Amortizable Intangible Assets - Amortizable Intangible Assets (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	$ 1,939,419	$ 1,837,873
Accumulated amortization	(1,016,001)	(939,066)
Net carrying value	923,418	898,807
Purchased Customer Accounts
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	1,905,638	1,804,404
Accumulated amortization	(985,436)	(909,415)
Net carrying value	$ 920,202	$ 894,989
Weighted average life (years)	15 years	14 years 10 months 24 days
Non-compete Agreements
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	$ 33,781	$ 33,469
Accumulated amortization	(30,565)	(29,651)
Net carrying value	$ 3,216	$ 3,818
Weighted average life (years)	4 years 6 months	4 years 6 months