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

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