Common Stock - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Aug. 05, 2016
Schedule Of Common Stock [Line Items]
Shares outstanding	30,958,581	30,769,063
Common Class A [Member]
Schedule Of Common Stock [Line Items]
Shares outstanding	30,958,581	30,769,063
Restricted issued but not yet vested, shares	107,786	146,494
New Board Authorized Share Repurchase Program
Schedule Of Common Stock [Line Items]
Stock repurchase program, authorized amount			$ 50.0
Remaining authorized amount	$ 12.6