Basis of Presentation and Significant Accounting Policies - ASU Adoptions (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments to retained earnings	$ 654,605	$ 616,149
Right of use assets	41,200
Lease liability	42,166
ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments to retained earnings			$ 1,200
ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments to retained earnings			(300)
ASU 2017-12
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments to retained earnings			$ (3)
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right of use assets	37,200
Lease liability	$ 38,200