Discontinued Operations (Components of Estimated Gain From Disposal of Chaucer Business) (Parenthetical) (Details) - USD ($) $ in Millions				3 Months Ended		12 Months Ended
		Feb. 14, 2019	Dec. 28, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total consideration							$ 936.7
Maximum [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Contingent proceeds							45.0
Chaucer Irish Entity [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total consideration							779.0
Proceeds from sale of businesses segment		$ 28.0
Australian Entities [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 [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total consideration	[1]						779.0
Contingent proceeds			$ 31.7			$ 22.0	31.7	[1],[2]
Proceeds from dividends received				$ 85.0
Adjustment	[1]						17.0
Deferred tax assets							7.9
Chaucer [Member] | China Re [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Adjustment							$ 17.0

[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.