Financial Instruments - Summary of Bond Activity (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Fair value at beginning of period	$ 56,556	$ 51,101
Purchases	9,470	18,426
Redemptions		(1,793)
Maturities and calls	(6,865)	(10,445)
Change in bond premium	(171)	(377)
Fair market value adjustment	697	(356)
Fair value at end of period	$ 59,687	$ 56,556