Discontinued Operations (Narrative) (Details) - USD ($) $ in Millions			3 Months Ended	12 Months Ended
		Feb. 14, 2019	Mar. 31, 2019	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total cash proceeds expected from sale of Chaucer Holdings Limited				$ 793.7
Pre-tax gain on sale				$ 174.4
Excess loss percentage of net earned premium				10.00%
Restricted assets, fixed maturities						$ 528.1
Restricted assets, cash and cash equivalents						7.1
Reinsurance recoverable on paid and unpaid losses and unearned premiums				$ 1,648.6		1,625.5
Assets held-for-sale				57.4		4,466.6
Liabilities held-for-sale				22.2		3,707.2
Net Income/gain (loss) from discontinued businesses, net of taxes				0.1		(16.8)	$ (1.0)
Income tax benefit related to discontinued operations						9.3	2.8
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net investment income for contract				8.0		8.0	3.0
Lloyd's Syndicates [Member] | Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Reinsurance recoverable on paid and unpaid losses and unearned premiums						443.0
Discontinued Accident and Health Insurance Business [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets held-for-sale				81.6		76.9
Liabilities held-for-sale				81.4		83.1
Chaucer Irish Entity[Member] | Scenario, Forecast [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from sale of businesses segment			$ 13.0
Chaucer Irish Entity[Member] | Subsequent Event [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from sale of businesses segment		$ 28.0
Australian Entities [Member] | Scenario, Forecast [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Cash proceeds to be received			$ 13.0
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total cash proceeds expected from sale of Chaucer Holdings Limited	[1]			793.7
Pre-tax gain on sale				174.4
Amount recognized as foreign currency transaction losses during the period				0.8		1.1	22.3
Net Income/gain (loss) from discontinued businesses, net of taxes				$ 20.0	[2]	$ (13.1)	$ 89.1

[1] Initial consideration for Chaucer Holdings Limited as determined in the sales and purchase agreement was $779 million. This amount, along with $28 million in cash proceeds received for the sale of the Irish entity on February 14, 2019, $13 million in cash proceeds to be received upon the closing of the sale of the Australian entities, estimated contingent consideration of $31.7 million, and an $85 million pre-signing dividend from Chaucer that was received in the second quarter of 2018, results in expected total proceeds from the entire transaction of $936.7 million.  These amounts were partially offset by $17.0 million paid to China Re to adjust the purchase price for amounts received by the Company from Chaucer prior to December 28, 2018.

[2] 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.