Fair Value of Marketable Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Trading Securities	$ 5.3	$ 3.6	$ 3.4
Fair Value, Inputs, Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Trading Securities	5.3	3.6	3.4
Fair Value, Inputs, Level 1 | Equity Funds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Trading Securities	3.0	2.1	1.9
Fair Value, Inputs, Level 1 | Fixed Income Funds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Trading Securities	$ 2.3	$ 1.5	$ 1.5