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: .
by how much did the unrecovered balance of project costs incurred decrease from 2017 to 2018?
.