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Share-Based Compensation Share-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2017
Share-based Compensation [Abstract]  
Summary of Components of Share-Based Compensation Expense
The components of share-based compensation expense for all share-based compensation plans and related tax benefits are as follows:
(in millions)
 
 
 
 
 
Years Ended December 31
2017
 
2016
 
2015
Total share-based compensation expense
$
39

 
$
34

 
$
38

Related tax benefits recognized in net income
$
14

 
$
13

 
$
14
Schedule of Unrecognized Compensation Expense
The following table summarizes unrecognized compensation expense for all share-based plans.
(dollar amounts in millions)
December 31, 2017
Total unrecognized share-based compensation expense
$
40

Weighted-average expected recognition period (in years)
2.8
Estimated Weighted-Average Grant-Date Fair Value per Option and the Underlying Model Assumptions
The estimated weighted-average grant-date fair value per option and the underlying binomial option-pricing model assumptions are summarized in the following table:
Years Ended December 31
2017
 
2016
 
2015
Weighted-average grant-date fair value per option
$
19.61

 
$
9.94

 
$
11.31

Weighted-average assumptions:
 
 
 
 
 
 Risk-free interest rates
2.47
%
 
2.01
%
 
1.83
%
 Expected dividend yield
3.00

 
3.00

 
3.00

Expected volatility factors of the market price of
   Comerica common stock
34

 
38

 
33

Expected option life (in years)
7.0

 
6.9

 
6.9
Summary of Stock Option Activity and Related Information
A summary of the Corporation’s stock option activity and related information for the year ended December 31, 2017 follows:
 
 
 
Weighted-Average
 
 
  
Number of
Options
(in thousands)
 
Exercise Price
per Share
 
Remaining
Contractual
Term (in years)
 
Aggregate
Intrinsic Value
(in millions)
Outstanding-January 1, 2017
6,892

 
$
37.24

 
 
 
 
Granted
430

 
67.67

 
 
 
 
Forfeited or expired
(57
)
 
48.74

 
 
 
 
Exercised
(3,092
)
 
37.45

 
 
 
 
Outstanding-December 31, 2017
4,173

 
40.06

 
5.9

 
$
195

Exercisable-December 31, 2017
2,347

 
$
36.27

 
4.3

 
$
119
Summary of Restricted Stock Activity and Related Information
A summary of the Corporation’s restricted stock activity and related information for the year ended December 31, 2017 follows:
 
Number of
Shares
(in thousands)
 
Weighted-Average
Grant-Date Fair 
Value per Share
Outstanding-January 1, 2017
1,591

 
$
37.20

Granted
237

 
67.83

Forfeited
(59
)
 
42.53

Vested
(526
)
 
35.28

Outstanding-December 31, 2017
1,243

 
$
43.59
Summary of Restricted Stock Unit Activity and Related Information
A summary of the Corporation's restricted stock unit activity and related information for the year ended December 31, 2017 follows:
 
Service-Based Units
 
Performance-Based Units
 
Number of
Units
(in thousands)
 
Weighted-Average
Grant-Date Fair 
Value per Share
 
Number of
Units
(in thousands)
 
Weighted-Average
Grant-Date Fair 
Value per Share
Outstanding-January 1, 2017
180

 
$
39.61

 
781

 
$
39.47

Granted
19

 
75.06

 
149

 
66.07

Vested
—

 
—

 
(212
)
 
48.32

Outstanding-December 31, 2017
199

 
43.00

 
718

 
42.39