Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
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-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Long-Term Debt and Capital Leases Schedule of Long-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term Debt	$ 2,233,281			$ 2,102,345
Secured Debt Montana Due 2049 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 150,000
Montana 3.98%, Due June 2049
Debt Instrument [Line Items]
Debt Instrument, Face Amount			$ 50,000
Montana 3.98%, Due September 2049
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 100,000
Secured Debt \| Secured Debt Montana Due 2049 [Member]
Debt Instrument [Line Items]
Long-term Debt				$ 0
Secured Debt \| Montana 3.98%, Due June 2049
Debt Instrument [Line Items]
Long-term Debt	50,000
Secured Debt \| Montana 3.98%, Due September 2049
Debt Instrument [Line Items]
Long-term Debt	$ 100,000