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, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Stock Options
Outstanding at beginning of the period (in shares)	2,826,500	2,484,400	2,484,400
Granted (in shares)	385,600	708,000
Exercised (in shares)	(62,655)	(172,250)
Forfeited (in shares)	(85,250)	(153,150)
Outstanding at end of the period (in shares)	3,064,195	2,867,000	2,826,500	2,484,400
Exercisable at end of the period (in shares)	1,681,595	1,051,125
Weighted Average Exercise Price
Outstanding at beginning of the period (in dollars per share)	$ 15.69	$ 16.22	$ 16.22
Granted (in dollars per share)	23.38	14.15
Exercised (in dollars per share)	15.77	16.12
Forfeited (in dollars per share)	16.24	16.36
Outstanding at end of the period (in dollars per share)	16.64	15.70	$ 15.69	$ 16.22
Exercisable at end of the period (in dollars per share)	$ 15.90	$ 16.04
Aggregate intrinsic value
Weighted average remaining life (in years)	7 years 1 month 6 days	7 years 9 months 18 days	7 years 7 months 6 days	8 years
Weighted average exercisable (in years)	6 years 3 months 18 days	7 years 1 month 6 days
Aggregate intrinsic value	$ 16,149	$ 17,021	$ 14,956	$ 1,225
Aggregate intrinsic value exercised	444	680
Aggregate intrinsic value exercisable	$ 9,764	$ 5,885