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	Mar. 31, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 28, 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
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 consideration	[1]				779.0
Contingent proceeds	[1],[2]				31.7
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 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] Contingent proceeds, as reflected in the sales and purchase agreement, may be up to $45 million and is determined based upon 2018 catastrophe losses. Based upon the Company’s best estimate of Chaucer’s 2018 catastrophe losses, expected contingent consideration is $31.7 million.