Summary of Significant Accounting Principles - Accounting Standards Adopted in 2019 (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset	$ 19,602		$ 0
Operating lease, liability	$ 20,225
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset		$ 23,300
Operating lease, liability		$ 24,000