SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs	[1]	$ 477.5	$ 467.4	$ 450.8
Future policy benefits, losses, claims and loss expenses	[1]	6,015.4	5,646.0	5,296.1
Unearned premiums	[1]	2,482.7	2,416.7	2,277.8
Other policy claims and benefits payable	[1]	8.6	8.4	8.0
Premium revenue	[1]	4,527.4	4,474.5	4,254.4
Net investment income	[1],[2]	265.1	281.3	267.4
Benefits, claims, losses and settlement expenses	[1]	2,845.2	2,865.5	2,724.6
Amortization of deferred acquisition costs	[1]	951.0	926.7	891.8
Other operating expenses	[1],[3]	586.3	576.4	595.4	[4]
Premiums written	[1]	4,598.5	4,581.7	4,384.8
Interest On Debt [Member]
Supplementary Insurance Information By Segment [Line Items]
Other operating expenses	[3]	37.1	37.5	45.1	[4]
Eliminations [Member]
Supplementary Insurance Information By Segment [Line Items]
Other operating expenses	[3]	(6.2)	(6.1)	(7.8)	[4]
Commercial Lines [Member] | Operating Segments [Member]
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs		312.1	307.4	296.9
Future policy benefits, losses, claims and loss expenses		4,067.4	3,768.9	3,550.6
Unearned premiums		1,513.9	1,446.7	1,369.5
Other policy claims and benefits payable		8.6	8.4	8.0
Premium revenue		2,683.3	2,654.2	2,548.4
Net investment income	[2]	175.3	180.1	182.2
Benefits, claims, losses and settlement expenses		1,647.6	1,610.0	1,568.3
Amortization of deferred acquisition costs		618.4	604.4	587.5
Other operating expenses	[3]	330.3	330.3	318.0	[4]
Premiums written		2,733.1	2,707.2	2,610.7
Personal Lines [Member] | Operating Segments [Member]
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs		165.4	160.0	153.9
Future policy benefits, losses, claims and loss expenses		1,907.6	1,839.4	1,707.6
Unearned premiums		968.8	970.0	908.3
Premium revenue		1,844.1	1,820.3	1,706.0
Net investment income	[2]	76.7	80.1	73.7
Benefits, claims, losses and settlement expenses		1,193.3	1,254.2	1,155.0
Amortization of deferred acquisition costs		332.6	322.3	304.3
Other operating expenses	[3]	193.7	191.6	185.8	[4]
Premiums written		1,865.4	1,874.5	1,774.1
Other Segment [Member] | Operating Segments [Member]
Supplementary Insurance Information By Segment [Line Items]
Future policy benefits, losses, claims and loss expenses		40.4	37.7	37.9
Net investment income	[2]	13.1	21.1	11.5
Benefits, claims, losses and settlement expenses		4.3	1.3	1.3
Other operating expenses	[3]	$ 31.4	$ 23.1	$ 54.3	[4]

[1] Information for all periods excludes results and balances related to the former Chaucer business (See Note 2 – “Discontinued Operations” in the Notes to Consolidated Financial Statements).

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

[3] For other operating expenses that are not directly attributable to a single segment for combined property and casualty insurance companies, expenses are generally allocated based upon either net premiums written or net premiums earned.

[4] In 2020 and 2018, the Other segment includes $6.2 million and $28.2 million, respectively, of losses from the repayment of debt.