Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset	$ 32,737
Operating lease, liability	$ 47,935
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset			$ 33,953
Operating lease, liability			$ 49,395
Software development
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Finite-lived intangible assets, useful life	2 years	1 year