SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs		$ 620.8	$ 604.8	$ 552.0
Future policy benefits, losses, claims and loss expenses		7,295.3	7,001.1	6,438.0
Unearned premiums		3,102.5	2,954.2	2,734.9
Other policy claims and benefits payable		12.8	11.5	9.6
Premium revenue		5,663.1	5,252.3	4,770.2
Net investment income	[1]	332.1	296.3	310.7
Benefits, claims, losses and settlement expenses		4,134.6	3,623.4	3,134.2
Amortization of deferred acquisition costs		1,176.0	1,093.2	982.7
Other operating expenses	[2]	641.8	608.0	589.6
Premiums written		5,810.2	5,476.5	4,993.4
Interest On Debt [Member]
Supplementary Insurance Information By Segment [Line Items]
Other operating expenses	[2]	34.1	34.1	34.0
Eliminations [Member]
Supplementary Insurance Information By Segment [Line Items]
Other operating expenses	[2]	(8.1)	(7.8)	(7.3)
Core Commercial [Member] | Operating Segments [Member]
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs		223.0	212.6	197.6
Future policy benefits, losses, claims and loss expenses		3,461.5	3,344.9	2,986.5
Unearned premiums		1,066.8	1,016.9	964.9
Other policy claims and benefits payable		6.9	6.0	4.5
Premium revenue		2,060.3	1,950.5	1,810.9
Net investment income	[1]	151.8	136.2	146.5
Benefits, claims, losses and settlement expenses		1,354.9	1,336.5	1,220.1
Amortization of deferred acquisition costs		443.8	417.7	380.0
Other operating expenses	[2]	250.9	229.6	222.6
Premiums written		2,107.0	1,999.9	1,864.8
Specialty [Member] | Operating Segments [Member]
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs		193.6	188.3	172.4
Future policy benefits, losses, claims and loss expenses		1,619.6	1,592.6	1,516.9
Unearned premiums		755.1	747.9	702.6
Other policy claims and benefits payable		5.9	5.5	5.1
Premium revenue		1,274.2	1,189.0	1,029.9
Net investment income	[1]	71.1	62.1	62.9
Benefits, claims, losses and settlement expenses		645.5	641.8	591.1
Amortization of deferred acquisition costs		329.5	303.9	261.2
Other operating expenses	[2]	133.2	124.8	115.0
Premiums written		1,293.3	1,243.7	1,118.9
Personal Lines [Member] | Operating Segments [Member]
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs		204.2	203.9	182.0
Future policy benefits, losses, claims and loss expenses		2,148.8	1,998.1	1,868.4
Unearned premiums		1,280.6	1,189.4	1,067.4
Premium revenue		2,328.6	2,112.8	1,929.4
Net investment income	[1]	96.8	86.8	89.4
Benefits, claims, losses and settlement expenses		2,131.9	1,643.9	1,322.0
Amortization of deferred acquisition costs		402.7	371.6	341.5
Other operating expenses	[2]	211.6	207.0	206.5
Premiums written		2,409.9	2,232.9	2,009.7
Other [Member] | Operating Segments [Member]
Supplementary Insurance Information By Segment [Line Items]
Future policy benefits, losses, claims and loss expenses		65.4	65.5	66.2
Net investment income	[1]	12.4	11.2	11.9
Benefits, claims, losses and settlement expenses		2.3	1.2	1.0
Other operating expenses	[2]	$ 20.1	$ 20.3	$ 18.8

[1] The Company manages investment assets for its Core Commercial, Specialty, Personal Lines and Other segments on a combined basis, based on the requirements of its combined insurance companies. Net investment income is allocated to these segments based on actuarial information related to the underlying businesses.

[2] For other operating expenses that are not directly attributable to a single segment, expenses are generally allocated based upon either net premiums written or net premiums earned.