Losses and Loss Expenses Payable - Schedule of Activity in Liability for Losses and Loss Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Losses and loss expenses payable, at beginning of year	$ 1,181.6	$ 1,053.0	$ 983.2
Less: reinsurance recoverable on losses and loss expenses payable	3.6	5.9	9.6
Net balance at beginning of year	1,178.0	1,047.1	973.6
Incurred related to:
Current year	964.9	915.4	852.8
Prior years	(46.6)	27.0	10.0
Total incurred	918.3	942.4	862.8
Paid related to:
Current year	445.2	417.8	421.5
Prior years	398.6	393.7	367.8
Total paid	843.8	811.5	789.3
Net balance at end of year	1,252.5	1,178.0	1,047.1
Plus: reinsurance recoverable on losses and loss expenses payable	3.1	3.6	5.9
Losses and loss expenses payable, at end of year (affiliates $711.4, $630.9, and $532.4, respectively)	$ 1,255.6	$ 1,181.6	$ 1,053.0