Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Mar. 31, 2019
Acquired Finite-Lived Intangible Assets
Finite-Lived Intangible Assets, Gross		$ 83,830	$ 83,830
Finite-Lived Intangible Assets, Accumulated Amortization		(56,035)	(57,400)
Finite-Lived Intangible Assets, Net		27,795	26,430
Value of Business Acquired
Acquired Finite-Lived Intangible Assets
Finite-Lived Intangible Assets, Gross		42,788	42,788
Finite-Lived Intangible Assets, Accumulated Amortization		(42,788)	(42,788)
Finite-Lived Intangible Assets, Net		0	0
Customer Relationships
Acquired Finite-Lived Intangible Assets
Finite-Lived Intangible Assets, Gross		34,661	34,661
Finite-Lived Intangible Assets, Accumulated Amortization		(11,164)	(12,288)
Finite-Lived Intangible Assets, Net		$ 23,497	22,373
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	7 years 7 months	8 years
Trade Names
Acquired Finite-Lived Intangible Assets
Finite-Lived Intangible Assets, Gross		$ 6,381	6,381
Finite-Lived Intangible Assets, Accumulated Amortization		(2,083)	(2,324)
Finite-Lived Intangible Assets, Net		$ 4,298	$ 4,057
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	5 years 7 months	6 years