Equity Incentive Plan - Unvested Shares (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Feb. 28, 2017	May 31, 2016	Jul. 31, 2012	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary of non-vested shares/units
Granted (in shares)						2,554
Weighted Average Grant Date Fair Value
Granted (in dollars per share)						$ 22.26
Share grants to trustees				$ 0	$ 0	$ (57)
Performance units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average period of recognition of unrecognized share-based compensation expense				2 years 1 month 12 days
Weighted Average Grant Date Fair Value
Share grants to trustees				$ (2,100)	$ (1,700)	900
Total fair value of shares vested						$ 6,700
Restricted share awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average period of recognition of unrecognized share-based compensation expense				2 years 3 months 24 days
Summary of non-vested shares/units
Unvested at the beginning of the period (in shares)				700,325	649,447	540,885
Granted (in shares)				592,673	425,076	675,375
Vested (in shares)				(438,881)	(363,160)	(272,780)
Forfeited (in shares)				(113,325)	(11,038)	(294,033)
Unvested at the end of the period (in shares)				740,792	700,325	649,447
Weighted Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share)				$ 22.88	$ 23.00	$ 26.73
Granted (in dollars per share)				21.42	23.15	20.59
Vested (in dollars per share)				22.92	23.41	23.66
Forfeited (in dollars per share)				21.58	23.24	23.70
Unvested at the end of the period (in dollars per share)				$ 21.89	$ 22.88	$ 23.00
Share grants to trustees				$ (10,200)	$ (8,900)	$ (6,900)
Total fair value of shares vested				$ 9,500	$ 7,700	$ 5,800
2012 Performance Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			4 years
Share Based Compensation Arrangement by Share Based Payment Award, Performance Based Vesting Period			3 years
Share Based Compensation Arrangement by Share Based Payment Award, Time Based Vesting Period			1 year
2016 Performance Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		4 years
Share Based Compensation Arrangement by Share Based Payment Award, Performance Based Vesting Period		3 years
Share Based Compensation Arrangement by Share Based Payment Award, Time Based Vesting Period		1 year
Vesting percentage upon satisfaction of performance-based vesting period		50.00%
Share-based compensation arrangement, by share-based payment award vesting rights, percentage after one year		50.00%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate		1.05%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate		23.82%
Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Share Based Awards Other Than Options Percentage Of Grant Date Fair Value Recognized Over Three Years		50.00%
Employee service share based compensation cost period of recognition		3 years
Percentage of grant date fair value to be recognized over four years		50.00%
Employee Service Share Based Compensation Nonvested Awards For Remaining Fifty Percent Cost Not Yet Recognized, Period For Recognition		4 years
Summary of non-vested shares/units
Granted (in shares)		280,000
Weighted Average Grant Date Fair Value
Granted (in dollars per share)		$ 10.31
2017 Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Share Based Compensation Arrangement by Share Based Payment Award, Performance Based Vesting Period	3 years
Share Based Compensation Arrangement by Share Based Payment Award, Time Based Vesting Period	1 year
Vesting percentage upon satisfaction of performance-based vesting period	50.00%
Share-based compensation arrangement, by share-based payment award vesting rights, percentage after one year	50.00%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate	1.57%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate	25.73%
Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Share Based Awards Other Than Options Percentage Of Grant Date Fair Value Recognized Over Three Years	50.00%
Employee service share based compensation cost period of recognition	3 years
Percentage of grant date fair value to be recognized over four years	50.00%
Employee Service Share Based Compensation Nonvested Awards For Remaining Fifty Percent Cost Not Yet Recognized, Period For Recognition	4 years
Summary of non-vested shares/units
Granted (in shares)	259,000
Weighted Average Grant Date Fair Value
Granted (in dollars per share)	$ 14.93
President and Chief Executive Officer | Performance units
Weighted Average Grant Date Fair Value
Share grants to trustees		$ 2,300
President and Chief Executive Officer | Restricted share awards
Weighted Average Grant Date Fair Value
Share grants to trustees		$ 500
President and Chief Executive Officer | 2012 Performance Units
Summary of non-vested shares/units
Forfeited (in shares)		(133,467)
Minimum | 2016 Performance Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance units that will convert into restricted shares		25.00%
Minimum | 2017 Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance units that will convert into restricted shares	25.00%
Maximum | 2016 Performance Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance units that will convert into restricted shares		150.00%
Maximum | 2017 Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance units that will convert into restricted shares	150.00%