Intangible Assets - Reconciliation of Intangible Assets (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Goodwill And Intangible Assets Disclosure [Abstract]
Gross carrying cost at beginning of period			$ 68,322,000	$ 68,322,000
Gross carrying cost at end of period	$ 68,322,000	$ 68,322,000	68,322,000	68,322,000
Accumulated amortization at beginning of period			(40,653,000)	(38,193,000)
Amortization expense	(615,000)	(615,000)	(1,845,000)	(1,845,000)
Accumulated amortization at end of period	(42,498,000)	(40,038,000)	(42,498,000)	(40,038,000)
Net book value at end of period	$ 25,824,000	$ 28,284,000	$ 25,824,000	$ 28,284,000	$ 27,669,000