Performance Share Awards Performance share awards are granted under the LTIP with the 2012 performance share awards being settled in 50% common stock and 50% cash over the three-year vesting term.
The 2011 performance share awards are being settled entirely in common stock over the three-year vesting term.
The performance shares granted prior to 2011 generally vest and settle over a three-year period with the holders receiving shares of common stock and/or cash annually during the vesting period.
These awards are recorded at fair value at the date of grant with the estimated grant date fair value based on the expected payout of the award, which may range from 75% to 125% of the payout target.
The common stock portion is considered an equity award with the 75% payout floor being valued based on Exelon’s stock price on the grant date.
The cash portion of the award is considered a liability award with the 75% payout floor being remeasured each reporting period based on Exelon’s current stock price.
The expected payout in excess of the 75% floor for the equity and liability portions are remeasured each reporting period based on Exelon’s current stock price and changes in the expected payout of the award; therefore these portions of the award are subject to volatility until the payout is established.
In 2010, the number of performance shares granted was determined based on the performance of Exelon’s common stock relative to certain stock market indices during the three-year period through the end of the year of grant.
These performance share awards generally vest and settle over a three-year period.
The holders of these performance share awards receive shares of common stock and/or cash annually during the vesting period.
Participants are eligible for partial or full distributions in cash if they meet certain stock ownership requirements.
The 2010 performance share awards that were settled in stock were recorded as common stock within the Consolidated Balance Sheets and recorded at fair value at the date of grant.
The grant date fair value of equity classified performance share awards granted during the year ended 2010 was estimated using historical data for the previous two plan years and a Monte Carlo simulation model for the current plan year.
This model requires assumptions regarding Exelon’s total shareholder return relative to certain stock market indices and the stock beta and volatility of Exelon’s common stock and all stocks represented in these indices.
Volatility for Exelon and all comparable companies is based on historical volatility over one year using daily stock price observation.
The 2010 performance share awards that were settled in cash were recorded as liabilities within the Consolidated Balance Sheets.
The grant date fair value of liability classified performance share awards granted during the year ended 2010 was based on historical data for the previous two plan years and actual results for the current plan year.
The liabilities were remeasured each reporting period throughout the requisite service period and as a result, the compensation costs for cash-settled awards were subject to volatility.
For non retirement-eligible employees, stock-based compensation costs are recognized over the vesting period of three years using the graded-vesting method, a method in which the compensation cost is recognized over the requisite service period for each separately vesting tranche of the award as though the award were multiple awards.
For performance shares granted to retirementeligible employees, the value of the performance shares is recognized ratably over the vesting period which is the year of grant.
The following table summarizes Exelon’s nonvested performance share awards activity for the year ended December 31, 2012:
## Table 0 ##
(a) Excludes 204,643 and 455,418 of performance share awards issued to retirement-eligible employees as of December 31, 2012 and December 31, 2011, respectively, as they are fully vested.
(b) Represents performance share awards that vested but were not distributed to retirement-eligible employees during 2012.
The weighted average grant date fair value (per share) of performance share awards granted during the years ended December 31, 2012, 2011 and 2010 was $39.71, $43.52 and $60.82 respectively.
During the years ended December 31, 2012, 2011 and 2010, Exelon settled performance shares with a fair value totaling $23 million, $22 million and $32 million, respectively, of which $3 million,
Other Purchase Obligations The Registrants’ other purchase obligations as of December 31, 2012, which primarily represent commitments for services, materials and information technology, are as follows:
## Table 1 ##
Commercial Commitments Exelon’s commercial commitments as of December 31, 2012, representing commitments potentially triggered by future events, were as follows:
## Table 2 ##
(a) Letters of credit (non-debt)—Exelon and certain of its subsidiaries maintain non-debt letters of credit to provide credit support for certain transactions as requested by third parties.
(b) Surety bonds—Guarantees issued related to contract and commercial agreements, excluding bid bonds.
(c) Performance guarantees—Guarantees issued to ensure performance under specific contracts, including $211 million issued on behalf of CENG nuclear generating facilities for credit support, $200 million of Trust Preferred Securities of ComEd Financing III, $178 million of Trust Preferred Securities of PECO Trust III and IV and $250 million of Trust Preferred Securities of BGE Capital Trust II.
(d) Energy marketing contract guarantees—Guarantees issued to ensure performance under energy commodity contracts.
Amount includes approximately $8.3 billion of guarantees previously issued by Constellation on behalf of its Generation and NewEnergy business to allow it the flexibility needed to conduct business with counterparties without having to post other forms of collateral.
The majority of these guarantees contain evergreen provisions that require the guarantee to remain in effect until cancelled.
Exelon’s estimated net exposure for obligations under commercial transactions covered by these guarantees is approximately $1.5 billion at December 31, 2012, which represents the total amount Exelon could be required to fund based on December 31, 2012 market prices.
(e) Lease guarantees—Guarantees issued to ensure payments on building leases.
(f) Middle market lending commitments—Represents commitments to investment in loans or managed funds which invest in private companies.
These commitments will be funded by Generation’s existing nuclear decommissioning trust funds.
See Note 9—Fair Value of Financial Assets and Liabilities for more information on nuclear decommissioning trust funds and middle market lending.
(g) Nuclear insurance premiums—Represents the maximum amount that Generation would be required to pay for retrospective premiums in the event of nuclear disaster at any domestic site under the Secondary Financial Protection pool as required under the Price-Anderson Act as well as the current aggregate annual retrospective premium obligation that could be imposed by NEIL.
See the Nuclear Insurance section within this note for additional details on Generation’s nuclear insurance premiums.
Construction Commitments Generation has committed to the construction of a solar PV facility in Los Angeles County, California.
The first portion of the project began operations in December 2012, with additional phases to come online and an expectation of full commercial operation by the end of the third quarter of 2013.
Generation’s estimated remaining commitment for the project is $636 million for 2013.
See Note 4— Merger and Acquisitions for additional information.
Refer to Note 3—Regulatory Matters for information on investment programs associated with regulatory mandates, such as ComEd’s Infrastructure Investment Plan under EIMA, PECO’s Smart Meter Procurement and Installation Plan, BGE’s comprehensive smart grid initiative and ComEd’s, PECO’s and BGE’s commitment to construct transmission facilities under their operating agreements with PJM.
The changes in operating and maintenance expense for 2011 compared to 2010, consisted of the following:
## Table 3 ##
(a) Reflects the impact of increased planned refueling outages during 2011.
(b) Includes $30 million in 2011 associated with labor, other benefits, contracting and materials at Exelon Wind.
(c) Reflects an increase in Generation’s decommissioning obligation for spent fuel at Zion station.
See Note 13 of the Combined Notes to Consolidated Financial Statements for further information.
(d) Reflects the impact of the return of property and business interruption insurance premiums in 2010.
No premiums were returned for 2011.
(e) Primarily reflects increased lobbying costs related to EPA and competitive market matters.
(f) Reflects increase in certain costs associated with the acquisitions of Constellation, Exelon Wind, Wolf Hollow and Antelope Valley incurred in 2011.
See Note 4 of the Combined Notes to Consolidated Financial Statements for further information.
(g) Includes additional environmental remediation costs recorded during 2011.
Depreciation and Amortization Year Ended December 31, 2012 Compared to Year Ended December 31, 2011.
The increase in depreciation and amortization expense was primarily a result of higher plant balances due to the addition of Constellation facilities; and capital additions and other upgrades to legacy plants.
Year Ended December 31, 2011 Compared to Year Ended December 31, 2010.
The increase in depreciation and amortization expense was primarily a result of higher plant balances due to the acquisition of Exelon Wind, capital additions and other upgrades to existing facilities.
Higher plant balances resulted in an increase in depreciation and amortization expense of $61 million.
The remaining increase in depreciation and amortization expense was due to the impact of increases in asset retirement costs (ARC) for Generation’s nuclear generating facilities.
Taxes Other Than Income Year Ended December 31, 2012 Compared to Year Ended December 31, 2011.
The increase was primarily due to the addition of Constellation’s financial results in 2012.
Year Ended December 31, 2011 Compared to Year Ended December 31, 2010.
The increase was primarily due to increased gross receipt taxes related to retail sales in the Mid-Atlantic region.
These gross receipt taxes are recovered in revenue, and as a result, have no impact to Generation’s results of operations.
Equity in Losses of Unconsolidated Affiliates Year Ended December 31, 2012 Compared to Year Ended December 31, 2011.
Equity in losses of unconsolidated affiliates in 2012 primarily reflected $172 million related to the amortization of the basis difference in CENG recorded at fair value at the merger date, partially offset by $73 million of net income generated from Exelon’s equity investment in CENG.
Interest Expense Year Ended December 31, 2012 Compared to Year Ended December 31, 2011.
The increase in interest expense is primarily due to the increase in long-term debt as a result of the merger.
Year Ended December 31, 2011 Compared to Year Ended December 31, 2010.
The increase in interest expense is primarily due to debt issuances in 2010, further described in Note 11 of the Combined Notes to Consolidated Financial Statements.
The increase in long-term debt resulted in higher interest expense of approximately $27 million.
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Asset Utilization – In response to economic conditions and lower revenue in 2009, we implemented productivity initiatives to improve efficiency and reduce costs, in addition to adjusting our resources to reflect lower demand.
Although varying throughout the year, our resource reductions included removing from service approximately 26% of our road locomotives and 18% of our freight car inventory by year end.
We also reduced shift levels at most rail facilities and closed or significantly reduced operations in 30 of our 114 principal rail yards.
These demand-driven resource adjustments and our productivity initiatives combined to reduce our workforce by 10%.  ?
Fuel Prices – As the economy worsened during the third and fourth quarters of 2008, fuel prices dropped dramatically, reaching $33.87 per barrel in December 2008, a near five-year low.
Throughout 2009, crude oil prices generally increased, ending the year around $80 per barrel.
Overall, our average fuel price decreased by 44% in 2009, reducing operating expenses by $1.3 billion compared to 2008.
We also reduced our consumption rate by 4% during the year, saving approximately 40 million gallons of fuel.
The use of newer, more fuel efficient locomotives; increased use of distributed locomotive power; fuel conservation programs; and improved network operations and asset utilization all contributed to this improvement.  ?
Free Cash Flow – Cash generated by operating activities totaled $3.2 billion, yielding free cash flow of $515 million in 2009.
Free cash flow is defined as cash provided by operating activities, less cash used in investing activities and dividends paid.
Free cash flow is not considered a financial measure under accounting principles generally accepted in the United States (GAAP) by SEC Regulation G and Item 10 of SEC Regulation S-K.  We believe free cash flow is important in evaluating our financial performance and measures our ability to generate cash without additional external financings.
Free cash flow should be considered in addition to, rather than as a substitute for, cash provided by operating activities.
The following table reconciles cash provided by operating activities (GAAP measure) to free cash flow (non-GAAP measure):

Tables referenced above, in order (Table 0 first):
<table><tr><td></td><td>Shares</td><td> Weighted Average Grant Date Fair Value (per share)</td></tr><tr><td>Nonvested at December 31, 2011<sup>(a)</sup></td><td>346,848</td><td>$45.37</td></tr><tr><td>Granted</td><td>1,429,189</td><td>39.72</td></tr><tr><td>Vested</td><td>-167,048</td><td>47.46</td></tr><tr><td>Forfeited</td><td>-116,388</td><td>39.78</td></tr><tr><td>Undistributed vested awards<sup>(b)</sup></td><td>-179,867</td><td>40.72</td></tr><tr><td>Nonvested at December 31, 2012<sup>(a)</sup></td><td>1,312,734</td><td>$40.08</td></tr></table>
<table><tr><td></td><td></td><td colspan="6"> Expiration within</td></tr><tr><td></td><td> Total</td><td> 2013</td><td> 2014</td><td> 2015</td><td> 2016</td><td> 2017</td><td> 2018 and beyond</td></tr><tr><td>Exelon</td><td>$716</td><td>$186</td><td>$167</td><td>$114</td><td>$51</td><td>$49</td><td>$149</td></tr><tr><td>Generation</td><td>487</td><td>127</td><td>120</td><td>94</td><td>32</td><td>29</td><td>85</td></tr><tr><td>ComEd</td><td>8</td><td>2</td><td>6</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>PECO</td><td>45</td><td>17</td><td>18</td><td>1</td><td>1</td><td>1</td><td>7</td></tr><tr><td>BGE</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr></table>
<table><tr><td></td><td></td><td colspan="6"> Expiration within</td></tr><tr><td></td><td> Total</td><td> 2013</td><td> 2014</td><td> 2015</td><td> 2016</td><td> 2017</td><td> 2018 and beyond</td></tr><tr><td>Letters of credit (non-debt)<sup>(a)</sup></td><td>$1,889</td><td>$1,325</td><td>$—</td><td>$564</td><td>$—</td><td>$—</td><td>$—</td></tr><tr><td>Surety bonds<sup>(b)</sup></td><td>286</td><td>225</td><td>—</td><td>1</td><td>6</td><td>4</td><td>50</td></tr><tr><td>Performance guarantees<sup>(c)</sup></td><td>1,897</td><td>908</td><td>203</td><td>—</td><td>—</td><td>—</td><td>786</td></tr><tr><td>Energy marketing contract guarantees<sup>(d)</sup></td><td>8,556</td><td>8,556</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Lease guarantees<sup>(e)</sup></td><td>48</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>48</td></tr><tr><td>Middle market lending commitments<sup>(f)</sup></td><td>180</td><td>180</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Nuclear insurance premiums<sup>(g)</sup></td><td>2,494</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>2,494</td></tr><tr><td>Total commercial commitments</td><td>$15,350</td><td>$11,194</td><td>$203</td><td>$565</td><td>$6</td><td>$4</td><td>$3,378</td></tr></table>
<table><tr><td></td><td> Increase (Decrease)</td></tr><tr><td>Labor, other benefits, contracting and materials</td><td>$113</td></tr><tr><td>Nuclear refueling outage costs, including the co-owned Salem Plant<sup>(a)</sup></td><td>74</td></tr><tr><td>Exelon Wind<sup>(b)</sup></td><td>39</td></tr><tr><td>Asset retirement obligation reduction<sup>(c)</sup></td><td>28</td></tr><tr><td>2010 nuclear insurance credit<sup>(d)</sup></td><td>20</td></tr><tr><td>Corporate allocations<sup>(e)</sup></td><td>19</td></tr><tr><td>Acquisition costs<sup>(f)</sup></td><td>14</td></tr><tr><td>Other<sup>(g)</sup></td><td>29</td></tr><tr><td>Increase in operating and maintenance expense</td><td>$336</td></tr></table>

Question: What's the difference of Surety bonds between 2016 and 2015?
Answer: 5
Reasoning program: subtract(6,1)