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Employee Benefit Plans and Stockholders' Equity (Tables)
 12 Months Ended
Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Stock Option Activity
A summary of our stock option activity and related information for the twelve months ended December 31, 2018 is as follows:
 
Number of
Shares
(in millions)
 
Weighted-
Average
Exercise
Price
per Share
 
Weighted-
Average
Remaining
Contractual
Term
(in years)
 
Aggregate 
Intrinsic
Value
(in millions)
Outstanding at December 31, 2017
0.4

 
$
6.71

 
 
 
 
Exercised
(0.3
)
 
6.34

 
 
 
 
Forfeited
—

 
—

 
 
 
 
Outstanding at December 31, 2018
0.1

 
8.02

 
0.95
 
$
9.6

Exercisable at December 31, 2018
0.1

 
$
8.02

 
0.95
 
$
9.6
Schedule of Intrinsic Value of Options Exercised and Fair Value of Options Vested
The total intrinsic value of stock options exercised as of the date of exercise was as follows:
 
Years Ended December 31,    
(In millions)
2018
 
2017
 
2016
Intrinsic value of options exercised
$
30.0

 
$
21.6

 
$
39.9
Schedule of Share-Based Compensation Expenses
The following table summarizes share-based compensation expense related to restricted stock units, stock options, and employee stock purchases under the ESPP for the twelve months ended December 31, 2018, 2017 and 2016:
 
Years Ended December 31,    
(In millions)
2018
 
2017
 
2016
Cost of sales
$
9.2

 
$
9.6

 
$
12.0

Research and development
33.0

 
37.5

 
39.8

Selling, general and administrative
59.7

 
59.1

 
59.0

Total share-based compensation expense included in net loss
$
101.9

 
$
106.2

 
$
110.8
Schedule of Valuation Assumptions for Employee Stock Purchase Plan
We estimate the fair value of stock options granted and ESPP purchase rights on the date of grant using the Black-Scholes option pricing model and the assumptions below. We did not have any stock option grants during the twelve months ended December 31, 2018, 2017 and 2016.
 
Years Ended December 31,
 ESPP
2018
 
2017
 
2016
Risk free interest rate
1.55 – 2.25

 
0.75 – 1.12

 
0.46 – 0.57

Dividend yield
—
%
 
—
%
 
—
%
Expected volatility of DexCom common stock
0.50 – 0.67

 
0.33 – 0.56

 
0.33 – 0.57

Expected life (in years)
1

 
1

 
1
Schedule of Restricted Stock Units Activity
A summary of our RSU activity for the twelve months ended December 31, 2018, 2017 and 2016 is as follows:
(In millions except weighted average grant date fair value)
Shares
 
Weighted 
Average
Grant Date
Fair Value
 
Aggregate
Intrinsic Value
Nonvested at December 31, 2015
4.1

 
$
50.60

 
 
Granted
1.9

 
68.16

 
 
Vested
(2.1
)
 
44.95

 
 
Forfeited
(0.2
)
 
56.37

 
 
Nonvested at December 31, 2016
3.7

 
62.51

 
$
218.6

Granted
1.3

 
75.78

 
 
Vested
(1.9
)
 
58.92

 
 
Forfeited
(0.4
)
 
67.97

 
 
Nonvested at December 31, 2017
2.7

 
70.68

 
154.5

Granted
1.7

 
66.07

 
 
Vested
(1.4
)
 
68.44

 
 
Forfeited
(0.3
)
 
68.56

 
 
Nonvested at December 31, 2018
2.7

 
$
69.19

 
$
319.0
Schedule of Stock Options Reserved for Future Issuance
Reserved Shares
Shares of common stock reserved for future issuance were as follows as of the dated indicated:
 
December 31,
(In millions)
2018
 
2017
Stock options and awards under our plans:
 
 
 
Stock options granted and outstanding
0.1

 
0.4

Unvested restricted stock units
2.7

 
2.7

Reserved for future grant
3.2

 
4.7

Employee Stock Purchase Plan
1.1

 
1.3

Total
7.1

 
9.1