Transactions with Affiliates - Reinsurance Transactions on Balance Sheets for Pooling Arrangement (Details) - USD ($) $ in Millions	Dec. 31, 2018	Mar. 01, 2018	Dec. 31, 2017
Deferred Policy Acquisition Costs:
Deferred Policy Acquisition Costs Ceded	$ (53.6)		$ (53.1)
Deferred Policy Acquisition Costs Assumed	101.9		110.3
Deferred Policy Acquisitions Costs from Affiliates	48.3		57.2
Losses and loss expenses payable:
Ceded	(547.7)		(541.1)
Assumed	1,141.3		1,252.5
Losses And Loss Expenses Payable From Affiliates	593.6		711.4
Unearned premiums:
Ceded	(465.2)	$ 9.7	(417.5)
Assumed	577.6		605.4
Unearned Premiums From Affiliates	112.4		187.9
Pension and Other Postretirement Benefits:
Other Liabilities Ceded	(24.6)		(43.0)
Other Liabilities Assumed	42.3		58.5
Other Liabilities	$ 17.7		$ 15.5