Shareholders' Equity - 2012 Equity Incentive Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Noncash share-based compensation	$ 3,636	$ 3,125	$ 2,586
Stock options and Restricted stock units
Class of Stock [Line Items]
Noncash share-based compensation	3,600	$ 3,100	$ 2,600
Stock options
Class of Stock [Line Items]
Total unrecognized compensation cost	$ 4,100
Weighted-average period of unvested cost	1 year 5 months 5 days
Restricted stock units
Class of Stock [Line Items]
Total unrecognized compensation cost	$ 1,300
Weighted-average period of unvested cost	2 years 1 month 17 days
Restricted stock units | 2012 Equity Incentive Plan
Class of Stock [Line Items]
Stock options granted (in shares)	74,100	44,000	92,000
Board of Trustees | 2012 Equity Incentive Plan
Class of Stock [Line Items]
Vesting period	4 years	4 years	4 years
Expiration period	10 years	10 years	10 years
Class A common shares | 2012 Equity Incentive Plan
Class of Stock [Line Items]
Shares available for issuance (in shares)	6,000,000
Class A common shares | Stock options | 2012 Equity Incentive Plan
Class of Stock [Line Items]
Stock options granted (in shares)	708,000	588,500	1,270,000