Common Stock - (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Aug. 05, 2016
Schedule Of Common Stock [Line Items]
Remaining authorized amount	$ 12.6
Shares outstanding	30,479,969	26,875,585
Common Class A [Member]
Schedule Of Common Stock [Line Items]
Shares outstanding	30,479,969	26,875,585
Restricted issued but not yet vested, shares	221,782	163,557
New Board Authorized Share Repurchase Program
Schedule Of Common Stock [Line Items]
Stock repurchase program, authorized amount			$ 50.0