SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net incurred losses and LAE in respect of losses occurring in current year		$ 2,877.8	$ 2,893.0	$ 2,733.5
Net incurred losses and LAE in respect of losses occurring in prior years		32.6	28.4	8.9
Reinsurance Recoverable On Unpaid Losses Property Casualty Liability		1,641.6	1,574.8	1,472.6	$ 1,455.0
Prepaid premiums, gross		144.2	149.3	117.6
Consolidated Property and Casualty Insurance Entity [Member]
Deferred acquisition costs	[1]	477.5	467.4	450.8
Reserves for unpaid claims and claim adjustment expenses	[1],[2]	6,024.0	5,654.4	5,304.1
Unearned premiums	[1],[2]	2,482.7	2,416.7	2,277.8
Earned premiums	[1]	4,527.4	4,474.5	4,254.4
Net investment income	[1]	265.1	281.3	267.4
Net incurred losses and LAE in respect of losses occurring in current year	[1],[2]	2,877.8	2,893.9	2,733.5
Net incurred losses and LAE in respect of losses occurring in prior years	[1],[3]	(32.6)	(28.4)	(8.9)
Amortization of deferred acquisition costs	[1],[2]	951.0	926.7	891.8
Paid claims and claim adjustment expenses	[1]	2,542.4	2,616.5	2,496.6
Premiums written	[1]	$ 4,598.5	$ 4,581.7	$ 4,384.8

[1] Information for all periods excludes results and balances related to the discontinued Chaucer business. See Note 2 – “Discontinued Operations” in the Notes to Consolidated Financial Statements for further information on that business.

[2] Reserves for unpaid claims and claim adjustment expenses are shown gross of $1,641.6 million, $1,574.8 million and $1,472.6 million of reinsurance recoverable on unpaid losses in 2020, 2019 and 2018, respectively. Unearned premiums are shown gross of prepaid premiums of $144.2 million, $149.3 million and $117.6 million in 2020, 2019 and 2018, respectively. Reserves for unpaid claims and claims adjustment expense also include policyholder dividends.

[3] The Company does not use discounting techniques.