Shareholders' Equity / Partners' Capital - Stock Options Activity (Details) - 2012 Equity Incentive Plan - Stock options - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Stock Options
Outstanding at beginning of the period (in shares)	2,252,275	3,052,450	3,052,450
Granted (in shares)	20,000	140,000
Exercised (in shares)	(650,375)	(171,875)
Forfeited (in shares)	(12,350)	(66,250)
Outstanding at end of the period (in shares)	1,609,550	2,954,325	2,252,275	3,052,450
Exercisable at end of the period (in shares)	1,240,400	2,174,550
Weighted Average Exercise Price
Outstanding at beginning of the period (in dollars per share)	$ 16.92	$ 16.65	$ 16.65
Granted (in dollars per share)	20.48	19.40
Exercised (in dollars per share)	15.89	16.04
Forfeited (in dollars per share)	20.80	17.69
Outstanding at end of the period (in dollars per share)	17.35	16.79	$ 16.92	$ 16.65
Exercisable at end of the period (in dollars per share)	$ 16.72	$ 16.20
Aggregate intrinsic value
Weighted average remaining life (in years)	5 years 9 months 18 days	6 years	6 years 1 month 6 days	6 years 10 months 24 days
Weighted average exercisable (in years)	5 years 3 months 18 days	5 years 3 months 18 days
Aggregate intrinsic value	$ 11,209	$ 16,347	$ 7,713	$ 16,421
Aggregate intrinsic value exercised	5,336	787
Aggregate intrinsic value exercisable	$ 9,413	$ 13,123