14. Stock-Based Compensation Plans

As of June 30, 2018, we had the 2014 Stock Incentive Plan, the Employee Stock Purchase Plan, the 2014 Directors Stock Plan, the Amended and Restated 2010 Stock Incentive Plan, the 2005 Directors Stock Plan, the Stock Incentive Plan and the Directors Stock Plan (“Stock-Based Compensation Plans”). As of May 20, 2014, no new grants will be made under the Amended and Restated 2010 Stock Incentive Plan or the 2005 Directors Stock Plan. No grants have been made under the Stock Incentive Plan or the Directors Stock Plan since at least 2005. Under the terms of the 2014 Stock Incentive Plan, grants may be nonqualified stock options, incentive stock options qualifying under Section 422 of the Internal Revenue Code, restricted stock, restricted stock units, stock appreciation rights, performance shares, performance units or other stock-based awards. The 2014 Directors Stock Plan provides for the grant of nonqualified stock options, restricted stock, restricted stock units or other stock-based awards to our nonemployee directors. To date, we have not granted any incentive stock options, restricted stock or performance units under any plans.

As of June 30, 2018, the maximum number of new shares of common stock available for grant under the 2014 Stock Incentive Plan and the 2014 Directors Stock Plan was 7.3 million.

For awards with graded vesting, we use an accelerated expense attribution method. The compensation cost that was charged against net income for stock-based awards granted under the Stock-Based Compensation Plans was as follows:

	For the six months ended June 30,
	2018		2017
	(in millions)
Compensation cost	$	42.7	$	43.2
Related income tax benefit	9.3		14.2
Capitalized as part of an asset	1.1		1.2

Nonqualified Stock Options

Nonqualified stock options were granted to certain employees under the 2014 Stock Incentive Plan. Total options granted were 0.8 million for the six months ended June 30, 2018. The fair value of stock options is estimated using the Black-Scholes option pricing model. The following is a summary of the assumptions used in this model for the stock options granted during the period:

	For the six months ended
	June 30, 2018
Expected volatility	26.0		%
Expected term (in years)	7.0
Risk-free interest rate	2.8		%
Expected dividend yield	3.19		%
Weighted average estimated fair value per common share	$	14.85

As of June 30, 2018, we had $7.6 million of total unrecognized compensation cost related to nonvested stock options. The cost is expected to be recognized over a weighted-average service period of approximately 1.2 years.

Performance Share Awards

Performance share awards were granted to certain employees under the 2014 Stock Incentive Plan. Total performance share awards granted were 0.2 million for the six months ended June 30, 2018. The performance share awards granted represent initial target awards and do not reflect potential increases or decreases resulting from the final performance results to be determined at the end of the performance period. The actual number of common shares to be awarded at the end of each performance period will range between 0% and 150% of the initial target awards. The fair value of performance share awards is determined based on the closing stock price of our common shares on the grant date. The weighted-average grant date fair value of these performance share awards granted was $63.98 per common share.

As of June 30, 2018, we had $8.4 million of total unrecognized compensation cost related to nonvested performance share awards granted. The cost is expected to be recognized over a weighted-average service period of approximately 1.1 years.

Restricted Stock Units

Restricted stock units were issued to certain employees pursuant to the 2014 Stock Incentive Plan and non-employee directors pursuant to the 2014 Directors Stock Plan. Total restricted stock units granted were 0.8 million for the six months ended June 30, 2018. The fair value of restricted stock units is determined based on the closing stock price of our common shares on the grant date. The weighted-average grant date fair value of these restricted stock units granted was $63.74 per common share.

As of June 30, 2018, we had $66.9 million of total unrecognized compensation cost related to nonvested restricted stock unit awards granted. The cost is expected to be recognized over a weighted-average period of approximately 1.9 years.

Employee Stock Purchase Plan

Under the Employee Stock Purchase Plan, employees purchased 0.4 million shares for the six months ended June 30, 2018. The weighted average fair value of the discount on the stock purchased was $8.64 per share.

As of June 30, 2018, a total of 2.5 million of new shares were available to be made issuable by us for this plan.