Discontinued Operations (Components of Estimated Gain From Disposal of Chaucer Business) (Details) - USD ($) $ in Millions			12 Months Ended
		Dec. 28, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Initial consideration received from sale					$ 936.7
Income tax expense			$ (0.5)		0.2
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Initial consideration received from sale	[1]				779.0
Adjustment	[1]				(17.0)
Contingent proceeds		$ 31.7	$ 22.0		31.7	[1],[2]
Total cash proceeds expected from sale of Chaucer Holdings Limited	[1]				793.7
Carrying value of Chaucer business	[3]				530.0
Transaction and other sale related costs	[4]				30.6
Net realized losses on securities, pension and currency translation obligations related to Chaucer business	[5]				58.7
Total pre-tax reductions					619.3
Pre-tax gain on sale		174.4			174.4
Income tax expense		$ 42.5		$ (5.3)	42.5	[6]
Gain on sale				$ (1.2)	$ 131.9

[1] Initial consideration for Chaucer as determined in the purchase and sales agreement was $779 million.  This amount, along with $28 million in cash proceeds received from the sale of the Irish entity on February 14, 2019, $13 million from the sale of the Australian entities on April 10, 2019, 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, resulted 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] Contingent proceeds, as reflected in the sales and purchase agreement, could have been up to $45 million and was determined based upon 2018 catastrophe losses.  In 2018, the Company’s best estimate of contingent consideration was $31.7 million.

[3] The carrying value of the Chaucer business reflects its U.S. GAAP book value at December 28, 2018, excluding $7.9 million of U.S.-related deferred tax assets that are no longer likely to be realized and therefore are reflected in the income tax expense category.

[4] Transaction and other sale related costs primarily include brokerage, legal, actuarial, tax and other professional fees, employee retention costs, costs for the purchase of aggregate excess of loss catastrophe coverage in consideration of the contingent proceeds provision, along with certain other miscellaneous charges related to the execution of the transaction.

[5] As part of the transaction, investments held by Chaucer were transferred to China Re resulting in the recognition of net realized investment losses that were previously reflected in accumulated other comprehensive income.  Additionally, Chaucer’s deferred pension obligations and currency translation obligations previously recognized in accumulated other comprehensive income were recognized as losses associated with the transaction.

[6] The income tax expense represents the current tax obligation on the sale and the derecognition of deferred tax assets that were no longer likely to be realized.