Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2015

Stock based compensation expense for the years ended December 31, 2015, 2014 and 2013 was as follows (in millions):

	2015		2014		2013
Stock based compensation	$	61.8	$	63.0	$	53.4
Tax benefit recognized in net income		21.6		22.1		18.7
Windfall tax benefit, net		22.2		21.5		16.0

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions

The Company estimates the fair value of its option awards using the Black-Scholes option valuation model. The stock volatility for each grant is measured using the weighted-average of historical daily price changes of the Company's common stock over the most recent period equal to the expected life of the grant. The expected term of options granted is derived from historical data to estimate option exercises and employee forfeitures, and represents the period of time that options granted are expected to be outstanding. The risk-free rate for periods within the contractual life of the option is based on the U.S. Treasury yield curve in effect at the time of grant. The weighted-average fair value of options granted in 2015, 2014 and 2013 were calculated using the following weighted-average assumptions:

	2015		2014		2013
Weighted-average fair value ($)		33.98		34.95		37.08
Risk-free interest rate (%)		1.53		1.63		0.86
Average expected option life (years)		5.10		5.22		5.19
Expected volatility (%)		22.17		27.01		36.09
Expected dividend yield (%)		0.62		0.58		0.56

The following table summarizes the Company's activities with respect to its stock option plans for the years ended December 31, 2015 and 2014:

Schedule of Share-based Compensation, Stock Option Activity

	Number of shares			Weighted-average exercise price per share		Weighted-average contractual term		Aggregate intrinsic value
Outstanding at January 1, 2014		2,988,436		$	74.00
Granted		650,000			137.05
Exercised		(587,661	)		55.98
Canceled		(69,664	)		116.29
Outstanding at December 31, 2014		2,981,111			90.48		6.37		$	196,378,239
Granted		628,155			162.77
Exercised		(400,050	)		82.50
Canceled		(91,600	)		142.36
Outstanding at December 31, 2015		3,117,616			104.54		6.08		$	265,782,636
Exercisable at December 31, 2015		1,935,351		$	78.76		4.55		$	214,879,670

Summary of Options Outstanding and Exercisable, by Range of Exercise Prices

The following table summarizes information for stock options outstanding at December 31, 2015:

		Outstanding options						Exercisable options
Exercise price		Number		Average exercise price		Average remaining life (years)		Number		Average exercise price
$	38.46 - 57.68		896,590	$	53.04		2.4		896,590	$	53.04
	57.69 - 76.91		298,733		72.65		5.1		298,733		72.65
	76.92 - 96.14		295,684		93.16		6.1		294,434		93.15
	96.15 - 115.37		310,597		114.82		7.0		200,319		114.60
	115.38 - 134.60		562,626		130.96		8.0		195,520		130.52
	134.61- 153.82		272,036		144.28		8.7		37,422		142.12
	153.83 - 173.05		413,350		165.00		9.2		11,000		156.57
	173.06 - 192.28		68,000		181.14		9.7		1,333		175.00
$	38.46 - 192.28		3,117,616	$	104.54		6.1		1,935,351	$	78.76

Schedule of Nonvested Restricted Stock Awards Activity

Restricted Stock Grants - During 2015 and 2014, the Company granted 437,035 and 375,060 shares, respectively, of restricted stock to certain employee and director participants under the 2006 Plan. Restricted stock grants generally vest over a period of 1 to 3 years. The Company recorded $46.5 million, $46.4 million and $36.5 million of compensation expense related to outstanding shares of restricted stock held by employees and directors during 2015, 2014 and 2013, respectively. A summary of the Company's nonvested shares activity for 2015 and 2014 is as follows:

	Number of shares			Weighted-average grant date fair value
Nonvested at December 31, 2013		573,850		$	103.44
Granted		375,060			142.30
Vested		(378,994	)		153.16
Forfeited		(27,361	)		106.60
Nonvested at December 31, 2014		542,555		$	130.29
Granted		437,035			159.32
Vested		(243,423	)		183.10
Forfeited		(26,892	)		148.82
Nonvested at December 31, 2015		709,275		$	183.90