Transactions with Affiliates - Schedule of Reinsurance Transactions on Balance Sheets for Pooling Arrangement (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred Policy Acquisition Costs:
Deferred Policy Acquisition Costs Ceded	$ (79.1)	$ (79.8)
Deferred Policy Acquisition Costs Assumed	129.8	129.1
Deferred Policy Acquisitions Costs from Affiliates	50.7	49.3
Losses and loss expenses payable:
Ceded	(547.1)	(514.7)
Assumed	1,178.0	1,047.1
Losses And Loss Expenses Payable From Affiliates	630.9	532.4
Unearned premiums:
Ceded	(390.8)	(395.3)
Assumed	611.7	609.5
Unearned Premiums From Affiliates	220.9	214.2
Pension and Other Postretirement Benefits:
Postretirement and Pension Benefits Ceded	(114.5)	(160.0)
Postretirement and Pension Benefits Assumed	74.4	104.0
Postretirement and Pension Benefits Ceded to Affiliates	(40.1)	(56.0)
Other Liabilities Ceded	(42.9)	(63.8)
Other Liabilities Assumed	53.9	55.4
Other Liabilities	11.0	(8.4)
Accumulated Other Comprehensive Income Ceded	(153.5)	(162.0)
Accumulated Other Comprehensive Income Assumed	99.8	105.3
Net Accumulated Other Comprehensive Income	$ (53.7)	$ (56.7)