Fair Value of Marketable Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Trading Securities	$ 3.6	$ 1.9
Fair Value, Inputs, Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Trading Securities	3.6	1.9
Fair Value, Inputs, Level 1 | Equity Funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Trading Securities	2.1	1.1
Fair Value, Inputs, Level 1 | Fixed Income Funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Trading Securities	$ 1.5	$ 0.8