Humana Inc.  NOTES TO CONSOLIDATED FINANCIAL STATEMENTS—(Continued) Gross unrealized losses and fair values aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position were as follows at December 31, 2012 and 2011, respectively:
## Table 0 ##
Approximately 94% of our debt securities were investment-grade quality, with a weighted average credit rating of AA- by S&P at December 31, 2012.
Most of the debt securities that were below investment-grade were rated BB, the higher end of the below investment-grade rating scale.
At December 31, 2012, 10% of our taxexempt municipal securities were pre-refunded, generally with U. S.  government and agency securities.
Taxexempt municipal securities that were not pre-refunded were diversified among general obligation bonds of U. S.  states and local municipalities as well as special revenue bonds.
General obligation bonds, which are backed by the taxing power and full faith of the issuer, accounted for 40% of the tax-exempt municipals that were not prerefunded in the portfolio.
Special revenue bonds, issued by a municipality to finance a specific public works project such as utilities, water and sewer, transportation, or education, and supported by the revenues of that project, accounted for the remaining 60% of these municipals.
Our general obligation bonds are diversified across the U. S.  with no individual state exceeding 11%.
In addition, 20% of our tax-exempt securities were insured by bond insurers and had an equivalent weighted average S&P credit rating of AA- exclusive of the bond insurers’ guarantee.
Our investment policy limits investments in a single issuer and requires diversification among various asset types.
surplus requirements associated with premium growth.
Refer to our parent company financial statements and accompanying notes in Schedule I – Parent Company Financial Information.
Regulatory requirements, including subsidiary dividends to the parent, are discussed in more detail in the following section.
Excluding Puerto Rico subsidiaries, the amount of ordinary dividends that may be paid to our parent company in 2014 is approximately $840 million in the aggregate.
However, actual dividends paid from the subsidiaries to the parent in 2014 could be reduced as a result of the proposed statutory accounting for the health insurance industry fee, discussed below, combined with higher surplus requirements associated with premium growth due to increases in membership.
The NAIC is continuing discussions regarding the statutory accounting for the health insurance industry fee required by the Health Care Reform Law which in its present form would restrict surplus in the year preceding payment, beginning in 2014.
Accordingly, in addition to requiring recognition of the full-year 2014 assessment in the first quarter of 2014, we may be required to restrict surplus for the 2015 assessment ratably in 2014.
This proposal does not affect our financial statements prepared in accordance with generally accepted accounting principles, under which the fee is expensed ratably throughout the payment year.
In September 2014, we expect to pay the federal government in the range of $525 million to $575 million for the annual health insurance industry fee.
In 2015, the health insurance industry fee increases by 41% for the industry taken as a whole.
Accordingly, absent changes in market share, we would expect a similar increase in our fee in 2015.
Regulatory Requirements For a detailed discussion of our regulatory requirements, including aggregate statutory capital and surplus as well as dividends paid from the subsidiaries to the parent, please refer to Note 14 to the consolidated financial statements included in Item 8.
– Financial Statements and Supplementary Data.
Contractual Obligations We are contractually obligated to make payments for years subsequent to December 31, 2013 as follows:
## Table 1 ##
(1) Interest includes the estimated contractual interest payments under our debt agreements.
(2) We lease facilities, computer hardware, and other furniture and equipment under long-term operating leases that are noncancelable and expire on various dates through 2025.
We sublease facilities or partial facilities to third party tenants for space not used in our operations which partially mitigates our operating lease commitments.
An operating lease is a type of off-balance sheet arrangement.
Assuming we acquired the asset, rather than leased such asset, we would have recognized a liability for the financing of these assets.
See also Note 15 to the consolidated financial statements included in Item 8.
– Financial Statements and Supplementary Data.
(3) Purchase obligations include agreements to purchase services, primarily information technology related services, or to make improvements to real estate, in each case that are enforceable and legally binding on us and that specify all significant terms, including: fixed or minimum levels of service to be purchased; fixed, minimum or variable price provisions; and the appropriate timing of the transaction.
Purchase obligations exclude agreements that are cancelable without penalty
Amortization expense, which is included in selling, general and administrative expenses, was $13.0 million, $13.9 million and $8.5 million for the years ended December 31, 2016, 2015 and 2014, respectively.
The following is the estimated amortization expense for the Company’s intangible assets as of December 31, 2016:
## Table 2 ##
At December 31, 2016, 2015 and 2014, the Company determined that its goodwill and indefinitelived intangible assets were not impaired.5.  Credit Facility and Other Long Term Debt Credit Facility The Company is party to a credit agreement that provides revolving commitments for up to $1.25 billion of borrowings, as well as term loan commitments, in each case maturing in January 2021.
As of December 31, 2016 there was no outstanding balance under the revolving credit facility and $186.3 million of term loan borrowings remained outstanding.
At the Company’s request and the lender’s consent, revolving and or term loan borrowings may be increased by up to $300.0 million in aggregate, subject to certain conditions as set forth in the credit agreement, as amended.
Incremental borrowings are uncommitted and the availability thereof, will depend on market conditions at the time the Company seeks to incur such borrowings.
The borrowings under the revolving credit facility have maturities of less than one year.
Up to $50.0 million of the facility may be used for the issuance of letters of credit.
There were $2.6 million of letters of credit outstanding as of December 31, 2016.
The credit agreement contains negative covenants that, subject to significant exceptions, limit the ability of the Company and its subsidiaries to, among other things, incur additional indebtedness, make restricted payments, pledge their assets as security, make investments, loans, advances, guarantees and acquisitions, undergo fundamental changes and enter into transactions with affiliates.
The Company is also required to maintain a ratio of consolidated EBITDA, as defined in the credit agreement, to consolidated interest expense of not less than 3.50 to 1.00 and is not permitted to allow the ratio of consolidated total indebtedness to consolidated EBITDA to be greater than 3.25 to 1.00 (“consolidated leverage ratio”).
As of December 31, 2016, the Company was in compliance with these ratios.
In addition, the credit agreement contains events of default that are customary for a facility of this nature, and includes a cross default provision whereby an event of default under other material indebtedness, as defined in the credit agreement, will be considered an event of default under the credit agreement.
Borrowings under the credit agreement bear interest at a rate per annum equal to, at the Company’s option, either (a) an alternate base rate, or (b) a rate based on the rates applicable for deposits in the interbank market for U. S.  Dollars or the applicable currency in which the loans are made (“adjusted LIBOR”), plus in each case an applicable margin.
The applicable margin for loans will

Tables referenced above, in order (Table 0 first):
<table><tr><td></td><td colspan="2">Less than 12 months</td><td colspan="2">12 months or more</td><td colspan="2">Total</td></tr><tr><td></td><td> Fair Value</td><td>Gross Unrealized Losses</td><td> Fair Value</td><td>Gross Unrealized Losses</td><td> Fair Value</td><td>Gross Unrealized Losses</td></tr><tr><td></td><td colspan="6">(in millions)</td></tr><tr><td> December 31, 2012</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>U.S. Treasury and other U.S. government corporations and agencies:</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>U.S. Treasury and agency obligations</td><td>$56</td><td>$0</td><td>$2</td><td>$0</td><td>$58</td><td>$0</td></tr><tr><td>Mortgage-backed securities</td><td>38</td><td>0</td><td>25</td><td>-1</td><td>63</td><td>-1</td></tr><tr><td>Tax-exempt municipal securities</td><td>233</td><td>-3</td><td>27</td><td>-1</td><td>260</td><td>-4</td></tr><tr><td>Mortgage-backed securities:</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Residential</td><td>0</td><td>0</td><td>4</td><td>-1</td><td>4</td><td>-1</td></tr><tr><td>Commercial</td><td>94</td><td>0</td><td>0</td><td>0</td><td>94</td><td>0</td></tr><tr><td>Asset-backed securities</td><td>2</td><td>0</td><td>4</td><td>0</td><td>6</td><td>0</td></tr><tr><td>Corporate debt securities</td><td>104</td><td>-2</td><td>4</td><td>0</td><td>108</td><td>-2</td></tr><tr><td>Total debt securities</td><td>$527</td><td>$-5</td><td>$66</td><td>$-3</td><td>$593</td><td>$-8</td></tr><tr><td> December 31, 2011</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>U.S. Treasury and other U.S. government corporations and agencies:</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>U.S. Treasury and agency obligations</td><td>$117</td><td>$0</td><td>$0</td><td>$0</td><td>$117</td><td>$0</td></tr><tr><td>Mortgage-backed securities</td><td>67</td><td>-1</td><td>18</td><td>-1</td><td>85</td><td>-2</td></tr><tr><td>Tax-exempt municipal securities</td><td>53</td><td>0</td><td>48</td><td>-2</td><td>101</td><td>-2</td></tr><tr><td>Mortgage-backed securities:</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Residential</td><td>3</td><td>0</td><td>24</td><td>-2</td><td>27</td><td>-2</td></tr><tr><td>Commercial</td><td>14</td><td>0</td><td>0</td><td>0</td><td>14</td><td>0</td></tr><tr><td>Asset-backed securities</td><td>16</td><td>0</td><td>4</td><td>0</td><td>20</td><td>0</td></tr><tr><td>Corporate debt securities</td><td>355</td><td>-10</td><td>41</td><td>-1</td><td>396</td><td>-11</td></tr><tr><td>Total debt securities</td><td>$625</td><td>$-11</td><td>$135</td><td>$-6</td><td>$760</td><td>$-17</td></tr></table>
<table><tr><td></td><td colspan="5"> Payments Due by Period</td></tr><tr><td></td><td> Total</td><td> Less than 1 Year</td><td> 1-3 Years</td><td> 3-5 Years</td><td> More than 5 Years</td></tr><tr><td></td><td colspan="5"> (in millions)</td></tr><tr><td>Debt</td><td>$2,550</td><td>0</td><td>500</td><td>800</td><td>1,250</td></tr><tr><td>Interest -1</td><td>1,548</td><td>146</td><td>277</td><td>208</td><td>917</td></tr><tr><td>Operating leases -2</td><td>904</td><td>228</td><td>346</td><td>187</td><td>143</td></tr><tr><td>Purchase obligations -3</td><td>253</td><td>132</td><td>91</td><td>30</td><td>0</td></tr><tr><td>Future policy benefits payable and other long-term liabilities -4</td><td>2,563</td><td>70</td><td>356</td><td>213</td><td>1,924</td></tr><tr><td>Total</td><td>$7,818</td><td>576</td><td>1,570</td><td>1,438</td><td>4,234</td></tr></table>
<table><tr><td>2017</td><td>$10,509</td></tr><tr><td>2018</td><td>9,346</td></tr><tr><td>2019</td><td>9,240</td></tr><tr><td>2020</td><td>7,201</td></tr><tr><td>2021</td><td>5,318</td></tr><tr><td>2022 and thereafter</td><td>16,756</td></tr><tr><td>Amortization expense of intangible assets</td><td>$58,370</td></tr></table>

Question: what was the difference in millions of amortization expense between 2015 and 2016?
Answer: -0.9
Reasoning program: subtract(13.0,13.9)