securities have historically returned approximately 10% ( 10 % ) annually over long periods of time , while u.s . debt securities have returned approximately 6% ( 6 % ) annually over long periods . application of these historical returns to the plan 2019s allocation ranges for equities and bonds produces a result between 7.25% ( 7.25 % ) and 8.75% ( 8.75 % ) and is one point of reference , among many other factors , that is taken into consideration . we also examine the plan 2019s actual historical returns over various periods and consider the current economic environment . recent experience is considered in our evaluation with appropriate consideration that , especially for short time periods , recent returns are not reliable indicators of future returns . while annual returns can vary significantly ( actual returns for 2012 , 2011 , and 2010 were +15.29% ( +15.29 % ) , +.11% ( +.11 % ) , and +14.87% ( +14.87 % ) , respectively ) , the selected assumption represents our estimated long-term average prospective returns . acknowledging the potentially wide range for this assumption , we also annually examine the assumption used by other companies with similar pension investment strategies , so that we can ascertain whether our determinations markedly differ from others . in all cases , however , this data simply informs our process , which places the greatest emphasis on our qualitative judgment of future investment returns , given the conditions existing at each annual measurement date . taking into consideration all of these factors , the expected long-term return on plan assets for determining net periodic pension cost for 2012 was 7.75% ( 7.75 % ) , the same as it was for 2011 . after considering the views of both internal and external capital market advisors , particularly with regard to the effects of the recent economic environment on long-term prospective fixed income returns , we are reducing our expected long-term return on assets to 7.50% ( 7.50 % ) for determining pension cost for under current accounting rules , the difference between expected long-term returns and actual returns is accumulated and amortized to pension expense over future periods . each one percentage point difference in actual return compared with our expected return causes expense in subsequent years to increase or decrease by up to $ 8 million as the impact is amortized into results of operations . we currently estimate a pretax pension expense of $ 73 million in 2013 compared with pretax expense of $ 89 million in 2012 . this year-over-year expected decrease reflects the impact of favorable returns on plan assets experienced in 2012 as well as the effects of the lower discount rate required to be used in the table below reflects the estimated effects on pension expense of certain changes in annual assumptions , using 2013 estimated expense as a baseline . table 27 : pension expense - sensitivity analysis change in assumption ( a ) estimated increase to 2013 pension expense ( in millions ) .
by what percentage did the pension pretax expenses decrease from 2012 to 2013?
( a ) the impact is the effect of changing the specified assumption while holding all other assumptions constant . our pension plan contribution requirements are not particularly sensitive to actuarial assumptions . investment performance has the most impact on contribution requirements and will drive the amount of required contributions in future years . also , current law , including the provisions of the pension protection act of 2006 , sets limits as to both minimum and maximum contributions to the plan . we do not expect to be required by law to make any contributions to the plan during 2013 . we maintain other defined benefit plans that have a less significant effect on financial results , including various nonqualified supplemental retirement plans for certain employees , which are described more fully in note 15 employee benefit plans in the notes to consolidated financial statements in item 8 of this report . the pnc financial services group , inc . 2013 form 10-k 77 .