Commitments and Contingent Liabilities - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Future Minimum Lease Payments Due	$ 148.1
Future Minimum Lease Payments Due Within Twelve Months	27.0
Future Minimum Lease Payments Due Within Two Years	24.7
Future Minimum Lease Payments Due Within Three Years	20.1
Future Minimum Lease Payments Due Within Four Years	15.9
Future Minimum Lease Payments Due Within Five Years	12.3
Future Minimum Lease Payments Due Thereafter	48.1
Rental Expense	54.2	$ 51.4	$ 42.5
Total Unfunded Commitments	444.0
Settled Litigation | Unfavorable Regulatory Action
Loss Contingencies
Loss Contingency Accrual, Insurance-related Assessment, Premium Tax Offset	44.0
Loss Contingency, Loss in Period, Tax	7.2
Net of Tax | Settled Litigation | Unfavorable Regulatory Action
Loss Contingencies
Loss Contingency, Loss in Period	$ 13.4