SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net incurred losses and LAE in respect of losses occurring in current year			$ 2,733.5	$ 2,579.8	$ 2,307.8
Net incurred losses and LAE in respect of losses occurring in prior years		$ 174.1	8.9	0.2	(238.2)
Reinsurance Recoverable On Unpaid Losses Property Casualty Liability		1,349.2	1,472.6	1,455.0	1,349.2	$ 1,295.3
Prepaid premiums, gross		90.9	117.6	101.8	90.9
Consolidated Property and Casualty Insurance Entity [Member]
Deferred acquisition costs	[1]	402.8	450.8	430.0	402.8
Reserves for unpaid claims and claim adjustment expenses	[1],[2]	4,660.0	5,304.1	5,058.5	4,660.0
Unearned premiums	[1],[2]	$ 1,992.0	2,277.8	2,131.7	1,992.0
Earned premiums	[1]		4,254.4	3,980.4	3,789.5
Net investment income	[1]		267.4	243.9	231.6
Net incurred losses and LAE in respect of losses occurring in current year	[1],[2]		2,733.5	2,575.1	2,307.8
Net incurred losses and LAE in respect of losses occurring in prior years	[1],[3]		(8.9)	(0.2)	238.2
Amortization of deferred acquisition costs	[1],[2]		891.8	840.7	803.6
Paid claims and claim adjustment expenses	[1]		2,496.6	2,282.2	2,141.8
Premiums written	[1]		$ 4,384.8	$ 4,109.1	$ 3,882.7

[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 $1,472.6 million, $1,455.0 million and $1,349.2 million of reinsurance recoverable on unpaid losses in 2018, 2017 and 2016, respectively. Unearned premiums are shown gross of prepaid premiums of $117.6 million, $101.8 million and $90.9 million in 2018, 2017 and 2016, respectively. Reserves for unpaid claims and claims adjustment expense also include policyholder dividends.

[3] The Company does not use discounting techniques.