Financial Instruments (Bonds) - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Corporate and municipal bonds	$ 50.2	$ 42.4
Maturity dates of bonds, start date	2017-11
Maturity dates of bonds, end date	2022-12