Goodwill and intangible assets (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 34,913	$ 34,913
AccumulatedAmortization	3,236	2,494
Net	$ 31,677	32,419
Licenses [Member]
Indefinite-lived Intangible Assets [Line Items]
Amortization Period(in years)	10 years
Gross Carrying Amount	$ 1,000	1,000
AccumulatedAmortization	342	317
Net	$ 658	683
Patents [Member]
Indefinite-lived Intangible Assets [Line Items]
Amortization Period(in years)	5 years
Gross Carrying Amount	$ 386	386
AccumulatedAmortization	376	374
Net	10	12
Covenant Not to Compete [Member]
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	1,270	1,270
AccumulatedAmortization	514	475
Net	$ 756	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	1,953	1,288
Net	$ 29,707	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	21	10
Net	$ 546	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