Commitments, Contingencies and Off-Balance Sheet Arrangements - Outstanding Letters of Credit, Financial Guarantees and Funding Commitments (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Maximum Exposure	$ 19.7
Liability Recorded	$ 13.5
Funding Commitment To One Fund - Funded In January 2018 [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Collateral	None
Compensation to Us	None
Maximum Exposure	$ 0.4
Credit Support Under Letters Of Credit For Deductibles Due By Us On Our Own Insurance Coverage's Expires After 2022 [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Collateral	None
Compensation to Us	None
Maximum Exposure	$ 9.7
Liability Recorded	$ 12.8
Credit Enhancement Under Letters Of Credit For Our Captive Insurance Operations To Meet Minimum Statutory Capital Requirements Expires After 2022 [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Collateral	None
Compensation to Us	Reimbursement of LOC fees
Maximum Exposure	$ 6.3
Credit Support for Our Obligation Under a Client's Insurance Program Expires 2018 [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Collateral	None
Compensation to Us	None
Maximum Exposure	$ 0.7
Liability Recorded	$ 0.7
Credit Support Under Letters of Credit in Lieu of Security Deposit for Acquisition Lease Expires 2023 [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Collateral	None
Compensation to Us	None
Maximum Exposure	$ 0.5
Financial Guarantees of Loans to Nineteen Canadian Based Employees Trigger [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Collateral	-1
Compensation to Us	None
Maximum Exposure	$ 2.1