Share-based compensation plans (Tables)
 9 Months Ended
Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan
Compensation expenses for the share-based awards were as follows:
 
Compensation Expense
 
Compensation Expense
 
Three Months Ended September 30,
 
Nine Months Ended September 30,
(in millions)
2018
 
2017
 
2018
 
2017
 
 
 
 
 
 
 
 
Performance Share Units
$
3.1

 
1.9

 
$
9.7

 
6.4

Market Share Units
—

 
0.1

 
0.1

 
0.2

Restricted Stock Units
1.6

 
1.1

 
4.9

 
3.5

Deferred Stock Units and fees paid in stock
0.4

 
0.3

 
0.9

 
0.8

Stock Options
1.2

 
0.6

 
3.2

 
1.6

Share-based payment expense
6.3

 
4.0

 
18.8

 
12.5

Income tax benefit
(1.5
)
 
(1.5
)
 
(4.4
)
 
(4.6
)
Share-based payment expense, net of tax
$
4.8

 
2.5

 
$
14.4

 
7.9
Option activity
The following table summarizes time-based stock option activity during the first nine months of 2018: 
 
Shares
(in thousands)
 
Weighted-Average Grant-Date Fair Value
Outstanding balance as of December 31, 2017
40.6

 
$
8.66

Granted
—

 
—

Forfeited
—

 
—

Exercised
(37.9
)
 
7.77

Outstanding balance as of September 30, 2018
2.7

 
$
21.09

The following table summarizes performance-based stock option activity during the first nine months of 2018: 
 
Shares
(in thousands)
 
Weighted-Average Grant-Date Fair Value
Outstanding balance as of December 31, 2017
879.8

 
$
8.04

Granted
417.6

 
16.73

Forfeited
—

 
—

Exercised
—

 
—

Outstanding balance as of September 30, 2018
1,297.4

 
$
10.83
Nonvested share activity
The following table summarizes all MSU activity during the first nine months of 2018: 
 
Shares
(in thousands)
 
Weighted-Average Grant-Date Fair Value
Nonvested balance as of December 31, 2017
74.2

 
$
30.37

Granted
—

 
—

Forfeited
—

 
—

Vested(a)
(74.2
)
 
30.37

Nonvested balance as of September 30, 2018
—

 
$
—


(a)
The vested MSUs presented are based on the target amount of the award. In accordance with the terms of the underlying award agreements, the actual shares earned and distributed for the performance period ended December 31, 2017 were 111.3. No additional compensation expense was required to be recognized for the additional shares distributed, as the market condition was included in the $30.37 grant date fair value.
The following table summarizes RSU activity during the first nine months of 2018: 
 
Shares
(in thousands)
 
Weighted-Average Grant-Date Fair Value
Nonvested balance as of December 31, 2017
265.8

 
$
39.80

Granted
83.5

 
72.51

Forfeited
(3.8
)
 
56.52

Vested
(95.8
)
 
35.71

Nonvested balance as of September 30, 2018
249.7

 
$
52.06

The following table summarizes all DSU activity during the first nine months of 2018:
 
Shares
(in thousands)
 
Weighted-Average Grant-Date Fair Value
Nonvested balance as of December 31, 2017
10.9

 
$
60.80

Granted
12.5

 
74.43

Forfeited
—

 
—

Vested
(10.9
)
 
60.80

Nonvested balance as of September 30, 2018
12.5

 
$
74.43

The following table summarizes all PSU activity during the first nine months of 2018:
 
Shares
(in thousands)
 
Weighted-Average Grant-Date Fair Value
Nonvested balance as of December 31, 2017
671.2

 
$
37.26

Granted
174.4

 
73.61

Forfeited
(5.8
)
 
50.63

Vested(a)
(137.7
)
 
29.17

Nonvested balance as of September 30, 2018
702.1

 
$
47.75


(a)
The vested Prior PSUs presented are based on the target amount of the award. In accordance with the terms of the underlying award agreements, the actual shares earned and distributed for the performance period ended December 31, 2017 were 344.3.