Goodwill and intangible assets (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 34,913	$ 34,913
AccumulatedAmortization	4,719	2,494
Net	$ 30,194	32,419
Licenses [Member]
Indefinite-lived Intangible Assets [Line Items]
Amortization Period(in years)	10 years
Gross Carrying Amount	$ 1,000	1,000
AccumulatedAmortization	392	317
Net	$ 608	683
Patents [Member]
Indefinite-lived Intangible Assets [Line Items]
Amortization Period(in years)	5 years
Gross Carrying Amount	$ 386	386
AccumulatedAmortization	379	374
Net	7	12
Covenant Not to Compete [Member]
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	1,270	1,270
AccumulatedAmortization	591	475
Net	$ 679	795
Covenant Not to Compete [Member] | Maximum [Member]
Indefinite-lived Intangible Assets [Line Items]
Amortization Period(in years)	10 years
Covenant Not to Compete [Member] | Minimum [Member]
Indefinite-lived Intangible Assets [Line Items]
Amortization Period(in years)	6 years
Customer Relationships [Member]
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 31,660	31,660
AccumulatedAmortization	3,284	1,288
Net	$ 28,376	30,372
Customer Relationships [Member] | Maximum [Member]
Indefinite-lived Intangible Assets [Line Items]
Amortization Period(in years)	12 years
Customer Relationships [Member] | Minimum [Member]
Indefinite-lived Intangible Assets [Line Items]
Amortization Period(in years)	10 years
Above Market Leases [Member]
Indefinite-lived Intangible Assets [Line Items]
Amortization Period(in years)	13 years
Gross Carrying Amount	$ 567	567
AccumulatedAmortization	43	10
Net	$ 524	557
Trade Names [Member]
Indefinite-lived Intangible Assets [Line Items]
Amortization Period(in years)	2 years
Gross Carrying Amount	$ 30	30
AccumulatedAmortization	30	30
Net	$ 0	$ 0