allows us to repurchase shares at times when we may otherwise be prevented from doing so under insider trading laws or because of self-imposed trading blackout periods .
subject to applicable regulations , we may elect to amend or cancel this repurchase program or the share repurchase parameters at our discretion .
as of december 31 , 2018 , we have repurchased an aggregate of 4510000 shares of common stock under this program .
credit facilities and short-term debt we have an unsecured revolving credit facility of $ 2.25 billion that expires in june 2023 .
in march 2018 , awcc and its lenders amended and restated the credit agreement with respect to awcc 2019s revolving credit facility to increase the maximum commitments under the facility from $ 1.75 billion to $ 2.25 billion , and to extend the expiration date of the facility from june 2020 to march 2023 .
all other terms , conditions and covenants with respect to the existing facility remained unchanged .
subject to satisfying certain conditions , the credit agreement also permits awcc to increase the maximum commitment under the facility by up to an aggregate of $ 500 million , and to request extensions of its expiration date for up to two , one-year periods .
interest rates on advances under the facility are based on a credit spread to the libor rate or base rate in accordance with moody investors service 2019s and standard & poor 2019s financial services 2019 then applicable credit rating on awcc 2019s senior unsecured , non-credit enhanced debt .
the facility is used principally to support awcc 2019s commercial paper program and to provide up to $ 150 million in letters of credit .
indebtedness under the facility is considered 201cdebt 201d for purposes of a support agreement between the company and awcc , which serves as a functional equivalent of a guarantee by the company of awcc 2019s payment obligations under the credit facility .
awcc also has an outstanding commercial paper program that is backed by the revolving credit facility , the maximum aggregate outstanding amount of which was increased in march 2018 , from $ 1.60 billion to $ 2.10 billion .
the following table provides the aggregate credit facility commitments , letter of credit sub-limit under the revolving credit facility and commercial paper limit , as well as the available capacity for each as of december 31 , 2018 and 2017 : credit facility commitment available credit facility capacity letter of credit sublimit available letter of credit capacity commercial paper limit available commercial capacity ( in millions ) december 31 , 2018 .
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.
.
.
.
.
.
$ 2262 $ 2177 $ 150 $ 69 $ 2100 $ 1146 december 31 , 2017 .
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.
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1762 1673 150 66 1600 695 the weighted average interest rate on awcc short-term borrowings for the years ended december 31 , 2018 and 2017 was approximately 2.28% ( 2.28 % ) and 1.24% ( 1.24 % ) , respectively .
capital structure the following table provides the percentage of our capitalization represented by the components of our capital structure as of december 31: .

2018
2017
2016
Total common shareholders' equity
40.4%
41.0%
42.1%
Long-term debt and redeemable preferred stock at redemption value
52.4%
49.6%
46.4%
Short-term debt and current portion of long-term debt
7.2%
9.4%
11.5%
Total
100%
100%
100%

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Question: Analyse this data from a financial earnings document. for the awcc commercial paper program that is backed by the revolving credit facility , what was the change in billions of the maximum aggregate outstanding amount from march 2018 , to december 2018?
Choices:
0.8
18.4
3.7
0.5
64.4
Correct choice index: 3