Goodwill and Other Intangibles - Amortization Expense Included in General and Administrative Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amortization Expense of Acquired Finite Lived Intangible Assets [Line Items]
Amortization expense	$ 380	$ 443	$ 1,213	$ 1,353
Subsequent Event [Member]
Amortization Expense of Acquired Finite Lived Intangible Assets [Line Items]
Amortization expense					$ 1,257	$ 1,316	$ 1,523	$ 1,721	$ 1,972	$ 521