Financial Instruments (Bonds) - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Corporate and municipal bonds	$ 49.5	$ 42.4
Maturity dates of bonds, start date	2018-01
Maturity dates of bonds, end date	2023-12