Insurance Claim Reserves (Details) - Prior Year Reserve Development - Property-casualty [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Liability for claims and claim adjustment expense
Net favorable (unfavorable) prior year reserve development included in estimated claims and claim adjustment expenses incurred for claims arising in prior years			$ 459.0	$ (16.0)
Net favorable (unfavorable) prior year reserve development impacting the Company's results of operations			499.0	29.0
Accretion expense			24.0	24.0
Business Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable (unfavorable) prior year reserve development impacting the Company's results of operations	$ 73.0	$ 0.0	207.0	5.0
Bond & Specialty Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable (unfavorable) prior year reserve development impacting the Company's results of operations	44.0	(33.0)	59.0	(33.0)
Personal Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable (unfavorable) prior year reserve development impacting the Company's results of operations	$ 65.0	$ 35.0	$ 233.0	$ 57.0