Financial Derivatives - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Maximum amount of loss due to credit risk	$ 1,300,000	$ 1,100,000
Total net settlement amount asset positions	1,300,000	1,100,000
Total net settlement amount liability positions	$ 900,000	$ 2,900,000