pre-construction costs , interim dam safety measures and environmental costs and construction costs . the authorized costs were being recovered via a surcharge over a twenty-year period which began in october 2012 . the unrecovered balance of project costs incurred , including cost of capital , net of surcharges totaled $ 85 million and $ 89 million as of december 31 , 2018 and 2017 , respectively . surcharges collected were $ 8 million and $ 7 million for the years ended december 31 , 2018 and 2017 , respectively . pursuant to the general rate case approved in december 2018 , approval was granted to reset the twenty-year amortization period to begin january 1 , 2018 and to establish an annual revenue requirement of $ 8 million to be recovered through base rates . debt expense is amortized over the lives of the respective issues . call premiums on the redemption of long- term debt , as well as unamortized debt expense , are deferred and amortized to the extent they will be recovered through future service rates . purchase premium recoverable through rates is primarily the recovery of the acquisition premiums related to an asset acquisition by the company 2019s utility subsidiary in california during 2002 , and acquisitions in 2007 by the company 2019s utility subsidiary in new jersey . as authorized for recovery by the california and new jersey pucs , these costs are being amortized to depreciation and amortization on the consolidated statements of operations through november 2048 . tank painting costs are generally deferred and amortized to operations and maintenance expense on the consolidated statements of operations on a straight-line basis over periods ranging from five to fifteen years , as authorized by the regulatory authorities in their determination of rates charged for service . as a result of the prepayment by american water capital corp. , the company 2019s wholly owned finance subsidiary ( 201cawcc 201d ) , of the 5.62% ( 5.62 % ) series c senior notes due upon maturity on december 21 , 2018 ( the 201cseries c notes 201d ) , 5.62% ( 5.62 % ) series e senior notes due march 29 , 2019 ( the 201cseries e notes 201d ) and 5.77% ( 5.77 % ) series f senior notes due december 21 , 2022 ( the 201cseries f notes , 201d and together with the series e notes , the 201cseries notes 201d ) , a make-whole premium of $ 10 million was paid to the holders of the series notes on september 11 , 2018 . substantially all of these early debt extinguishment costs were allocable to the company 2019s utility subsidiaries and recorded as regulatory assets , as the company believes they are probable of recovery in future rates . other regulatory assets include certain construction costs for treatment facilities , property tax stabilization , employee-related costs , deferred other postretirement benefit expense , business services project expenses , coastal water project costs , rate case expenditures and environmental remediation costs among others . these costs are deferred because the amounts are being recovered in rates or are probable of recovery through rates in future periods . regulatory liabilities regulatory liabilities generally represent amounts that are probable of being credited or refunded to customers through the rate-making process . also , if costs expected to be incurred in the future are currently being recovered through rates , the company records those expected future costs as regulatory liabilities . the following table provides the composition of regulatory liabilities as of december 31: .

Table: 2018 2017 income taxes recovered through rates $ 1279 $ 1242 removal costs recovered through rates 309 315 postretirement benefit liability 209 33 pension and other postretirement benefit balancing accounts 46 48 tcja reserve on revenue 36 2014 other 28 26 total regulatory liabilities $ 1907 $ 1664

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Question: by how much did the unrecovered balance of project costs incurred decrease from 2017 to 2018?
Answer: 4.5%
Calculation: divide(subtract(89, 85), 89)