Share-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

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

Summary of activity for outstanding stock options

The following table summarizes activity for our outstanding stock options, for the years ended December 31, 2018, 2017 and 2016 (options in thousands):

	2018				2017				2016
	Number of Options		Weighted Average Exercise Price		Number of Options		Weighted Average Exercise Price		Number of Options		Weighted Average Exercise Price
Outstanding at beginning of year	648		$	40.08	675		$	29.55	1,394		$	30.85
Granted	—		—		184		44.07		216		38.73
Exercised	(2	)	28.33		(211	)	9.90		(934	)	33.61
Forfeited	—		—		—		—		(1	)	29.11
Outstanding at end of year	646		$	40.12	648		$	40.08	675		$	29.55
Exercisable at end of year	186		$	38.18	128		$	37.59	280		$	16.38

Summary of activity for restricted stock awards

The following table summarizes activity for Time-Based Restricted Stock awards for the years ended December 31, 2018, 2017 and 2016 (shares in thousands):

	2018				2017				2016
	Number of Shares		Weighted Average Grant-Date Fair Value		Number of Shares		Weighted Average Grant-Date Fair Value		Number of Shares		Weighted Average Grant-Date Fair Value
Unvested at beginning of year	160		$	37.63	249		$	33.61	339		$	29.96
Granted	51		40.01		45		44.07		91		40.03
Vested	(86	)	34.42		(134	)	32.35		(181	)	29.99
Unvested at end of year	125		$	40.82	160		$	37.63	249		$	33.61

The following table summarizes activity for TSR Restricted Stock awards for the years ended December 31, 2018, 2017 and 2016 (shares in thousands):

	2018				2017			2016
	Number of Shares		Weighted Average Grant-Date Fair Value		Number of Shares	Weighted Average Grant-Date Fair Value		Number of Shares	Weighted Average Grant-Date Fair Value
Unvested at beginning of year	253		$	40.70	214	$	39.66	123	$	39.72
Granted	45		41.71		39	46.39		91	39.59
Vested	(123	)	39.72		—	—		—	—
Unvested at end of year	175		$	41.65	253	$	40.70	214	$	39.66

The following table summarizes activity for TSR LTIP II units for the years ended December 31, 2018 and 2017 (numbers of units in thousands):

	2018
	Number of Units	Weighted Average Grant-Date Fair Value
Unvested at beginning of year	—	$	—
Granted	243	41.84
Unvested at end of year	243	$	41.84

The following table summarizes activity for TSR LTIP I units for the years ended December 31, 2018 and 2017 (units in thousands):

	2018			2017
	Number of Units	Weighted Average Grant-Date Fair Value		Number of Units	Weighted Average Grant-Date Fair Value
Unvested at beginning of year	45	$	46.21	—	$	—
Granted	48	41.48		45	46.21
Unvested at end of year	93	$	43.78	45	$	46.21

Assumptions used in the determination of grant-date fair value of awards

The midpoints of our valuation assumptions for the 2018, 2017 and 2016 grants were as follows:

	2018			2017			2016
Grant date market value of a common share	$	40.95		$	44.07		$	38.73
Risk-free interest rate	2.32		%	1.57		%	1.15		%
Dividend yield	3.52		%	3.27		%	3.41		%
Expected volatility	18.02		%	21.33		%	21.24		%
Derived vesting period of TSR Restricted Stock and TSR LTIP I units	3.4 years			3.4 years			3.4 years
Weighted average expected term of TSR Stock Options and LTIP II units	5.6 years			5.8 years			5.8 years