Stock-based Compensation - (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of non-vested restricted common shares

The following is summary of the status of the Trust’s non-vested restricted common shares during 2017, 2016, and 2015:

	Common Shares		Weighted Average Grant Date Fair Value
Non-vested at December 31, 2014	319,654		$	12.60
Granted	162,522		15.95
Vested	(170,337	)	12.93
Non-vested at December 31, 2015	311,839		14.17
Granted	155,306		17.96
Vested	(170,034	)	14.16
Forfeited	(326	)	15.36
Non-vested at December 31, 2016	296,785		16.16
Granted	143,593		19.74
Vested	(266,552	)	16.00
Forfeited	(550	)	18.78
Non-vested at December 31, 2017	173,276		$	19.36

Schedule of weighted average grant date fair value assumptions

The Company utilized a Monte Carlo simulation to calculate the weighted average grant date fair values in 2017, 2016, and 2015 of $33.43, $28.50, and $20.06 per unit, respectively, using the following assumptions:

	2017			2016			2015
Volatility	21.5		%	20.3		%	20.7		%
Dividend assumption	reinvested			reinvested			reinvested
Expected term in years	2.8 years			2.8 years			2.8 years
Risk-free rate	1.68		%	1.07		%	1.14		%
Stock price (per share)	$	19.80		$	17.67		$	15.87

Summary of the activity in the restricted share units

The following is a summary of the activity in the Trust’s restricted share units during 2017, 2016, and 2015: 

	Executive Awards				Trustee Awards
	Restricted Share Units		Weighted Average Grant Date Fair Value		Restricted Share Units		Weighted Average Grant Date Fair Value
Non-vested at December 31, 2014	55,680		$	16.94	—		$	—
Granted	75,250		19.22		40,957		15.87
Non-vested at December 31, 2015	130,930		18.48		40,957		15.87
Granted	104,553		26.33		36,784		17.67
Vested	—		—		(20,481	)	15.87
Non-vested at December 31, 2016	235,483		21.84		57,260		17.03
Granted	174,320		29.34		32,831		19.80
Vested	(55,680	)	16.94		(38,871	)	16.72
Non-vested at December 31, 2017	354,123		$	26.30	51,220		$	19.04