Amortizable Intangible Assets - Amortizable Intangible Assets (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 1,872,339	$ 1,837,873
Accumulated Amortization	(964,701)	(939,066)
Net Carrying Value	907,638	898,807
Purchased customer accounts
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	1,838,732	1,804,404
Accumulated Amortization	(934,756)	(909,415)
Net Carrying Value	$ 903,976	$ 894,989
Weighted Average Life (Years)	14 years 10 months 24 days	14 years 10 months 24 days
Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 33,607	$ 33,469
Accumulated Amortization	(29,945)	(29,651)
Net Carrying Value	$ 3,662	$ 3,818
Weighted Average Life (Years)	4 years 6 months	4 years 6 months