Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2016

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

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

The significant assumptions underlying determination of fair values for market-based awards granted were as follows:

	Year ended December 31,
	2016	2015	2014
Volatility	18.50%	17.10%	24.60%
Risk free interest rate	0.88%	0.78%	0.64%

Disclosure of Share-based Compensation Arrangements by Share-based Payment Award

The Company recorded stock-based compensation in general and administrative expenses in the accompanying Consolidated Statements of Operations, the components of which are further described below:

	Year ended December 31,
(in thousands)	2016			2015		2014
Restricted stock (1)	$	13,422		13,869		12,161
Directors' fees paid in common stock (1)	193			200		208
Capitalized stock-based compensation (2)	(2,963		)	(2,988	)	(2,707	)
Stock-based compensation, net of capitalization	$	10,652		11,081		9,662

⁽¹⁾ Includes amortization of the grant date fair value of restricted stock awards over the respective vesting periods.

⁽²⁾ Includes compensation expense specifically identifiable to development and leasing activities.

Schedule of Share-based Compensation, Stock Options, Activity

The following table summarizes stock options outstanding:

	Year ended December 31, 2016
	Number of Options	Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2015	8,741	$	88.45	1.1	$	(178	)
Outstanding as of December 31, 2016	8,741	$	88.45	0.1	$	(170	)
Vested as of December 31, 2016	8,741	$	88.45	0.1	$	(170	)
Exercisable as of December 31, 2016 (1)	8,741	$	88.45	0.1	$	(170	)

⁽¹⁾ The Company issues new shares to fulfill option exercises from its authorized shares available. The total intrinsic value of options exercised during the year ended December 31,2014 was approximately $1.3 million.

Schedule of Nonvested Share Activity

The following table summarizes non-vested restricted stock activity:

	Year ended December 31, 2016
	Number of Shares	Intrinsic Value (in thousands)	Weighted Average Grant Price
Non-vested as of December 31, 2015	615,420
Add: Time-based awards granted (1) (4)	117,275		$73.22
Add: Performance-based awards granted (2) (4)	6,521		$73.22
Add: Market-based awards granted (3) (4)	67,332		$90.49
Less: Vested and Distributed (5)	222,915		$70.27
Less: Forfeited	22,372		$60.53
Non-vested as of December 31, 2016 (6)	561,261	$39,699

⁽¹⁾ Time-based awards vest beginning on the first anniversary following the grant date over a three or four year service period. These grants are subject only to continued employment and are not dependent on future performance measures. Accordingly, if such vesting criteria are not met, compensation cost previously recognized would be reversed.

⁽²⁾ Performance-based awards are earned subject to future performance measurements. Once the performance criteria are achieved and the actual number of shares earned is determined, shares vest over a required service period. The Company considers the likelihood of meeting the performance criteria based upon management's estimates from which it determines the amounts recognized as expense on a periodic basis.

⁽³⁾ Market-based awards are earned dependent upon the Company's total shareholder return in relation to the shareholder return of a NAREIT index over a three-year period. Once the performance criteria are met and the actual number of shares earned is determined, the shares are immediately vested and distributed. The probability of meeting the criteria is considered when calculating the estimated fair value on the date of grant using a Monte Carlo simulation. These awards are accounted for as awards with market criteria, with compensation cost recognized over the service period, regardless of whether the performance criteria are achieved and the awards are ultimately earned. The significant assumptions underlying determination of fair values for market-based awards granted were as follows:

	Year ended December 31,
	2016	2015	2014
Volatility	18.50%	17.10%	24.60%
Risk free interest rate	0.88%	0.78%	0.64%

⁽⁴⁾ The weighted-average grant price for restricted stock granted during the years ended December 31, 2016, 2015, and 2014 was $79.40, $69.80, and $48.18, respectively.

⁽⁵⁾ The total intrinsic value of restricted stock vested during the years ended December 31, 2016, 2015, and 2014 was $15.4 million, $18.6 million, and $12.4 million, respectively.

⁽⁶⁾ As of December 31, 2016, there was $12.8 million of unrecognized compensation cost related to non-vested restricted stock granted under the Parent Company's Plan. When recognized, this compensation results in additional paid in capital in the accompanying Consolidated Statements of Equity of the Parent Company and in general partner preferred and common units in the accompanying Consolidated Statements of Capital of the Operating Partnership. This unrecognized compensation cost is expected to be recognized over the next three years. The Company issues new restricted stock from its authorized shares available at the date of grant.