Amortizable Intangible Assets - Amortizable Intangible Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 1,494,561	$ 1,477,348
Accumulated Amortization	(853,556)	(769,894)
Net Carrying Value	641,005	707,454
Purchased customer accounts
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	1,464,274	1,447,680
Accumulated Amortization	(824,584)	(741,770)
Net Carrying Value	$ 639,690	$ 705,910
Weighted Average Life (Years)	15 years	15 years
Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 30,287	$ 29,668
Accumulated Amortization	(28,972)	(28,124)
Net Carrying Value	$ 1,315	$ 1,544
Weighted Average Life (Years)	6 years 9 months 18 days	6 years 9 months 18 days