Amortizable Intangible Assets - Amortizable Intangible Assets (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 1,595,939	$ 1,494,561
Accumulated Amortization	(915,713)	(853,556)
Net Carrying Value	680,226	641,005
Purchased customer accounts
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	1,565,186	1,464,274
Accumulated Amortization	(886,229)	(824,584)
Net Carrying Value	$ 678,957	$ 639,690
Weighted Average Life (Years)	15 years	15 years
Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 30,753	$ 30,287
Accumulated Amortization	(29,484)	(28,972)
Net Carrying Value	$ 1,269	$ 1,315
Weighted Average Life (Years)	6 years 8 months 12 days	6 years 9 months 18 days