Transactions with Affiliates - Schedule of Reinsurance Transactions on Balance Sheets for Pooling Arrangement (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Deferred Policy Acquisition Costs:
Deferred Policy Acquisition Costs Ceded	$ (144.1)	$ (79.7)
Deferred Policy Acquisition Costs Assumed	129.1	126.5
Deferred Policy Acquisitions Costs from Affiliates	(15.0)	46.8
Losses and loss expenses payable:
Ceded	(514.7)	(479.3)
Assumed	1,047.1	973.6
Losses And Loss Expenses Payable From Affiliates	532.4	494.3
Unearned premiums:
Ceded	(395.3)	(404.6)
Assumed	609.5	606.3
Unearned Premiums From Affiliates	214.2	201.7
Pension and Other Postretirement Benefits:
Postretirement and Pension Benefits Ceded	(160.0)	(180.5)
Postretirement and Pension Benefits Assumed	104.0	117.3
Postretirement and Pension Benefits Ceded to Affiliates	(56.0)	(63.2)
Other Liabilities Ceded	(63.8)	(70.7)
Other Liabilities Assumed	55.4	65.6
Other Liabilities	(8.4)	(5.1)
Accumulated Other Comprehensive Income Ceded	(162.0)	(186.0)
Accumulated Other Comprehensive Income Assumed	105.3	120.9
Net Accumulated Other Comprehensive Income	$ (56.7)	$ (65.1)