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 4,510,000 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’s 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’s and Standard & Poor’s Financial Services’ then applicable credit rating on AWCC’s senior unsecured, non-credit enhanced debt.
The facility is used principally to support AWCC’s commercial paper program and to provide up to $150 million in letters of credit.
Indebtedness under the facility is considered “debt” 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’s 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:
## Table 0 ##
The weighted average interest rate on AWCC short-term borrowings for the years ended December 31, 2018 and 2017 was approximately 2.28% and 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:
The effective income tax rate from continuing operations for the years ended December 31 varies from the U. S.  statutory federal income tax rate as follows:
## Table 1 ##
The Company’s effective tax rate for each of 2017, 2016 and 2015 differs from the U. S.  federal statutory rate of 35.0% due principally to the Company’s earnings outside the United States that are indefinitely reinvested and taxed at rates lower than the U. S.  federal statutory rate.
In addition: ?
The effective tax rate of 16.0% in 2017 includes 500 basis points of net tax benefits related to the revaluation of net U. S.  deferred tax liabilities from 35.0% to 21.0% due to the TCJA and release of reserves upon statute of limitation expiration, partially offset by income tax expense related to the Transition Tax on foreign earnings due to the TCJA and changes in estimates associated with prior period uncertain tax positions.  ?
The effective tax rate of 17.5% in 2016 includes 350 basis points of net tax benefits from permanent foreign exchange losses and the release of reserves upon the expiration of statutes of limitation and audit settlements, partially offset by income tax expense related to repatriation of earnings and legal entity realignments associated with the Separation and changes in estimates associated with prior period uncertain tax positions.  ?
The effective tax rate of 14.4% in 2015 includes 290 basis points of net tax benefits from permanent foreign exchange losses, releases of valuation allowances related to foreign operating losses and the release of reserves upon the expiration of statutes of limitation, partially offset by changes in estimates associated with prior period uncertain tax positions.
The Company made income tax payments related to both continuing and discontinued operations of $689 million, $767 million and $584 million in 2017, 2016 and 2015, respectively.
Current income taxes payable related to both continuing and discontinued operations has been reduced by $85 million, $99 million, and $147 million in 2017, 2016 and 2015, respectively, for tax deductions attributable to stock-based compensation, of which, the excess tax benefit over the amount recorded for financial reporting purposes for both continuing and discontinued operations was $55 million, $50 million and $88 million, respectively.
The excess tax benefits realized have been recorded as increases to additional paid-in capital for the years ended December 31, 2016 and 2015 and are reflected as a financing cash inflow in the accompanying Consolidated Statements of Cash Flows.
As a result of the adoption of ASU 2016-09, Compensation—Stock Compensation, the excess tax benefit for the year ended December 31, 2017 has been recorded as a reduction to the current income tax provision and is reflected as an operating cash inflow in the accompanying Consolidated Statement of Cash Flows.
Included in deferred income taxes related to continuing operations as of December 31, 2017 are tax benefits for U. S.  and nonU.
S.  net operating loss carryforwards totaling $502 million ($283 million of which the Company does not expect to realize and have corresponding valuation allowances).
Certain of the losses can be carried forward indefinitely and others can be carried forward to various dates from 2018 through 2037.
In addition, the Company had general business and foreign tax credit carryforwards related to continuing operations of $171 million ($30 million of which the Company does not expect to realize and have corresponding valuation allowances) as of December 31, 2017, which can be carried forward to various dates from 2018 to 2027.
In addition, as of December 31, 2017, the Company had $12 million of valuation allowances related to other deferred tax asset balances that are not more likely than not of being realized.
As of December 31, 2017, gross unrecognized tax benefits related to continuing operations totaled $737 million ($736 million, net of the impact of $104 million of indirect tax benefits offset by $103 million associated with potential interest and penalties).
Incurred Losses and LAE.
The following table presents the incurred losses and LAE for the U. S.  Reinsurance segment for the periods indicated.
## Table 2 ##
Incurred losses decreased by 13.6% to $825.1 million in 2015 compared to $954.5 million in 2014, primarily due to an increase in favorable development of $147.6 million on prior year attritional losses in 2015 compared to 2014 related to treaty property, treaty casualty, marine lines of business and less year over year development on A&E reserves.
This favorable development was partially offset by the increase in current year attritional losses of $7.3 million resulting primarily from $14.2 million related to the explosion at the Chinese port of Tianjin.
Current year catastrophe losses were $16.7 million in 2015 mainly due to the US storms ($16.2 million).
The $12.5 million of current year catastrophe losses in 2014 related to the Japan snowstorm ($7.8 million) and Hurricane Odile ($4.7 million).
Incurred losses increased by 17.2% to $954.5 million in 2014 compared to $814.7 million in 2013, primarily due to the increase in current year attritional losses of $151.5 million resulting primarily from the impact of the increase in premiums earned and less favorable development of $61.2 million on prior years’ attritional losses in 2014 compared to 2013, mainly related to an increase in A&E reserves.
This increase was partially offset by a decrease in current year catastrophe losses (outlined above) and favorable development of $33.5 million on prior year catastrophe losses in 2014 compared to 2013, mainly related to Superstorm Sandy.
The $51.8 million of current year catastrophe losses in 2013 were mainly due to U. S.  Storms ($44.8 million), the European floods ($5.0 million) and the Canadian Floods ($2.0 million).
Segment Expenses.
Commission and brokerage expenses increased by 5.8% to $493.3 million in 2015 compared to $466.3 million in 2014.
The variance was primarily due to the impact of changes in the mix of business.
Segment other underwriting expenses increased to $50.1 million in 2015 from $45.6 million in 2014.
The increase was primarily due to the impact of changes in the mix of business and higher employee benefit costs.

Tables referenced above, in order (Table 0 first):
<table><tr><td></td><td>2018</td><td>2017</td><td>2016</td></tr><tr><td>Total common shareholders' equity</td><td>40.4%</td><td>41.0%</td><td>42.1%</td></tr><tr><td>Long-term debt and redeemable preferred stock at redemption value</td><td>52.4%</td><td>49.6%</td><td>46.4%</td></tr><tr><td>Short-term debt and current portion of long-term debt</td><td>7.2%</td><td>9.4%</td><td>11.5%</td></tr><tr><td>Total</td><td>100%</td><td>100%</td><td>100%</td></tr></table>
<table><tr><td></td><td colspan="3">Percentage of Pretax Earnings</td></tr><tr><td></td><td>2017</td><td>2016</td><td>2015</td></tr><tr><td>Statutory federal income tax rate</td><td>35.0%</td><td>35.0%</td><td>35.0%</td></tr><tr><td>Increase (decrease) in tax rate resulting from:</td><td></td><td></td><td></td></tr><tr><td>State income taxes (net of federal income tax benefit)</td><td>0.8%</td><td>0.6%</td><td>0.7%</td></tr><tr><td>Foreign income taxed at lower rate than U.S. statutory rate</td><td>-11.6%</td><td>-10.2%</td><td>-17.1%</td></tr><tr><td>Resolution and expiration of statutes of limitation of uncertain tax positions</td><td>-6.5%</td><td>-3.1%</td><td>-0.7%</td></tr><tr><td>Permanent foreign exchange losses</td><td>-0.6%</td><td>-8.2%</td><td>-4.6%</td></tr><tr><td>Research credits, uncertain tax positions and other</td><td>-1.0%</td><td>3.4%</td><td>1.1%</td></tr><tr><td>Revaluation of U.S. deferred income taxes</td><td>-41.5%</td><td>—%</td><td>—%</td></tr><tr><td>TCJA - Transition Tax</td><td>41.4%</td><td>—%</td><td>—%</td></tr><tr><td>Effective income tax rate</td><td>16.0%</td><td>17.5%</td><td>14.4%</td></tr></table>
<table><tr><td></td><td colspan="8">Years Ended December 31,</td><td></td></tr><tr><td>(Dollars in millions)</td><td>Current Year</td><td colspan="2">Ratio %/ Pt Change</td><td>Prior Years</td><td colspan="2">Ratio %/ Pt Change</td><td>Total Incurred</td><td colspan="2">Ratio %/ Pt Change</td></tr><tr><td>2015</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Attritional</td><td>$940.6</td><td>48.2%</td><td></td><td>$-123.1</td><td>-6.3%</td><td></td><td>$817.5</td><td>41.9%</td><td></td></tr><tr><td>Catastrophes</td><td>16.7</td><td>0.9%</td><td></td><td>-9.2</td><td>-0.5%</td><td></td><td>7.6</td><td>0.4%</td><td></td></tr><tr><td>Total segment</td><td>$957.4</td><td>49.1%</td><td></td><td>$-132.3</td><td>-6.8%</td><td></td><td>$825.1</td><td>42.3%</td><td></td></tr><tr><td>2014</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Attritional</td><td>$933.3</td><td>47.0%</td><td></td><td>$24.5</td><td>1.2%</td><td></td><td>$957.8</td><td>48.2%</td><td></td></tr><tr><td>Catastrophes</td><td>12.5</td><td>0.6%</td><td></td><td>-15.8</td><td>-0.8%</td><td></td><td>-3.3</td><td>-0.2%</td><td></td></tr><tr><td>Total segment</td><td>$945.8</td><td>47.6%</td><td></td><td>$8.7</td><td>0.4%</td><td></td><td>$954.5</td><td>48.0%</td><td></td></tr><tr><td>2013</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Attritional</td><td>$781.8</td><td>46.7%</td><td></td><td>$-36.7</td><td>-2.2%</td><td></td><td>$745.2</td><td>44.5%</td><td></td></tr><tr><td>Catastrophes</td><td>51.8</td><td>3.1%</td><td></td><td>17.7</td><td>1.1%</td><td></td><td>69.5</td><td>4.2%</td><td></td></tr><tr><td>Total segment</td><td>$833.6</td><td>49.8%</td><td></td><td>$-18.9</td><td>-1.1%</td><td></td><td>$814.7</td><td>48.7%</td><td></td></tr><tr><td>Variance 2015/2014</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Attritional</td><td>$7.3</td><td>1.2</td><td>pts</td><td>$-147.6</td><td>-7.5</td><td>pts</td><td>$-140.3</td><td>-6.3</td><td>pts</td></tr><tr><td>Catastrophes</td><td>4.2</td><td>0.3</td><td>pts</td><td>6.6</td><td>0.3</td><td>pts</td><td>10.9</td><td>0.6</td><td>pts</td></tr><tr><td>Total segment</td><td>$11.6</td><td>1.5</td><td>pts</td><td>$-141.0</td><td>-7.2</td><td>pts</td><td>$-129.4</td><td>-5.7</td><td>pts</td></tr><tr><td>Variance 2014/2013</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Attritional</td><td>$151.5</td><td>0.3</td><td>pts</td><td>$61.2</td><td>3.4</td><td>pts</td><td>$212.6</td><td>3.7</td><td>pts</td></tr><tr><td>Catastrophes</td><td>-39.3</td><td>-2.5</td><td>pts</td><td>-33.5</td><td>-1.9</td><td>pts</td><td>-72.8</td><td>-4.4</td><td>pts</td></tr><tr><td>Total segment</td><td>$112.2</td><td>-2.2</td><td>pts</td><td>$27.7</td><td>1.5</td><td>pts</td><td>$139.9</td><td>-0.7</td><td>pts</td></tr><tr><td colspan="2">(Some amounts may not reconcile due to rounding.)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></table>

Question: Which year is Catastrophes for Prior Years the lowest?
Answer: 2014