Discontinued Operations (Schedule of Effects of Reinsurance) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Property and casualty premiums written, Direct		$ 4,816.0		$ 4,540.0	$ 4,265.7
Property and casualty premiums written, Assumed		27.6		23.6	25.1
Property and casualty premiums written, Ceded		(458.8)		(454.5)	(408.1)
Property and casualty premiums written, Net		4,384.8		4,109.1	3,882.7
Property and casualty premiums earned, Direct		4,673.6		4,399.9	4,162.6
Property and casualty premiums earned, Assumed		26.6		23.6	24.4
Property and casualty premiums earned, Ceded		(445.8)		(443.1)	(397.5)
Premiums Earned, Net, Total		4,254.4		3,980.4	3,789.5
Property and casualty losses and LAE, Direct		2,986.2		2,934.6	2,785.1
Property and casualty losses and LAE, Assumed		25.2		20.5	30.6
Property and casualty losses and LAE, Ceded	[1]	(286.8)		(375.5)	(269.7)
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Property and casualty premiums written, Direct		625.3		597.6	560.3
Property and casualty premiums written, Assumed		685.3		646.9	546.3
Property and casualty premiums written, Ceded	[2]	(459.9)		(395.4)	(290.5)
Property and casualty premiums written, Net		850.7		849.1	816.1
Property and casualty premiums earned, Direct		611.9		568.7	584.0
Property and casualty premiums earned, Assumed		667.2		628.3	577.3
Property and casualty premiums earned, Ceded	[2]	(429.1)		(344.0)	(322.7)
Premiums Earned, Net, Total		850.0	[3]	853.0	838.6
Property and casualty losses and LAE, Direct		362.2		365.2	403.1
Property and casualty losses and LAE, Assumed		568.7		570.7	258.8
Property and casualty losses and LAE, Ceded	[4]	(415.4)		(386.8)	(243.2)
Policyholder Benefits and Claims Incurred, Net, Total		$ 515.5		$ 549.1	$ 418.7

[1] The higher level of ceded losses and LAE in 2017 as compared to 2016 and 2018 is primarily due to higher non-catastrophe large loss activity occurring in certain domestic lines

[2] The increase in ceded reinsurance premiums from 2016 through 2018 is primarily due to Chaucer’s planned increase in reinsurance purchases.

[3] 2018 reflects the results of operations for the period in which THG owned Chaucer, which concluded with the sale of the Chaucer business on December 28, 2018.

[4] The increase in ceded losses and LAE from 2016 through 2018 is primarily due to higher catastrophe loss activity in certain Chaucer lines and due to the aforementioned increase in reinsurance purchases.