Supplemental Insurance Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reinsurance Segment
Supplementary Insurance Information By Segment [Line Items]
Deferred Policy Acquisition Costs	$ 460.7	$ 406.7	$ 361.2
Future Policy Benefits, Losses, Claims and Loss Expenses	9,814.0	9,323.8	9,442.2
Unearned Premiums	1,873.7	1,668.1	1,523.9
Other Policy Claims and Benefits Payable	0.0	0.0	0.0
Premium Revenue	4,644.7	4,327.0	3,939.0
Net Investment Income	330.2	371.9	327.0
Benefits, Claims, Losses and Settlement Expenses	3,386.9	2,961.1	2,869.3
Amortization of Deferred Policy Acquisition Costs	1,225.3	1,178.7	1,075.9
Other Operating Expenses	199.7	228.1	206.4
Premiums Written	4,845.0	4,495.0	3,969.1
Insurance Segment
Supplementary Insurance Information By Segment [Line Items]
Deferred Policy Acquisition Costs	134.4	115.9	103.3
Future Policy Benefits, Losses, Claims and Loss Expenses	3,226.9	2,670.3	2,874.9
Unearned Premiums	1,124.2	910.1	754.2
Other Policy Claims and Benefits Payable	0.0	0.0	0.0
Premium Revenue	1,355.5	1,151.1	1,037.2
Net Investment Income	135.5	161.3	153.9
Benefits, Claims, Losses and Settlement Expenses	952.2	725.3	651.1
Amortization of Deferred Policy Acquisition Costs	231.7	214.1	195.5
Other Operating Expenses	133.1	137.8	139.4
Premiums Written	$ 1,499.4	$ 1,256.7	$ 1,079.3