Losses and Loss Expenses Payable - Schedule of Activity in Liability for Losses and Loss Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Losses and loss expenses payable, at beginning of year	$ 1,053.0	$ 983.2	$ 959.9
Less: reinsurance recoverable on losses and loss expenses payable	5.9	9.6	9.1
Net balance at beginning of year	1,047.1	973.6	950.8
Incurred related to:
Current year	915.4	852.8	726.2
Prior years	27.0	10.0	45.1
Total incurred	942.4	862.8	771.3
Paid related to:
Current year	417.8	421.5	373.2
Prior years	393.7	367.8	375.3
Total paid	811.5	789.3	748.5
Net balance at end of year	1,178.0	1,047.1	973.6
Plus: reinsurance recoverable on losses and loss expenses payable	3.6	5.9	9.6
Losses and loss expenses payable, at end of year (affiliates $630.9, $532.4, and $494.3, respectively)	$ 1,181.6	$ 1,053.0	$ 983.2