Goodwill and Other Intangible Assets - Intangible Assets by Major Class (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Amortized intangible assets, Weighted Average Useful Life in Years	13 years 7 months 6 days
Amortized intangible assets, Gross Carrying Amount	$ 120,456	$ 124,530
Amortized intangible assets, Accumulated Amortization	81,906	78,010
Amortized intangible assets, Net Carrying Amount	38,550	46,520
Intangible assets, Gross Carrying Amount	126,056	130,130
Intangible assets, Net Carrying Amount	44,150	52,120
Trade names
Finite-Lived Intangible Assets [Line Items]
Unamortized intangible assets, Gross Carrying Amount	$ 5,600	5,600
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Amortized intangible assets, Weighted Average Useful Life in Years	13 years 8 months 12 days
Amortized intangible assets, Gross Carrying Amount	$ 117,192	119,736
Amortized intangible assets, Accumulated Amortization	80,523	75,212
Amortized intangible assets, Net Carrying Amount	36,669	44,524
Non-competition agreements
Finite-Lived Intangible Assets [Line Items]
Amortized intangible assets, Gross Carrying Amount	0	1,263
Amortized intangible assets, Accumulated Amortization	0	1,246
Amortized intangible assets, Net Carrying Amount	$ 0	17
Acquired software
Finite-Lived Intangible Assets [Line Items]
Amortized intangible assets, Weighted Average Useful Life in Years	9 years 9 months 18 days
Amortized intangible assets, Gross Carrying Amount	$ 3,264	3,171
Amortized intangible assets, Accumulated Amortization	1,383	1,292
Amortized intangible assets, Net Carrying Amount	1,881	1,879
Trade names
Finite-Lived Intangible Assets [Line Items]
Amortized intangible assets, Gross Carrying Amount	0	360
Amortized intangible assets, Accumulated Amortization	0	260
Amortized intangible assets, Net Carrying Amount	$ 0	$ 100