Financial report context:
the pension plan investments are held in a master trust , with the northern trust company . investments in the master trust are valued at fair value , which has been determined based on fair value of the underlying investments of the master trust . investments in securities traded on public security exchanges are valued at their closing market prices on the valuation date ; where no sale was made on the valuation date , the security is generally valued at its most recent bid price . certain short-term investments are carried at cost , which approximates fair value . investments in registered investment companies and common trust funds , which primarily invest in stocks , bonds , and commodity futures , are valued using publicly available market prices for the underlying investments held by these entities . the majority of pension plan assets are invested in equity securities , because equity portfolios have historically provided higher returns than debt and other asset classes over extended time horizons , and are expected to do so in the future . correspondingly , equity investments also entail greater risks than other investments . equity risks are balanced by investing a significant portion of the plan 2019s assets in high quality debt securities . the average quality rating of the debt portfolio exceeded aa as of december 31 , 2008 and 2007 . the debt portfolio is also broadly diversified and invested primarily in u.s . treasury , mortgage , and corporate securities with an intermediate average maturity . the weighted-average maturity of the debt portfolio was 5 years at both december 31 , 2008 and 2007 , respectively . the investment of pension plan assets in securities issued by union pacific is specifically prohibited for both the equity and debt portfolios , other than through index fund holdings . other retirement programs thrift plan 2013 we provide a defined contribution plan ( thrift plan ) to eligible non-union employees and make matching contributions to the thrift plan . we match 50 cents for each dollar contributed by employees up to the first six percent of compensation contributed . our thrift plan contributions were $ 14 million in 2008 , $ 14 million in 2007 , and $ 13 million in 2006 . railroad retirement system 2013 all railroad employees are covered by the railroad retirement system ( the system ) . contributions made to the system are expensed as incurred and amounted to approximately $ 620 million in 2008 , $ 616 million in 2007 , and $ 615 million in 2006 . collective bargaining agreements 2013 under collective bargaining agreements , we provide certain postretirement healthcare and life insurance benefits for eligible union employees . premiums under the plans are expensed as incurred and amounted to $ 49 million in 2008 and $ 40 million in both 2007 and 5 . other income other income included the following for the years ended december 31 : millions of dollars 2008 2007 2006 .

Financial table:
millions of dollars: rental income; 2008: $ 87; 2007: $ 68; 2006: $ 83
millions of dollars: net gain on non-operating asset dispositions; 2008: 41; 2007: 52; 2006: 72
millions of dollars: interest income; 2008: 21; 2007: 50; 2006: 29
millions of dollars: sale of receivables fees; 2008: -23 ( 23 ); 2007: -35 ( 35 ); 2006: -33 ( 33 )
millions of dollars: non-operating environmental costs and other; 2008: -34 ( 34 ); 2007: -19 ( 19 ); 2006: -33 ( 33 )
millions of dollars: total; 2008: $ 92; 2007: $ 116; 2006: $ 118

Additional report context:
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Question: what was the percentage change in rental income from 2006 to 2007?
Answer: -18%