		(51.5)	(43.7)	(21.1)
Pre-Tax Income (Loss)		(183.2)	5.3	(46.4)
U.S. Operations
Segment Reporting Information [Line Items]
Earned premiums		1,209.0	1,283.7	1,207.6
Net investment income		88.4	119.4	80.3
International Operations
Segment Reporting Information [Line Items]
Earned premiums		530.5	625.8	572.5
Net investment income		39.1	50.6	26.7
Run-off Lines
Segment Reporting Information [Line Items]
Earned premiums		0.9	0.6	0.4
Net investment income		2.3	3.6	4.0
Operating Segments
Segment Reporting Information [Line Items]
Pre-Tax Income (Loss)		39.1	83.7	28.2
Operating Segments | U.S. Operations
Segment Reporting Information [Line Items]
Earned premiums		1,209.0	1,283.7	1,207.6
Net investment income		88.4	119.4	80.3
Pre-Tax Income (Loss)		(22.9)	61.1	113.1
Operating Segments | International Operations
Segment Reporting Information [Line Items]
Earned premiums		530.5	625.8	572.5
Net investment income		39.1	50.6	26.7
Pre-Tax Income (Loss)		63.8	64.1	(75.3)
Operating Segments | Run-off Lines
Segment Reporting Information [Line Items]
Earned premiums		0.9	0.6	0.4
Net investment income		2.3	3.6	4.0
Pre-Tax Income (Loss)		(1.8)	(41.5)	(9.6)
Corporate and Other
Segment Reporting Information [Line Items]
Net investment income		0.0	14.0	1.7
Pre-Tax Income (Loss)		$ (32.0)	$ (22.5)	$ (34.5)