Financial Instruments (Bonds) - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Corporate and municipal bonds	$ 58.3	$ 55.7
Maturity dates of bonds, start date	2019-04
Maturity dates of bonds, end date	2024-02