Shareholders' Equity / Partners' Capital - Stock Options Activity (Details) - 2012 Equity Incentive Plan - Stock options - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Stock Options
Outstanding at beginning of the period (in shares)	3,052,450	2,826,500	2,826,500
Granted (in shares)	140,000	385,600
Exercised (in shares)	(684,875)	(62,655)
Forfeited (in shares)	(112,750)	(85,250)
Outstanding at end of the period (in shares)	2,394,825	3,064,195	3,052,450	2,826,500
Exercisable at end of the period (in shares)	1,659,050	1,681,595
Weighted Average Exercise Price
Outstanding at beginning of the period (in dollars per share)	$ 16.65	$ 15.69	$ 15.69
Granted (in dollars per share)	19.40	23.38
Exercised (in dollars per share)	16.04	15.77
Forfeited (in dollars per share)	17.84	16.24
Outstanding at end of the period (in dollars per share)	16.92	16.64	$ 16.65	$ 15.69
Exercisable at end of the period (in dollars per share)	$ 16.23	$ 15.90
Aggregate intrinsic value
Weighted average remaining life (in years)	6 years 4 months 10 days	7 years 1 month 6 days	6 years 10 months 24 days	7 years 7 months 6 days
Weighted average exercisable (in years)	5 years 7 months 25 days	6 years 3 months 18 days
Aggregate intrinsic value	$ 12,388	$ 16,149	$ 16,421	$ 14,956
Aggregate intrinsic value exercised	4,388	444
Aggregate intrinsic value exercisable	$ 9,521	$ 9,764