XML 51 R59.htm IDEA: XBRL DOCUMENT
v2.4.0.8

Available-for-sale securities (Details) (USD $) | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2014 | Sep. 30, 2013 | Dec. 31, 2013
Available-for-sale securities [Line Items] |  |  |  |  |
Investments used to satisfy obligations under nonqualified benefit plans | $ 63,600,000 | $ 58,100,000 | $ 63,600,000 | $ 58,100,000 | $ 62,400,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans | (800,000) | 4,100,000 | 1,200,000 | 9,200,000 |
Available-for-sale securities [Abstract] |  |  |  |  |
Cost | 10,206,000 | 9,963,000 | 10,206,000 | 9,963,000 | 10,057,000
Gross unrealized gains |  |  | 79,000 | 85,000 | 84,000
Gross unrealized losses |  |  | (10,000) | (24,000) | (31,000)
Fair value | 10,275,000 | 10,024,000 | 10,275,000 | 10,024,000 | 10,110,000
Mortgage backed securities [Member] |  |  |  |  |
Available-for-sale securities [Abstract] |  |  |  |  |
Cost | 7,838,000 | 8,051,000 | 7,838,000 | 8,051,000 | 8,151,000
Gross unrealized gains |  |  | 71,000 | 70,000 | 69,000
Gross unrealized losses |  |  | (8,000) | (20,000) | (27,000)
Fair value | 7,901,000 | 8,101,000 | 7,901,000 | 8,101,000 | 8,193,000
US Treasury securities [Member] |  |  |  |  |
Available-for-sale securities [Abstract] |  |  |  |  |
Cost | 2,368,000 | 1,912,000 | 2,368,000 | 1,912,000 | 1,906,000
Gross unrealized gains |  |  | 8,000 | 15,000 | 15,000
Gross unrealized losses |  |  | (2,000) | (4,000) | (4,000)
Fair value | $ 2,374,000 | $ 1,923,000 | $ 2,374,000 | $ 1,923,000 | $ 1,917,000

X
- Definition Investments used to satisfy obligations under unfunded, nonqualified benefit plans for executive officers and certain key management employees. + References No definition available. + Details
Name: | mdu_Investmentsusedtosatisfynonqualifiedbenefitplansobligations
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Net unrealized gain (loss) of investments used to satisfy obligations under unfunded, nonqualified benefit plans for executive officers and certain key management employees. + References No definition available. + Details
Name: | mdu_Netunrealizedgainlossofinvestmentsusedtosatisfyobligationsundernonqualifiedbenefitplans
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 + Details
Name: | us-gaap_AvailableForSaleSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_AvailableForSaleSecuritiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
Name: | us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
Name: | us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration