each clearing firm is required to deposit and maintain balances in the form of cash , u.s . government securities , certain foreign government securities , bank letters of credit or other approved investments to satisfy performance bond and guaranty fund requirements . all non-cash deposits are marked-to-market and haircut on a daily basis . securities deposited by the clearing firms are not reflected in the consolidated financial statements and the clearing house does not earn any interest on these deposits . these balances may fluctuate significantly over time due to investment choices available to clearing firms and changes in the amount of contributions required . in addition , the rules and regulations of cbot require that collateral be provided for delivery of physical commodities , maintenance of capital requirements and deposits on pending arbitration matters . to satisfy these requirements , clearing firms that have accounts that trade certain cbot products have deposited cash , u.s . treasury securities or letters of credit . the clearing house marks-to-market open positions at least once a day ( twice a day for futures and options contracts ) , and require payment from clearing firms whose positions have lost value and make payments to clearing firms whose positions have gained value . the clearing house has the capability to mark-to-market more frequently as market conditions warrant . under the extremely unlikely scenario of simultaneous default by every clearing firm who has open positions with unrealized losses , the maximum exposure related to positions other than credit default and interest rate swap contracts would be one half day of changes in fair value of all open positions , before considering the clearing houses 2019 ability to access defaulting clearing firms 2019 collateral deposits . for cleared credit default swap and interest rate swap contracts , the maximum exposure related to cme 2019s guarantee would be one full day of changes in fair value of all open positions , before considering cme 2019s ability to access defaulting clearing firms 2019 collateral . during 2017 , the clearing house transferred an average of approximately $ 2.4 billion a day through the clearing system for settlement from clearing firms whose positions had lost value to clearing firms whose positions had gained value . the clearing house reduces the guarantee exposure through initial and maintenance performance bond requirements and mandatory guaranty fund contributions . the company believes that the guarantee liability is immaterial and therefore has not recorded any liability at december 31 , 2017 . at december 31 , 2016 , performance bond and guaranty fund contribution assets on the consolidated balance sheets included cash as well as u.s . treasury and u.s . government agency securities with maturity dates of 90 days or less . the u.s . treasury and u.s . government agency securities were purchased by cme , at its discretion , using cash collateral . the benefits , including interest earned , and risks of ownership accrue to cme . interest earned is included in investment income on the consolidated statements of income . there were no u.s . treasury and u.s . government agency securities held at december 31 , 2017 . the amortized cost and fair value of these securities at december 31 , 2016 were as follows : ( in millions ) amortized .

Table: ( in millions ) 2016 amortizedcost 2016 fairvalue u.s . treasury securities $ 5548.9 $ 5549.0 u.s . government agency securities 1228.3 1228.3

cme has been designated as a systemically important financial market utility by the financial stability oversight council and maintains a cash account at the federal reserve bank of chicago . at december 31 , 2017 and december 31 , 2016 , cme maintained $ 34.2 billion and $ 6.2 billion , respectively , within the cash account at the federal reserve bank of chicago . clearing firms , at their option , may instruct cme to deposit the cash held by cme into one of the ief programs . the total principal in the ief programs was $ 1.1 billion at december 31 , 2017 and $ 6.8 billion at december 31 .

Question: what was the average balance within the cash account at the federal reserve bank of chicago for december 31 , 2017 and december 31 , 2016 , in billions?
Answer: 20.2
Calculation: divide(add(34.2, 6.2), const_2)