Commitments, Contingencies and Off-Balance Sheet Arrangements - Outstanding Letters of Credit, Financial Guarantees and Funding Commitments (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Maximum Exposure	$ 62.8
Liability Recorded	$ 11.8
Funding Commitment To Two Funds Expire In 2019 And 2023 Trigger [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Collateral	None
Compensation to Us	None
Maximum Exposure	$ 2.9
Credit Support Under Letters of Credit for Deductibles Due by Us on Our Own Insurance Coverage's Expires After 2020 Trigger [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Collateral	None
Compensation to Us	None
Maximum Exposure	$ 9.7
Liability Recorded	$ 11.8
Credit Enhancement Under Letters of Credit for Our Captive Insurance Operations to Meet Minimum Statutory Capital Requirements Expires After 2020 [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 Under Letters of Credit for Clients Claim Funds Held by Our Bermuda Captive Insurance Operation in Fiduciary Capacity Expires After 2020 [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Collateral	None
Compensation to Us	Reimbursement of LOC fees
Maximum Exposure	$ 5.0
Funding Commitments Related to Expenditures on New Corporate Headquarters Building [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Collateral	None
Compensation to Us	None
Maximum Exposure	$ 31.4
Funding Commitments on Clean Energy Investment [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Collateral	None
Compensation to Us	None
Maximum Exposure	$ 2.2
Credit Support Under Letters of Credit in Lieu of Security Deposit for Acquisition Lease [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	$ 4.8