Summary of Significant Accounting Policies - Summary of Changes in Company's Allowance for Doubtful Accounts (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Abstract]
Balance at Beginning of Year	$ 1,783	$ 1,961	$ 1,715
Provision	986	806	894
Acquired in Business Combinations	0	0	0
Write-offs, Net of Recoveries	(670)	(984)	(648)
Balance at End of Year	$ 2,099	$ 1,783	$ 1,961