Supplemental Insurance Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reinsurance Segment
Supplementary Insurance Information By Segment [Line Items]
Deferred Policy Acquisition Costs	$ 406.7	$ 361.2	$ 355.9
Future Policy Benefits, Losses, Claims and Loss Expenses	9,323.8	9,442.2	9,128.0
Unearned Premiums	1,668.1	1,523.9	1,503.6
Other Policy Claims and Benefits Payable	0.0	0.0	0.0
Premium Revenue	4,327.0	3,939.0	3,808.7
Net Investment Income	371.9	327.0	312.0
Benefits, Claims, Losses and Settlement Expenses	2,961.1	2,869.3	2,785.4
Amortization of Deferred Policy Acquisition Costs	1,178.7	1,075.9	1,074.3
Other Operating Expenses	228.1	206.4	212.4
Premiums Written	4,495.0	3,969.1	3,810.1
Insurance Segment
Supplementary Insurance Information By Segment [Line Items]
Deferred Policy Acquisition Costs	115.9	103.3	97.4
Future Policy Benefits, Losses, Claims and Loss Expenses	2,670.3	2,874.9	2,811.1
Unearned Premiums	910.1	754.2	689.0
Other Policy Claims and Benefits Payable	0.0	0.0	0.0
Premium Revenue	1,151.1	1,037.2	1,146.3
Net Investment Income	161.3	153.9	122.6
Benefits, Claims, Losses and Settlement Expenses	725.3	651.1	834.8
Amortization of Deferred Policy Acquisition Costs	214.1	195.5	187.4
Other Operating Expenses	137.8	139.4	177.1
Premiums Written	$ 1,256.7	$ 1,079.3	$ 1,155.8