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 .

<i>Millions of Dollars</i>
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)
(35)
(33)
Non-operating environmental costs and other
(34)
(19)
(33)
Total
$92
$116
$118

.

Question: Analyse this data from a financial earnings document. what was the percentage change in rental income from 2006 to 2007?
Choices:
-0.18072
-0.17241
-5.53333
68.18072
-3
Correct choice index: 0