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Regulatory matters and revenues subject to refund (Details) (MTPSC [Member], USD $) In Millions, unless otherwise specified	Sep. 26, 2012
MTPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Total gas rate increase requested	$ 3.5
Percent above current rates requested	5.90%
Amount of requested interim increase in annual gas rates	$ 1.7
Requested percentage of interim gas rate increase	2.90%
Number of days to be effective within	30 days