Fair Value of Marketable Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Trading Securities	$ 5.1	$ 3.6	$ 3.3
Fair Value, Inputs, Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Trading Securities	5.1	3.6	3.3
Fair Value, Inputs, Level 1 | Equity Funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Trading Securities	1.9	2.1	1.1
Fair Value, Inputs, Level 1 | Fixed Income Funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Trading Securities	1.6	$ 1.5	0.9
Fair Value, Inputs, Level 1 | Cash Equivalents
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Trading Securities	$ 1.6		$ 1.3