BIOGEN INC.  AND SUBSIDIARIES NOTES TO CONSOLIDATED FINANCIAL STATEMENTS — (Continued) F- 27 As of December 31, 2015, our inventory included $24.7 million associated with our ZINBRYTA program, $24.2 million associated with our FLIXABI program and $18.4 million associated with our BENEPALI program, which had been capitalized in advance of regulatory approval.
The European Commission (EC) approved the marketing authorization applications for BENEPALI and FLIXABI, two anti-tumor necrosis factor (TNF) biosimilars, for marketing in the E. U.  in January 2016 and May 2016, respectively.
In addition, ZINBRYTA was approved for the treatment of relapsing forms of MS in the U. S.  in May 2016 and in the E. U.  in July 2016.
For information on our pre-approval inventory policy, please read Note 1, Summary of Significant Accounting Policies to these consolidated financial statements Inventory amounts written down as a result of excess, obsolescence, unmarketability or other reasons are charged to cost of sales, and totaled $48.2 million, $41.9 million and $50.6 million for the years ended December 31, 2016, 2015 and 2014, respectively.6.
Intangible Assets and Goodwill Intangible Assets Intangible assets, net of accumulated amortization, impairment charges and adjustments, are summarized as follows:
## Table 0 ##
Amortization of acquired intangible assets totaled $385.6 million, $382.6 million and $489.8 million for the years ended December 31, 2016, 2015 and 2014, respectively.
Amortization of acquired intangible assets during 2016 included impairment charges of $12.2 million related to two of our IPR&D intangible assets.
Amortization of acquired intangible assets during 2014 included impairment charges of $34.7 million related to one of our outlicensed patents and $16.2 million related to one of our IPR&D intangible assets.
Out-licensed Patents Out-licensed patents to third-parties primarily relate to patents acquired in connection with the merger of Biogen, Inc.  and IDEC Pharmaceuticals Corporation in 2003.
During 2014 we recorded a charge of $34.7 million related to the impairment of one of our out-licensed patents to reflect a change in its estimated fair value due to a change in the underlying competitive market for that product.
The charge was included in amortization of acquired intangible assets in our consolidated statements of income.
The fair value of the intangible asset was based on a discounted cash flow calculated using Level 3 fair value measurements and inputs including estimated revenues.
There were no impairment charges related to our out-licensed patents during 2016 or 2015.
Developed Technology Developed technology primarily relates to our AVONEX product, which was recorded in connection with the merger of Biogen, Inc.  and IDEC Pharmaceuticals Corporation in 2003.
The net book value of this asset as of December 31, 2016, was $363.3 million.
At May 31, 2015, our interest rate swap agreement effectively converted $500 million of our variable-rate debt to a fixed rate of 1.52% plus a leverage-based margin.
The fair value of our interest rate swap as of May 31, 2015 was a liability of $6.2 million, which is reflected in accounts payable and accrued liabilities in the consolidated balance sheet.
Net amounts to be received or paid under the swap agreement are reflected as adjustments to interest expense.
Since we have designated the interest rate swap agreement as a cash flow hedge, unrealized gains or losses resulting from adjusting the swap to fair value are recorded as components of other comprehensive income except for any ineffective portion of the change in fair value, which would be immediately recorded in interest expense.
During the year ended May 31, 2015, there was no ineffectiveness.
The fair value of the swap agreement was determined based on the present value of the estimated future net cash flows using implied rates in the applicable yield curve as of the valuation date.
This derivative instrument was classified within Level 2 of the valuation hierarchy.
During the year ended May 31, 2015, we recognized $4.0 million in interest expense related to settlements on the interest rate swap.
At May 31, 2015, the amount in accumulated other comprehensive income related to our interest rate swap that is expected to be reclassified into interest expense during the next 12 months was not material.
Interest Expense Interest expense was $39.9 million, $37.5 million and $31.7 million for the years ended May 31, 2015, 2014 and 2013, respectively.
Interest expense is comprised primarily of interest on our long-term debt and short-term lines of credit.
Interest expense also includes settlements on our interest rate swap, amortization of deferred debt issuance costs, commitment fees on the unused portions of our Corporate Credit Facility and short-term lines of credit and interest related to unrecognized income tax benefits.
Interest expense of $3.6 million that had been accrued related to an unrecognized tax benefit was eliminated as of May 31, 2015 and reflected as a reduction to Interest and other expense in the year ended May 31, 2015 in our consolidated statement of income.
See “Note 8 — Income Tax” for further discussion.
Interest expense during the year ended May 31, 2014 includes a loss on extinguishment of debt of $2.1 million in connection with the refinancing of our term loan and revolving credit facilities.
Interest expense for the years ended May 31, 2014 and 2013 includes dividend payments to HSBC Asia in the amounts of $3.3 million and $8.4 million, respectively, related to a redeemable noncontrolling interest that HSBC Asia held in Global Payments Asia-Pacific.
See “Note 3 — Business and Intangible Asset Acquisitions and Joint Ventures-Redeemable Noncontrolling Interest Acquisition” for further discussion.
NOTE 7—ACCOUNTS PAYABLE AND ACCRUED LIABILITIES As of May 31, 2015 and 2014, accounts payable and accrued liabilities consisted of the following:
## Table 1 ##
reach in the United States, adding a 1,400-person direct sales force, over 300,000 merchants and $130 billion in annual payments volume.
Goodwill of $3.2 billion arising from the merger, included in the North America segment, was attributable to expected growth opportunities, potential synergies from combining our existing businesses and an assembled workforce, and is not deductible for income tax purposes.
Due to the timing of our merger with Heartland, we are still in the process of assigning goodwill to our reporting units.
During the year ended May 31, 2016, we incurred transaction costs in connection with the merger of $24.4 million, which are recorded in selling, general and administrative expenses in the consolidated statements of income.
The following reflects the preliminary estimated fair values of the identified intangible assets (in thousands):
## Table 2 ##
The preliminary estimated fair value of customer-related intangible assets was determined using the income approach, which is based on projected cash flows discounted to their present value using discount rates that consider the timing and risk of the forecasted cash flows.
The discount rate used is the average estimated value of a market participant’s cost of capital and debt, derived using customary market metrics.
Other significant assumptions include terminal value margin rates, future capital expenditures and future working capital requirements.
Acquired technology was valued using the replacement cost method, which required us to estimate the cost to construct an asset of equivalent utility at prices available at the time of the valuation analysis, with adjustments in value for physical deterioration and functional and economic obsolescence.
Trademarks and trade names were valued using the relief-from-royalty approach.
This method assumes that trade marks and trade names have value to the extent that their owner is relieved of the obligation to pay royalties for the benefits received from them.
This method required us to estimate the future revenue for the related brands, the appropriate royalty rate and the weighted-average cost of capital.
The discount rate used is the average estimated value of a market participant’s cost of capital and debt, derived using customary market metrics.
The weighted-average estimated amortization period for the total estimated acquired intangible assets is approximately 11 years.
The customer-related intangible assets have an estimated amortization period range of 7-20 years.
The acquired technology has an estimated amortization period of 5 years.
The trademarks and trade names have an estimated amortization period of 7 years.
Covenants-not-to-compete have an estimated amortization period range of 1-4 years.
Heartland’s revenues and operating income represented approximately 4% and less than 0.5% of our total consolidated revenues and operating income, respectively, for the year ended May 31, 2016.
The following unaudited pro forma information shows the results of our operations for the years ended May 31, 2016 and May 31, 2015 as if our merger with Heartland had occurred on June 1, 2014.
The unaudited pro forma information reflects the effects of applying our accounting policies and certain pro forma adjustments to the combined historical financial information of Global Payments and Heartland.
The pro forma adjustments include incremental amortization and depreciation expense, incremental interest expense associated with new long-term debt, a reduction of revenues and operating expenses associated with fair value adjustments made in applying the acquisition-method of accounting and the elimination of nonrecurring transaction costs directly related to the merger.
The following table sets forth our private fixed maturity portfolios by NAIC rating attributable to the Closed Block Business as of the dates indicated.
## Table 3 ##
(1) Includes, as of December 30, 2007 and December 31, 2006, respectively, 106 securities with amortized cost of $1,578 million (fair value, $1,582 million) and 119 securities with amortized cost of $1,386 million (fair value, $1,421 million) that have been categorized based on expected NAIC designations pending receipt of SVO ratings.
Credit Derivative Exposure to Public Fixed Maturities In addition to the credit exposure from public fixed maturities noted above, we sell credit derivatives to enhance the return on our investment portfolio by creating credit exposure similar to an investment in public fixed maturity cash instruments.
In a credit derivative we sell credit protection on an identified name, or a basket of names in a first to default structure, and in return receive a quarterly premium.
With single name credit default derivatives, this premium or credit spread generally corresponds to the difference between the yield on the referenced name’s public fixed maturity cash instruments and swap rates, at the time the agreement is executed.
With first-to-default baskets, because of the additional credit risk inherent in a basket of named credits, the premium generally corresponds to a high proportion of the sum of the credit spreads of the names in the basket.
If there is an event of default by the referenced name or one of the referenced names in a basket, as defined by the agreement, then we are obligated to pay the counterparty the referenced amount of the contract and receive in return the referenced defaulted security or similar security.
Subsequent defaults on the remaining names within such instruments require no further payment to counterparties.
The majority of referenced names in the credit derivatives where we have sold credit protection, as well as all the counterparties to these agreements, are investment grade credit quality and our credit derivatives generally have maturities of five years or less.
As of December 31, 2007 and December 31, 2006, we had $1.5 billion and $1.6 billion, respectively, in outstanding notional amounts of credit derivative contracts where we have sold credit protection.
The Financial Services Businesses had $1.1 billion and $1.2 billion of outstanding notional amounts as of December 31, 2007 and December 31, 2006, respectively.
The Closed Block Business had $328 million and $378 million of outstanding notional amounts, as of December 31, 2007 and December 31, 2006, respectively.
Credit derivative contracts are recorded at fair value with changes in fair value, including the premium received, recorded in “Realized investment gains (losses), net.
” The premium received for the credit derivatives we sell attributable to the Financial Services Businesses was $12 million for both the years ended December 31, 2007 and 2006, and is included in adjusted operating income as an adjustment to “Realized investment gains (losses), net” over the life of the derivative.
The following table sets forth our exposure where we have sold credit protection through credit derivatives in the Financial Services Businesses by NAIC rating of the underlying credits as of the dates indicated.
## Table 4 ##
(1) First-to-default credit swap baskets, which may include credits of varying qualities, are grouped above based on the lowest credit in the basket.
However, such basket swaps may entail greater credit risk than the rating level of the lowest credit.

<table><tr><td></td><td></td><td colspan="3">As of December 31, 2016</td><td colspan="3">As of December 31, 2015</td></tr><tr><td>(In millions)</td><td>Estimated Life</td><td>Cost</td><td>AccumulatedAmortization</td><td>Net</td><td>Cost</td><td>AccumulatedAmortization</td><td>Net</td></tr><tr><td>Out-licensed patents</td><td>13-23 years</td><td>$543.3</td><td>$-523.6</td><td>$19.7</td><td>$543.3</td><td>$-506.0</td><td>$37.3</td></tr><tr><td>Developed technology</td><td>15-23 years</td><td>3,005.3</td><td>-2,634.3</td><td>371.0</td><td>3,005.3</td><td>-2,552.9</td><td>452.4</td></tr><tr><td>In-process research and development</td><td>Indefinite until commercialization</td><td>648.0</td><td>—</td><td>648.0</td><td>730.5</td><td>—</td><td>730.5</td></tr><tr><td>Trademarks and trade names</td><td>Indefinite</td><td>64.0</td><td>—</td><td>64.0</td><td>64.0</td><td>—</td><td>64.0</td></tr><tr><td>Acquired and in-licensed rights and patents</td><td>6-18 years</td><td>3,481.7</td><td>-776.1</td><td>2,705.6</td><td>3,303.2</td><td>-502.3</td><td>2,800.9</td></tr><tr><td>Total intangible assets</td><td></td><td>$7,742.3</td><td>$-3,934.0</td><td>$3,808.3</td><td>$7,646.3</td><td>$-3,561.2</td><td>$4,085.1</td></tr></table>
<table><tr><td></td><td>2015</td><td>2014</td></tr><tr><td></td><td colspan="2">(in thousands)</td></tr><tr><td>Trade accounts payable</td><td>$22,836</td><td>$13,076</td></tr><tr><td>Compensation and benefits</td><td>57,238</td><td>61,193</td></tr><tr><td>Third party processing expenses</td><td>4,399</td><td>11,062</td></tr><tr><td>Commissions to third parties</td><td>63,737</td><td>66,551</td></tr><tr><td>Accrued fees and assessment expenses</td><td>39,417</td><td>51,649</td></tr><tr><td>Other</td><td>125,020</td><td>86,575</td></tr><tr><td></td><td>$312,647</td><td>$290,106</td></tr></table>
<table><tr><td>Customer-related intangible assets</td><td>$977,400</td></tr><tr><td>Acquired technology</td><td>457,000</td></tr><tr><td>Trademarks and trade names</td><td>176,000</td></tr><tr><td>Covenants-not-to-compete</td><td>28,640</td></tr><tr><td>Total estimated acquired intangible assets</td><td>$1,639,040</td></tr></table>
<table><tr><td> -1</td><td></td><td colspan="4"> December 31, 2007</td><td colspan="4"> December 31, 2006</td></tr><tr><td> NAIC Designation</td><td> Rating Agency Equivalent</td><td> Amortized Cost</td><td> Gross Unrealized Gains</td><td> Gross Unrealized Losses</td><td> Fair Value</td><td> Amortized Cost</td><td> Gross Unrealized Gains</td><td> Gross Unrealized Losses</td><td> Fair Value</td></tr><tr><td></td><td></td><td colspan="8"> (in millions)</td></tr><tr><td>1</td><td>Aaa, Aa, A</td><td>$3,197</td><td>$219</td><td>$23</td><td>$3,393</td><td>$3,098</td><td>$180</td><td>$26</td><td>$3,252</td></tr><tr><td>2</td><td>Baa</td><td>6,495</td><td>363</td><td>28</td><td>6,830</td><td>6,620</td><td>355</td><td>47</td><td>6,928</td></tr><tr><td></td><td>Subtotal Investment Grade</td><td>9,692</td><td>582</td><td>51</td><td>10,223</td><td>9,718</td><td>535</td><td>73</td><td>10,180</td></tr><tr><td>3</td><td>Ba</td><td>1,246</td><td>63</td><td>21</td><td>1,288</td><td>1,173</td><td>75</td><td>7</td><td>1,241</td></tr><tr><td>4</td><td>B</td><td>442</td><td>6</td><td>5</td><td>443</td><td>413</td><td>18</td><td>7</td><td>424</td></tr><tr><td>5</td><td>C and lower</td><td>214</td><td>8</td><td>4</td><td>218</td><td>131</td><td>14</td><td>2</td><td>143</td></tr><tr><td>6</td><td>In or near default</td><td>56</td><td>18</td><td>—</td><td>74</td><td>26</td><td>7</td><td>—</td><td>33</td></tr><tr><td></td><td>Subtotal Below Investment Grade</td><td>1,958</td><td>95</td><td>30</td><td>2,023</td><td>1,743</td><td>114</td><td>16</td><td>1,841</td></tr><tr><td></td><td>Total Private Fixed Maturities</td><td>$11,650</td><td>$677</td><td>$81</td><td>$12,246</td><td>$11,461</td><td>$649</td><td>$89</td><td>$12,021</td></tr></table>
<table><tr><td> -1</td><td></td><td colspan="2">December 31, 2007</td><td colspan="2"> December 31, 2006</td></tr><tr><td> NAIC Designation</td><td> Rating Agency Equivalent</td><td> Notional</td><td>Fair Value</td><td> Notional</td><td> Fair Value</td></tr><tr><td></td><td></td><td colspan="4"> (in millions)</td></tr><tr><td>1</td><td>Aaa, Aa, A</td><td>$392</td><td>$-4</td><td>$952</td><td>$12</td></tr><tr><td>2</td><td>Baa</td><td>672</td><td>-65</td><td>162</td><td>2</td></tr><tr><td></td><td>Subtotal Investment Grade</td><td>1,064</td><td>-69</td><td>1,114</td><td>14</td></tr><tr><td>3</td><td>Ba</td><td>20</td><td>-1</td><td>20</td><td>—</td></tr><tr><td>4</td><td>B</td><td>38</td><td>-3</td><td>38</td><td>—</td></tr><tr><td>5</td><td>C and lower</td><td>20</td><td>-2</td><td>—</td><td>—</td></tr><tr><td>6</td><td>In or near default</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td></td><td>Subtotal Below Investment Grade</td><td>78</td><td>-6</td><td>58</td><td>—</td></tr><tr><td></td><td>Total</td><td>$1,142</td><td>$-75</td><td>$1,172</td><td>$14</td></tr></table>

Question: What's the average of Subtotal Below Investment Grade for national in 2007? (in million)