Losses and Loss Expenses Payable - Activity in Liability for Losses and Loss Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Losses and loss expenses payable, at beginning of year	$ 1,255.6	$ 1,181.6	$ 1,053.0
Less: reinsurance recoverable on losses and loss expenses payable	3.1	3.6	5.9
Net balance at beginning of year	1,252.5	1,178.0	1,047.1
Incurred related to:
Current year	876.6	964.9	915.3
Prior years	(80.2)	(46.6)	27.0
Total incurred	796.4	918.3	942.3
Paid related to:
Current year	456.5	445.2	417.7
Prior years	451.1	398.6	393.7
Total paid	907.6	843.8	811.4
Net balance at end of year	1,141.3	1,252.5	1,178.0
Plus: reinsurance recoverable on losses and loss expenses payable	5.5	3.1	3.6
Losses and loss expenses payable, at end of year (affiliates $711.4, $630.9, and $532.4, respectively)	$ 1,146.8	$ 1,255.6	$ 1,181.6