STOCK-BASED COMPENSATION

3 Months Ended

Mar. 31, 2019

NOTE 4. STOCK-BASED COMPENSATION

Presented below is a summary of the stock-based compensation cost and associated tax benefit included in the accompanying consolidated statements of operations:

	Three Months Ended March 31,
	2019		2018
Stock-based compensation expense	$	35.7	$	35.1
Tax benefit	$	7.8	$	7.2

During the first three months of 2019, the Company granted 0.2 million employee stock options, which had a weighted average grant date fair value of $43.10 per share based on the Black-Scholes option-pricing model. The Company also granted 0.8 million shares of restricted stock in the first three months of 2019, which had a weighted average grant date fair value of $173.58 per share. Both the employee stock options and restricted stock generally vest ratably over a four-year period. Additionally, the Company granted 0.1 million shares of performance-based awards whereby the number of shares that ultimately vest are based on the achievement of certain non-market based performance metrics of the Company over a three-year period. The weighted average grant date fair value of these awards was $167.82 per share.

The following weighted average assumptions were used in determining the fair value for options granted in 2019:

Expected dividend yield	1.15%
Expected stock volatility	23.62%
Risk-free interest rate	2.60%
Expected holding period	6.2 years
Grant date fair value	$43.10

Unrecognized stock-based compensation expense at March 31, 2019 was $11.4 million and $239.2 million for stock options and unvested restricted stock, respectively, which is expected to be recognized over a weighted average period of 2.3 years and 2.7 years, respectively. Additionally, there was $40.8 million of unrecognized stock-based compensation expense relating to the aforementioned non-market based performance-based awards, which is expected to be recognized over a weighted average period of 2.2 years.

The following table summarize information relating to stock option exercises and restricted stock vesting:

		Three Months Ended March 31,
		2019		2018
Exercise of stock options:
	Proceeds from stock option exercises	$	11.9	$	26.6
	Aggregate intrinsic value	$	35.6	$	61.9
	Tax benefit realized upon exercise	$	8.5	$	15.0
	Number of shares exercised		0.3		0.5
Vesting of restricted stock:
	Fair value of shares vested	$	146.5	$	146.7
	Tax benefit realized upon vesting	$	33.6	$	33.9
	Number of shares vested		0.8		0.9
Vesting of performance-based restricted stock:
	Fair value of shares vested	$	47.5	$	23.0
	Tax benefit realized upon vesting	$	11.5	$	5.5
	Number of shares vested		0.3		0.1