Discontinued Operations (Narrative) (Details) - USD ($) $ in Millions				3 Months Ended									12 Months Ended
	Apr. 10, 2019	Feb. 14, 2019	Dec. 28, 2018	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Mar. 31, 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]
Income tax expense (benefit)													$ (0.5)		$ 0.2
Revenues				$ 1,297.0	$ 1,249.8	$ 1,224.3	$ 1,056.2	$ 1,257.9	$ 1,214.7	$ 1,198.6	$ 1,219.5		4,827.3	$ 4,890.7	4,494.3
Pre-tax operating losses													484.7	453.6	406.5
Assets held-for-sale				110.2				102.8					110.2	102.8
Liabilities held-for-sale				120.8				116.9					120.8	116.9
Accident and Health Insurance Business [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Income (loss) from discontinued businesses, net of taxes													(3.7)	(4.3)	0.1
Assets held-for-sale				90.3				82.4					90.3	82.4
Liabilities held-for-sale				$ 89.8				$ 84.4					89.8	84.4
Income tax benefit related to discontinued operations													1.7	0.9
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Pre-tax gain on sale			$ 174.4												174.4
Income tax expense (benefit)			42.5											(5.3)	42.5	[1]
Contingent proceeds			$ 31.7										22.0		31.7	[2],[3]
Increase (decrease) in estimate of pre-tax contingent proceeds													9.7
Recognized tax benefits related to prior years									$ 3.4
Revenues														6.7	912.4
Pre-tax operating losses														(0.5)
Income (loss) from discontinued businesses, net of taxes													0.4	1.6	20.0
Decrease in uncertain tax positions due to expiration of statute of limitations														2.0
Amount recognized as foreign currency transaction losses during the period															0.8
Income tax benefit related to discontinued operations															$ (4.7)
Chaucer [Member] | Federal [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Recognized income tax charge related to new tax regulations													$ 1.2	$ 1.2
Chaucer Irish Entity [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Pre-tax gain on sale		$ 0.4
Income tax expense (benefit)		(0.5)
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]
Pre-tax gain on sale	$ 1.2
Income tax expense (benefit)	0.1
Proceeds from sale of businesses segment												$ 13.0
Cash proceeds to be received	$ 13.0

[1] 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.  

[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] 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.