Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2015

Summary of Activity Under Stock Option Plans

A summary of the activity under our stock option plans as of June 30, 2015 and changes during the three and six month periods then ended, is presented below:

	Options Outstanding		Weighted- Average Exercise Price Per Share		Weighted-Average Remaining Contractual Life in Years	Aggregate Intrinsic Value
Options outstanding at March 31, 2015	251,400		$	41.06	3.7	$	10,344,139
Options exercised	(26,562	)	$	46.09
Options forfeited	(600	)	$	34.83
Options outstanding at June 30, 2015	224,238		$	40.49	3.5	5,752,806
Options exercisable at June 30, 2015	223,427		$	39.98	3.2	5,845,097
Options vested or expected to vest at June 30, 2015*	224,214		$	40.47	3.5	5,755,574

* In addition to the vested options, we expect a portion of the unvested options to vest at some point in the future. Options expected to vest are calculated by applying an estimated forfeiture rate to the unvested options.

	Options Outstanding		Weighted- Average Exercise Price Per Share
Options outstanding at December 31, 2014	393,347		$	40.72
Options exercised	(167,659	)	$	40.91
Options forfeited	(1,450	)	$	39.62
Options outstanding at June 30, 2015	224,238

Assumptions Used in Calculation of Fair Value

Below are the assumptions used in the Monte Carlo calculation:

	June 30, 2015	June 30, 2014
Expected volatility	28.2%	33.7%
Expected term (in years)	3.0	3.0
Risk-free interest rate	0.96%	0.67%
Expected dividend yield	—	—

Performance-Based Restricted Stock [Member]

Restricted Stock Activities

	Performance-Based Restricted Stock Awards
Non-vested awards outstanding at December 31, 2014	92,437
Awards granted	50,798
Stock issued	(20,910	)
Awards forfeited	(8,605	)
Non-vested awards outstanding at June 30, 2015	113,720

Time Based Restricted Stock [Member]

Restricted Stock Activities

	Time-Based Restricted Stock Awards
Non-vested awards outstanding at December 31, 2014	238,386
Awards granted	65,285
Stock issued	(69,352	)
Awards forfeited	(4,969	)
Non-vested awards outstanding at June 30, 2015	229,350

Deferred Stock Units [Member]

Restricted Stock Activities

	Deferred Stock Units
Awards outstanding at December 31, 2014	30,150
Awards granted	9,100
Stock issued	(15,400	)
Awards outstanding at June 30, 2015	23,850