Otter Tail (OTTR) 10-K 22 Feb 19 2018 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 15, 2019	Jun. 29, 2018
Document Information [Line Items]
Entity Registrant Name	Otter Tail Corp
Entity Central Index Key	1,466,593
Trading Symbol	ottr
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	Yes
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding (in shares)		39,729,708
Entity Public Float			$ 1,810,041,170
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Shell Company	false

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets
Cash and Cash Equivalents	$ 861	$ 16,216
Accounts Receivable:
Trade (less allowance for doubtful accounts of $1,407 for 2018 and $1,094 for 2017)	75,144	68,466
Other	9,741	7,761
Inventories	106,270	88,034
Unbilled Receivables	23,626	22,427
Income Taxes Receivable	2,439	1,181
Regulatory Assets	17,225	22,551
Other	6,114	12,491
Total Current Assets	241,420	239,127
Investments	8,961	8,629
Other Assets	35,759	36,006
Goodwill	37,572	37,572
Other Intangibles–Net	12,450	13,765
Regulatory Assets	135,257	129,576
Plant
Electric Plant in Service	2,019,721	1,981,018
Nonelectric Operations	228,120	216,937
Construction Work in Progress	181,626	141,067
Total Gross Plant	2,429,467	2,339,022
Less Accumulated Depreciation and Amortization	848,369	799,419
Net Plant	1,581,098	1,539,603
Total Assets	2,052,517	2,004,278
Liabilities and Equity
Short-Term Debt	18,599	112,371
Current Maturities of Long-Term Debt	172	186
Accounts Payable	96,291	84,677
Accrued Salaries and Wages	24,857	21,534
Accrued Taxes	17,287	16,808
Regulatory Liabilities	738	9,688
Other Accrued Liabilities	12,149	11,389
Total Current Liabilities	170,093	256,653
Pensions Benefit Liability	98,358	109,708
Other Postretirement Benefits Liability	71,561	69,774
Other Noncurrent Liabilities	24,326	22,769
Commitments and Contingencies (note 9)
Deferred Credits
Deferred Income Taxes	120,976	100,501
Deferred Tax Credits	19,974	21,379
Regulatory Liabilities	226,469	232,893
Other	1,895	3,329
Total Deferred Credits	369,314	358,102
Capitalization (page 68)
Long-Term Debt—Net	590,002	490,380
Common Shares, Par Value $5 Per Share–Authorized, 50,000,000 Shares; Outstanding, 2018—39,664,884 Shares; 2017—39,557,491 Shares	198,324	197,787
Premium on Common Shares	344,250	343,450
Retained Earnings	190,433	161,286
Accumulated Other Comprehensive Loss	(4,144)	(5,631)
Total Common Equity	728,863	696,892
Total Capitalization	1,318,865	1,187,272
Total Liabilities and Equity	2,052,517	2,004,278
Cumulative Preferred Shares [Member]
Capitalization (page 68)
Cumulative Shares
Cumulative Preference Shares [Member]
Capitalization (page 68)
Cumulative Shares

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Trade, allowance for doubtful accounts	$ 1,407	$ 1,094
Common shares, par value (in dollars per share)	$ 5	$ 5
Common shares, authorized (in shares)	50,000,000	50,000,000
Common shares, outstanding (in shares)	39,664,884	39,557,491
Cumulative Preferred Shares [Member]
Cumulative shares, authorized (in shares)	1,500,000	1,500,000
Cumulative shares, without par value (in dollars per share)	$ 0	$ 0
Cumulative shares, outstanding (in shares)	0	0
Cumulative Preference Shares [Member]
Cumulative shares, authorized (in shares)	1,000,000	1,000,000
Cumulative shares, without par value (in dollars per share)	$ 0	$ 0
Cumulative shares, outstanding (in shares)	0	0

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Revenues:
Total	$ 916,447	$ 849,350	$ 803,539
Operating Expenses
Production Fuel – Electric	66,815	59,690	54,792
Purchased Power – Electric System Use	354,559	316,562	295,222
Electric Operation and Maintenance Expenses	155,534	146,914	147,274
Other Nonelectric Expenses	51,544	41,492	38,683
Depreciation and Amortization	74,666	72,545	73,445
Property Taxes – Electric	15,585	15,053	14,266
Total Operating Expenses	787,058	717,063	686,908
Operating Income	129,389	132,287	116,631
Interest Charges	30,408	29,604	31,886
Nonservice Cost Components of Postretirement Benefits	5,509	5,620	5,110
Other Income	3,461	2,632	2,905
Income Before Income Taxes	96,933	99,695	82,540
Income Tax Expense	14,588	27,256	20,219
Net Income	$ 82,345	$ 72,439	$ 62,321
Average Number of Common Shares Outstanding–Basic (in shares)	39,599,944	39,457,261	38,546,459
Average Number of Common Shares Outstanding–Diluted (in shares)	39,892,196	39,748,347	38,731,010
Basic Earnings Per Common Share (in dollars per share)	$ 2.08	$ 1.84	$ 1.62
Diluted Earnings Per Common Share (in dollars per share)	2.06	1.82	1.61
Dividends Declared Per Common Share (in dollars per share)	$ 1.34	$ 1.28	$ 1.25
Electricity [Member]
Operating Revenues:
Revenues from Contracts with Customers	$ 450,637	$ 436,477	$ 425,245
Changes in Accrued Revenues under Alternative Revenue Programs	(439)	(1,971)	2,104
Total Electric	450,198	434,506	427,349
Product [Member]
Operating Revenues:
Revenues from Contracts with Customers	466,249	414,844	376,190
Electricity, Purchased [Member]
Operating Expenses
Purchased Power – Electric System Use	$ 68,355	$ 64,807	$ 63,226

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Income	$ 82,345	$ 72,439	$ 62,321
Unrealized Loss on Available-for-Sale Securities:
Reversal of Previously Recognized Gains Realized on Sale of Investments and Included in Other Income During Period	(105)	(15)	(3)
(Losses) Gains Arising During Period	(61)	115	(14)
Income Tax Benefit (Expense)	35	(35)	6
Change in Unrealized Losses on Available-for-Sale Securities – net-of-tax	(131)	65	(11)
Pension and Postretirement Benefit Plans:
Actuarial Gains (Losses) net of Regulatory Allocation Adjustment	1,919	(3,791)	(445)
Amortization of Unrecognized Postretirement Benefit Losses and Costs (note 11)	985	629	628
Income Tax (Expense) Benefit	(755)	1,266	(74)
Adjustment to Income Tax Expense Related to 2017 Tax Cuts and Jobs Act	(531)
Pension and Postretirement Benefit Plans – net-of-tax	1,618	(1,896)	109
Total Other Comprehensive Income (Loss)	1,487	(1,831)	98
Total Comprehensive Income	$ 83,832	$ 70,608	$ 62,419

Consolidated Statements of Comm

Consolidated Statements of Common Shareholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]		Total
Balance (in shares) at Dec. 31, 2015	37,857,186
Common Stock Issuances, Net of Expenses (in shares)	1,494,618
Common Stock Retirements and Forfeitures (in shares)	(3,668)
Common Stock Issuances, Net of Expenses	$ 7,473	$ 38,490				$ 45,963
Common Stock Retirements and Forfeitures	(18)	(86)				(104)
Employee Stock Incentive Plan Expense		3,178				3,178
Other Comprehensive Income (Loss)				98		98
Balance at Dec. 31, 2016	196,741	337,684	139,479	(3,800)	[1]	670,104
Balance at Dec. 31, 2015	$ 189,286	293,610	126,025	(3,898)	[1]	605,023
Net Income			62,321			62,321
Common Dividends			(48,244)			(48,244)
ASU 2016-09 Adoption		2,492	(623)			1,869
Balance (in shares) at Dec. 31, 2016	39,348,136
Common Stock Issuances, Net of Expenses (in shares)	257,059
Common Stock Retirements and Forfeitures (in shares)	(47,704)
Common Stock Issuances, Net of Expenses	$ 1,285	3,684				4,969
Common Stock Retirements and Forfeitures	(239)	(1,560)				(1,799)
Employee Stock Incentive Plan Expense		3,642				3,642
Other Comprehensive Income (Loss)				(1,831)		(1,831)
Balance at Dec. 31, 2017	197,787	343,450	161,286	(5,631)	[1]	696,892
Balance at Dec. 31, 2016	$ 196,741	337,684	139,479	(3,800)	[1]	670,104
Net Income			72,439			72,439
Common Dividends			(50,632)			(50,632)
Balance (in shares) at Dec. 31, 2017	39,557,491
Common Stock Issuances, Net of Expenses (in shares)	178,601
Common Stock Retirements and Forfeitures (in shares)	(71,208)
Common Stock Issuances, Net of Expenses	$ 893	(986)				(93)
Common Stock Retirements and Forfeitures	(356)	(2,655)				(3,011)
Employee Stock Incentive Plan Expense		4,441				4,441
Other Comprehensive Income (Loss)				1,487		1,487
Balance at Dec. 31, 2018	198,324	344,250	190,433	(4,144)	[1]	728,863
Balance at Dec. 31, 2017	$ 197,787	$ 343,450	161,286	$ (5,631)	[1]	696,892
Net Income			82,345			82,345
Common Dividends			$ (53,198)			$ (53,198)
Balance (in shares) at Dec. 31, 2018	39,664,884
[1]	Accumulated Other Comprehensive Loss on December 31 is comprised of the following: (in thousands) 2018 2017 2016 Unrealized (Loss) Gain on Marketable Equity Securities: Before Tax $ (95) $ 71 $ (29) Tax Effect 20 (15) 10 Stranded Tax Effect (10) (10) -- Unrealized (Loss) Gain on Marketable Equity Securities - net-of-tax (85) 46 (19) Unamortized Actuarial Losses and Prior Service Costs Related to Pension and Postretirement Benefits: Before Tax (6,558) (9,462) (6,300) Tax Effect 1,705 2,991 2,519 Stranded Tax Effect 794 794 -- Unamortized Actuarial Losses and Prior Service Costs Related to Pension and Postretirement Benefits - net-of-tax (4,059) (5,677) (3,781) Accumulated Other Comprehensive Loss: Before Tax (6,653) (9,391) (6,329) Tax Effect 1,725 2,976 2,529 Stranded Tax Effect 784 784 -- Net Accumulated Other Comprehensive Loss $ (4,144) $ (5,631) $ (3,800)

Consolidated Statements of Co_2

Consolidated Statements of Common Shareholders' Equity (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Retained Earnings [Member]
Dividends Declared Per Common Share (in dollars per share)	$ 1.34	$ 1.28	$ 1.25
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss), Before Tax	$ (95)	$ 71	$ (29)
Accumulated Other Comprehensive Income (Loss), Tax Effect	20	(15)	10
Accumulated Other Comprehensive Income (Loss), Net of Tax	(85)	46	(19)
Accumulated Other Comprehensive Income (Loss), Stranded Tax Effect	(10)	(10)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss), Before Tax	(6,558)	(9,462)	(6,300)
Accumulated Other Comprehensive Income (Loss), Tax Effect	1,705	2,991	2,519
Accumulated Other Comprehensive Income (Loss), Net of Tax	(4,059)	(5,677)	(3,781)
Accumulated Other Comprehensive Income (Loss), Stranded Tax Effect	794	794
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss), Before Tax	(6,653)	(9,391)	(6,329)
Accumulated Other Comprehensive Income (Loss), Tax Effect	1,725	2,976	2,529
Accumulated Other Comprehensive Income (Loss), Net of Tax	(4,144)	(5,631)	(3,800)
Accumulated Other Comprehensive Income (Loss), Stranded Tax Effect	$ 784	$ 784
Dividends Declared Per Common Share (in dollars per share)	$ 1.34	$ 1.28	$ 1.25
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (4,144)	$ (5,631)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Flows from Operating Activities
Net Income	$ 82,345	$ 72,439	$ 62,321
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Depreciation and Amortization	74,666	72,545	73,445
Deferred Tax Credits	(1,405)	(1,470)	(1,657)
Deferred Income Taxes	19,224	24,001	19,124
Change in Deferred Debits and Other Assets	941	(2,173)	(10,090)
Discretionary Contribution to Pension Plan	(20,000)		(10,000)
Change in Noncurrent Liabilities and Deferred Credits	(2,414)	19,257	14,685
Allowance for Equity/Other Funds Used During Construction	(2,194)	(986)	(857)
Stock Compensation Expense – Equity Awards	4,441	3,642	3,178
Other—Net		10	7
Cash (Used for) Provided by Current Assets and Current Liabilities:
Change in Receivables	(8,559)	(2,135)	(944)
Change in Inventories	(18,236)	(4,294)	1,874
Change in Other Current Assets	(754)	(3,060)	(2,541)
Change in Payables and Other Current Liabilities	14,997	(3,013)	11,502
Change in Interest Payable and Income Taxes Receivable/Payable	396	(1,186)	3,339
Net Cash Provided by Operating Activities	143,448	173,577	163,386
Cash Flows from Investing Activities
Capital Expenditures	(105,425)	(132,913)	(161,259)
Proceeds from Disposal of Noncurrent Assets	2,378	4,491	4,837
Acquisition Purchase Price Cash Received			1,500
Cash Used for Investments and Other Assets	(4,372)	(4,168)	(4,402)
Net Cash (Used in) Provided by Investing Activities	(107,419)	(132,590)	(159,324)
Cash Flows from Financing Activities
Change in Checks Written in Excess of Cash	(345)	2,434	(3,363)
Net Short-Term (Repayments) Borrowings	(93,772)	69,488	(37,789)
Proceeds from Issuance of Common Stock		4,349	44,435
Common Stock Issuance Expenses	(108)		(562)
Payments for Retirement of Capital Stock	(3,011)	(1,799)	(104)
Proceeds from Issuance of Long-Term Debt	100,000		130,000
Short-Term and Long-Term Debt Issuance Expenses	(761)	(380)	(888)
Payments for Retirement of Long-Term Debt	(189)	(48,231)	(87,547)
Dividends Paid and Other Distributions	(53,198)	(50,632)	(48,244)
Net Cash Used in Financing Activities	(51,384)	(24,771)	(4,062)
Net Change in Cash and Cash Equivalents	(15,355)	16,216
Cash and Cash Equivalents at Beginning of Period	16,216
Cash and Cash Equivalents at End of Period	$ 861	$ 16,216

Consolidated Statements of Capi

Consolidated Statements of Capitalization - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt	$ 18,599	$ 112,371
Long-term Debt	592,523	492,711
Less: Current Maturities--net of Unamortized Debt Issuance Costs	172	186
Unamortized Long-Term Debt Issuance Costs	2,349	2,145
Long-Term Debt—Net	590,002	490,380
Total Common Shareholders’ Equity	728,863	696,892
Total Capitalization	1,318,865	1,187,272
Parent Company [Member]
Short-term Debt	9,215
Long-term Debt	80,523	80,711
Less: Current Maturities--net of Unamortized Debt Issuance Costs	172	186
Unamortized Long-Term Debt Issuance Costs	407	461
Long-Term Debt—Net	79,944	80,064
Total Capitalization	808,807	776,956
Otter Tail Power Company [Member]
Short-term Debt	9,384	112,371
Long-term Debt	512,000	412,000
Less: Current Maturities--net of Unamortized Debt Issuance Costs
Unamortized Long-Term Debt Issuance Costs	1,942	1,684
Long-Term Debt—Net	510,058	410,316
Otter Tail Corporation Credit Agreement [Member] | Parent Company [Member]
Short-term Debt	9,215
Otter Tail Power Company Credit Agreement [Member] | Parent Company [Member]
Short-term Debt	9,384	112,371
The 3.55% Guaranteed Senior Notes, due December 15, 2026 [Member] | Parent Company [Member]
Long-term Debt	80,000	80,000
North Dakota Development Note, 3.95%, due April 1, 2018 [Member] | Parent Company [Member]
Long-term Debt		27
Partnership in Assisting Community Expansion Note, 2.54%, due March 18, 2021 [Member] | Parent Company [Member]
Long-term Debt	523	684
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member]
Long-term Debt	140,000	140,000
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member] | Otter Tail Power Company [Member]
Long-term Debt	140,000	140,000
Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member]
Long-term Debt	30,000	30,000
Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member] | Otter Tail Power Company [Member]
Long-term Debt	30,000	30,000
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member]
Long-term Debt	42,000	42,000
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member] | Otter Tail Power Company [Member]
Long-term Debt	42,000	42,000
Senior Unsecured Notes 4.68%, Series A, Due February 27, 2029 [Member]
Long-term Debt	60,000	60,000
Senior Unsecured Notes 4.68%, Series A, Due February 27, 2029 [Member] | Otter Tail Power Company [Member]
Long-term Debt	60,000	60,000
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member]
Long-term Debt	50,000	50,000
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member] | Otter Tail Power Company [Member]
Long-term Debt	50,000	50,000
Senior Unsecured Notes 5.47%, Series B, Due February 27, 2044 [Member]
Long-term Debt	90,000	90,000
Senior Unsecured Notes 5.47%, Series B, Due February 27, 2044 [Member] | Otter Tail Power Company [Member]
Long-term Debt	90,000	$ 90,000
Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member]
Long-term Debt	100,000
Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member] | Parent Company [Member]
Long-term Debt
Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member] | Otter Tail Power Company [Member]
Long-term Debt	$ 100,000

Consolidated Statements of Ca_2

Consolidated Statements of Capitalization (Parentheticals)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
The 3.55% Guaranteed Senior Notes, due December 15, 2026 [Member] | Parent Company [Member]
Long-term Debt, Interest Rate	3.55%	3.55%
Long-term Debt, Due Date	Dec. 15, 2026	Dec. 15, 2026
North Dakota Development Note, 3.95%, due April 1, 2018 [Member] | Parent Company [Member]
Long-term Debt, Interest Rate	3.95%	3.95%
Long-term Debt, Due Date	Apr. 1, 2018	Apr. 1, 2018
Partnership in Assisting Community Expansion Note, 2.54%, due March 18, 2021 [Member] | Parent Company [Member]
Long-term Debt, Interest Rate	2.54%	2.54%
Long-term Debt, Due Date	Mar. 18, 2021	Mar. 18, 2021
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member]
Long-term Debt, Interest Rate	4.63%	4.63%
Long-term Debt, Due Date	Dec. 1, 2021	Dec. 1, 2021
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member] | Otter Tail Power Company [Member]
Long-term Debt, Interest Rate	4.63%	4.63%
Long-term Debt, Due Date	Dec. 1, 2021	Dec. 1, 2021
Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member]
Long-term Debt, Interest Rate	6.15%	6.15%
Long-term Debt, Due Date	Aug. 20, 2022	Aug. 20, 2022
Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member] | Otter Tail Power Company [Member]
Long-term Debt, Interest Rate	6.15%	6.15%
Long-term Debt, Due Date	Aug. 20, 2022	Aug. 20, 2022
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member]
Long-term Debt, Interest Rate	6.37%	6.37%
Long-term Debt, Due Date	Aug. 20, 2027	Aug. 20, 2027
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member] | Otter Tail Power Company [Member]
Long-term Debt, Interest Rate	6.37%	6.37%
Long-term Debt, Due Date	Aug. 20, 2027	Aug. 20, 2027
Senior Unsecured Notes 4.68%, Series A, Due February 27, 2029 [Member]
Long-term Debt, Interest Rate	4.68%	4.68%
Long-term Debt, Due Date	Feb. 27, 2029	Feb. 27, 2029
Senior Unsecured Notes 4.68%, Series A, Due February 27, 2029 [Member] | Otter Tail Power Company [Member]
Long-term Debt, Interest Rate	4.68%	4.68%
Long-term Debt, Due Date	Feb. 27, 2029	Feb. 27, 2029
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member]
Long-term Debt, Interest Rate	6.47%	6.47%
Long-term Debt, Due Date	Aug. 20, 2037	Aug. 20, 2037
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member] | Otter Tail Power Company [Member]
Long-term Debt, Interest Rate	6.47%	6.47%
Long-term Debt, Due Date	Aug. 20, 2037	Aug. 20, 2037
Senior Unsecured Notes 5.47%, Series B, Due February 27, 2044 [Member]
Long-term Debt, Interest Rate	5.47%	5.47%
Long-term Debt, Due Date	Feb. 27, 2044	Feb. 27, 2044
Senior Unsecured Notes 5.47%, Series B, Due February 27, 2044 [Member] | Otter Tail Power Company [Member]
Long-term Debt, Interest Rate	5.47%	5.47%
Long-term Debt, Due Date	Feb. 27, 2044	Feb. 27, 2044
Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member]
Long-term Debt, Interest Rate	4.07%
Long-term Debt, Due Date	Feb. 7, 2048
Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member] | Otter Tail Power Company [Member]
Long-term Debt, Interest Rate	4.07%
Long-term Debt, Due Date	Feb. 7, 2048

Note 1 - Summary of Significant

Note 1 - Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Significant Accounting Policies and New Accounting Pronouncements [Text Block]	1. Summary of Significant Accounting Policies Principles of Consolidation The consolidated financial statements of Otter Tail Corporation and its wholly owned subsidiaries (the Company) include the accounts of the following segments: Electric, Manufacturing and Plastics. See note 2 980, Regulated Operations 980 Prior Period Re classificatio ns In 2018, 2014 09, Revenues from Contracts with Customers (Topic 606 2014 09 2017 07, Compensation—Retirement Benefits (Topic 715 2017 07 The updates in ASU 2014 09, not The updates in ASU 2017 07 $4,405,000 2017 $3,951,000 2016, $1,215,000 2017 $1,159,000 2016 $5,620,000 2017 $5,110,000 2016. Additionally, in 2018 no December 31, 2018, 2017 2016. The above reclassifications resulted in no December 31, 2017 2016. Regulation and ASC 980 The Company’s regulated electric utility company, Otter Tail Power Company (OTP), accounts for the financial effects of regulation in accordance with ASC 980. 4 OTP is subject to various state and federal agency regulations. The accounting policies followed by this business are subject to the Uniform System of Accounts of the Federal Energy Regulatory Commission (FERC). These accounting policies differ in some respects from those used by the Company’s nonelectric businesses. Plant, Retirements and Depreciation Utility plant is stated at original cost. The cost of additions includes contracted work, direct labor and materials, allocable overheads and allowance for funds used during construction. The amount of interest capitalized on electric utility plant was $1,206,000 2018, $741,000 2017 $495,000 2016. 5 82 2.76% 2018, 2.74% 2017 2.88% 2016. Property and equipment of nonelectric operations are carried at historical cost or at fair value if acquired in a business combination and are depreciated on a straight-line basis over the assets’ estimated useful lives ( 2 40 No 2018, 2017 2016. Recoverability of Long-Lived Assets The Company reviews its long-lived assets whenever events or changes in circumstances indicate the carrying amount of the assets may not Jointly Owned Facilities OTP is a joint owner in two four one December 31, 2018 2017 Jointly Owned Facilities ( dollars in thousands) OTP Ownership Percentage Electric Plant in Service Construction Work in Progress Accumulated Depreciation Net Plant December 31, 201 8 Big Stone Plant 53.9 % $ 336,051 $ 361 $ (92,007 ) $ 244,405 Coyote Station 35.0 % 177,713 2,588 (100,997 ) 79,304 Fargo–Monticello 345 kV line 14.2 % 78,184 -- (5,891 ) 72,293 Brookings–Southeast Twin Cities 345 kV line 4.8 % 26,281 -- (1,713 ) 24,568 Bemidji–Grand Rapids 230 kV line 14.8 % 16,331 -- (2,091 ) 14,240 Big Stone South–Brookings 345 kV line 50.0 % 53,235 (150 ) (1,264 ) 51,821 Big Stone South–Ellendale 345 kV line 1 50.0 % -- 106,490 -- 106,490 December 31, 2017 Big Stone Plant 53.9 % $ 329,942 $ 1,074 $ (74,165 ) $ 256,851 Coyote Station 35.0 % 177,721 158 (103,944 ) 73,935 Fargo–Monticello 345 kV line 14.2 % 78,192 -- (4,667 ) 73,525 Brookings–Southeast Twin Cities 345 kV line 4.8 % 26,269 -- (1,293 ) 24,976 Bemidji–Grand Rapids 230 kV line 14.8 % 16,331 -- (1,753 ) 14,578 Big Stone South–Brookings 345 kV line 50.0 % 53,225 -- (434 ) 52,791 Big Stone South–Ellendale 345 kV line 1 50.0 % -- 89,980 -- 89,980 1 Midcontinent Independent System Operator, Inc. (MISO) Multi-Value Project (MVP) designation provides for a return on invested funds while under construction under the MISO Open Access Transmission, Energy and Operating Reserve Markets Tariff (MISO Tariff). The Company’s share of direct revenue and expenses of the jointly owned facilities is included in operating revenue and expenses in the consolidated statements of income. Coyote Station Lignite Supply Agreement – Variable Interest Entity October 2012 May 2016 December 2040. May 2016 December 2040 No none, none not If the LSA terminates prior to the expiration of its term or the production period terminates prior to December 31, 2040 December 31, 2018 $53.9 35% Income Taxes Comprehensive interperiod income tax allocation is used for substantially all book and tax temporary differences. Deferred income taxes arise for all temporary differences between the book and tax basis of assets and liabilities. Deferred taxes are recorded using the tax rates scheduled by tax law to be in effect in the periods when the temporary differences reverse. The Company amortizes investment tax credits over the estimated lives of related property. The Company records income taxes in accordance with ASC Topic 740, Income Taxes, not” not” 50%. 14 The Company also is required to assess the realizability of its deferred tax assets, taking into consideration the Company’s forecast of future taxable income, the reversal of other existing temporary differences, available net operating loss carryforwards and available tax planning strategies that could be implemented to realize the deferred tax assets. Based on this assessment, management must evaluate the need for, and amount of, valuation allowances against the Company’s deferred tax assets. To the extent facts and circumstances change in the future, adjustments to the valuation allowance may On December 22, 2017, 2017 14 Revenue Recognition In May 2014 2014 09, Revenue from Contracts with Customers (Topic 606 606 606 January 1, 2018 not 606 no 606 606 Due to the diverse business operations of the Company, recognition of revenue from contracts with customers depends on the product produced and sold or service performed. The Company recognizes revenue from contracts with customers, at prices that are fixed or determinable as evidenced by an agreement with the customer, when the Company has met its performance obligation under the contract and it is probable that the Company will collect the amount to which it is entitled in exchange for the goods or services transferred or to be transferred to the customer. Depending on the product produced and sold or service performed and the terms of the agreement with the customer, the Company recognizes revenue either over time, in the case of delivery or transmission of electricity or related services or the production and storage of certain custom-made products, or at a point in time for the delivery of standardized products and other products made to the customers specifications where the terms of the contract require transfer of the completed product. Based on review of the Company’s revenue streams, the Company has not 606. In addition to recognizing revenue from contracts with customers under ASC 606, 980. not Electric Segment Revenues two 1 2 Most Electric segment revenues are earned from the generation, transmission and sale of electricity to retail customers at rates approved by regulatory commissions in the states where OTP provides service. OTP also earns revenue from the transmission of electricity for others over the transmission assets it owns separately, or jointly with other transmission service providers, under rate tariffs established by the independent transmission system operator and approved by the FERC. These revenues account for over 80% 2. third ARPs provide for adjustments to rates outside of a general rate case proceeding, usually as a surcharge applied to future billings typically through the use of rate riders subject to periodic adjustments, to encourage or incentivize investments in certain areas such as conservation, renewable energy, pollution reduction or control, improved infrastructure of the transmission grid or other programs that provide benefits to the general public under public policy, laws or regulations. ARP riders generally provide for the recovery of specified costs and investments and include an incentive component to provide the regulated utility with a return on amounts invested. OTP has recovered costs and earned incentives or returns on investments subject to recovery under several ARP rate riders, including: ● In Minnesota: Transmission Cost Recovery (TCR), Environmental Cost Recovery (ECR), Renewable Resource Adjustment (RRA) and Conservation Improvement Program riders. ● In North Dakota: TCR, ECR and RRA riders. ● In South Dakota: TCR, ECR and Energy Efficiency Plan (conservation) riders. OTP accrues ARP revenue on the basis of costs incurred, investments made and returns on those investments that qualify for recovery through established riders. Amounts billed under riders in effect at the time of the billing are included in revenues from contracts with customers net of amounts billed that are subject to refund through future rider adjustments. Amounts accrued and subject to recovery through future rider rate updates and adjustments are reported as ARP revenue adjustments on a separate line in the revenue section of the Company’s consolidated statement of income. See table in note 3 December 31, 2018, 2017 2016. Manufacturing Segment Revenues company expects the customer will take. For revenue recognized on products when shipped, the operating companies have no Plastics Segment Revenues no one See operating revenue table in note 2 December 31, 2018, 2017 2016. Agreements Subject to Legally Enforceable Netting Arrangements OTP has certain derivative contracts that are designated as normal purchases. Individual counterparty exposures for these contracts can be offset according to legally enforceable netting arrangements. The Company does not Warranty Reserves Certain products sold by the Company’s manufacturing and plastics companies carry product warranties for one no not three Shipping and Handling Costs The Company includes revenues received for shipping and handling in operating revenues. Expenses paid for shipping and handling are recorded as part of cost of goods sold. U se of Estimates The Company uses estimates based on the best information available in recording transactions and balances resulting from business operations. As better information becomes available (or actual amounts are known), the recorded estimates are revised. Consequently, operating results can be affected by revisions to prior accounting estimates. Cash Equivalents The Company considers all highly liquid debt instruments purchased with maturity of 90 Investments The following table provides a breakdown of the Company’s investments at December 31: (in thousands) 2018 2017 Cost Method: Economic Development Loan Pools $ 34 $ 45 Other 123 115 Equity Method Partnerships 26 24 Marketable Debt Securities Classified as Available-for-Sale 7,484 7,160 Marketable Equity Securities Classified as Available-for-Sale 1,294 1,285 Total Investments $ 8,961 $ 8,629 The Company’s marketable securities classified as available-for-sale are held for insurance purposes and are reflected at their fair values on December 31, 2018. Fair Value Measurements The Company follows ASC Topic 820, Fair Value Measurements and Disclosures 820 820 three Level 1 1 Level 2 2 Level 3 no 3 may The following tables present, for each of the hierarchy levels, the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2018 December 31, 2017: Dec ember 3 1 , 201 8 (in thousands) Level 1 Level 2 Level 3 Assets: Investments: Equity Funds – Held by Captive Insurance Company $ 1,294 Corporate Debt Securities – Held by Captive Insurance Company $ 5,898 Government-Backed and Government-Sponsored Enterprises’ Debt Securities – Held by Captive Insurance Company 1,586 Other Assets: Money Market and Mutual Funds – Nonqualified Retirement Savings Plan 838 Total Assets $ 2,132 $ 7,484 December 31, 201 7 (in thousands) Level 1 Level 2 Level 3 Assets: Investments: Equity Funds – Held by Captive Insurance Company $ 1,285 Corporate Debt Securities – Held by Captive Insurance Company $ 5,373 Government-Backed and Government-Sponsored Enterprises’ Debt Securities – Held by Captive Insurance Company 1,787 Other Assets: Money Market and Mutual Funds – Nonqualified Retirement Savings Plan 823 Total Assets $ 2,108 $ 7,160 The valuation techniques and inputs used for the Level 2 Government-Backed and Government-Sponsored Enterprises’ and Corporate Debt Securities Held by the Company’s Captive Insurance Company third may Inventories Inventories, valued at the lower of cost or net realizable value, consist of the following: December 31, December 31, (in thousands) 2018 2017 Finished Goods $ 37,130 $ 26,605 Work in Process 20,393 14,222 Raw Material, Fuel and Supplies 48,747 47,207 Total Inventories $ 106,270 $ 88,034 Goodwill and Other Intangible Assets The Company accounts for goodwill and other intangible assets in accordance with the requirements of ASC Topic 350, Intangibles—Goodwill and Other, fourth may not The following tables summarize changes to goodwill by business segment during 2018 2017: (in thousands) Gross Balance December 31, 2017 Accumulated Impairments Balance (net of impairments) December 31, 2017 Adjustments to Goodwill in 2018 Balance (net of impairments) December 31, 2018 Manufacturing $ 18,270 $ -- $ 18,270 $ -- $ 18,270 Plastics 19,302 -- 19,302 -- 19,302 Total $ 37,572 $ -- $ 37,572 $ -- $ 37,572 (in thousands) Gross Balance December 31, 2016 Accumulated Impairments Balance (net of impairments) December 31, 2016 Adjustments to Goodwill in 2017 Balance (net of impairments) December 31, 2017 Manufacturing $ 18,270 $ -- $ 18,270 $ -- $ 18,270 Plastics 19,302 -- 19,302 -- 19,302 Total $ 37,572 $ -- $ 37,572 $ -- $ 37,572 Intangible assets with finite lives are amortized over their estimated useful lives and reviewed for impairment in accordance with requirements under ASC Topic 360 10 35, Property, Plant, and Equipment—Overall—Subsequent Measurement The following table summarizes the components of the Company’s intangible assets at December 31, 2018 December 31, 2017: December 31, 201 8 (in thousands) Gross Carrying Amount Accumulated Amortization Net Carrying Amount Remaining Amortization Periods (months) Amortizable Intangible Assets: Customer Relationships $ 22,491 $ 10,127 $ 12,364 12-200 Other 154 68 86 20 Total $ 22,645 $ 10,195 $ 12,450 December 31, 201 7 (in thousands) Amortizable Intangible Assets: Customer Relationships $ 22,491 $ 8,994 $ 13,497 24-212 Covenant not to Compete 590 459 131 8 Other 154 17 137 32 Total $ 23,235 $ 9,470 $ 13,765 The amortization expense for these intangible assets was: (in thousands) 2018 2017 2016 Amortization Expense – Intangible Assets $ 1,315 $ 1,347 $ 1,436 The estimated annual amortization expense for these intangible assets for the next five (in thousands) 2019 2020 2021 2022 2023 Estimated Amortization Expense – Intangible Assets $ 1,184 $ 1,133 $ 1,099 $ 1,099 $ 1,099 Supplemental Disclosures of Cash Flow Information As of December 31, (in thousands) 2018 2017 Noncash Investing Activities: Transactions Related to Capital Additions not Settled in Cash $ 13,757 $ 13,887 (in thousands) 2018 2017 2016 Cash Paid (Received) During the Year for: Interest (net of amount capitalized) $ 28,109 $ 29,791 $ 31,269 Income Taxes $ 6,109 $ 5,064 $ (1,291 ) New Accounting Standards Adopted ASU 2014 09 May 2014 2014 09, Revenue from Contracts with Customers (Topic 606 606 January 1, 2018 ASU 2016 01 January 2016 No. 2016 01, Financial Instruments—Overall (Subtopic 825 10 2016 01 2016 01 2016 01 December 15, 2017, 2016 01 first 2018, ASU 2017 07 March 2017 2017 07 715, Compensation—Retirement Benefits 715 , not not 2017 07 715 2017 07 2017 07 December 15, 2017, January 1, 2018. The majority of the Company’s benefit costs to which the amendments in ASU 2017 07 2017 07 2017 07. The Company’s nonservice cost components of net periodic postretirement benefit costs that were capitalized to plant in service in 2017 2017 07 2017 $0.8 2017 2016 December 31, 2018, 2017 2016 11 New Accounting Standards Pending Adoption ASU 2016 02 February 2016 No. 2016 02, Leases (Topic 842 2016 02 2016 02 842, 840 842 842 842 842 2016 02 December 15, 2018, 2016 02 2016 02 first 2019. first not 8 ASU 2017 04 January 2017 No. 2017 04, Intangibles—Goodwill and Other (Topic 350 2017 04 2 2 2, 2017 04, not The amendments in ASU 2017 04 no 2 2017 04 December 15, 2019. January 1, 2017. 2017 04 2019. 2018, no no 2 December 31, 2018. ASU 2018 02 February 2018 No. 2018 02, Income Statement—Reporting Comprehensive Income (Topic 220 2018 02 2018 02, 2018 02 December 15, 2018, 2018 02 2018 02 first 2019 not $784,000

Note 2 - Business Segment Infor

Note 2 - Business Segment Information	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Segment Reporting Disclosure [Text Block]	2 . Business Segment Information The accounting policies of the segments are described under note 1 three three Electric includes the production, transmission, distribution and sale of electric energy in Minnesota, North Dakota and South Dakota by OTP. In addition, OTP is a participant in the Midcontinent Independent System Operator, Inc. (MISO) markets. OTP’s operations have been the Company’s primary business since 1907. Manufacturing consists of businesses in the following manufacturing activities: contract machining, metal parts stamping, fabrication and painting, and production of plastic thermoformed horticultural containers, life science and industrial packaging, and material handling components. These businesses have manufacturing facilities in Georgia, Illinois and Minnesota and sell products primarily in the United States. Plastics consists of businesses producing PVC pipe at plants in North Dakota and Arizona. The PVC pipe is sold primarily in the upper Midwest and Southwest regions of the United States. OTP is a wholly owned subsidiary of the Company. All of the Company’s other businesses are owned by its wholly owned subsidiary, Varistar Corporation. The Company’s Corporate operating costs include items such as corporate staff and overhead costs, the results of the Company’s captive insurance company and other items excluded from the measurement of operating segment performance. Corporate assets consist primarily of cash, prepaid expenses, investments and fixed assets. Corporate is not No 10% 2018, 2017 2016. no 10% 2018, 2018 one 11.2% 2018 one 22.2% 2018 one 11.2% 2018 five 52% 2018 two 22.1% 17.0% 2018 one All the Company’s long-lived assets are within the United States and sales within the United States accounted for 98.4% 2018, 98.2% 2017 98.6% 2016. The Company evaluates the performance of its business segments and allocates resources to them based on earnings contribution and return on total invested capital. Information for the business segments for 2018, 2017 2016 (in thousands) 2018 2017 2016 Operating Revenue Electric Segment: Retail Sales Revenue from Contracts with Customers $ 388,690 $ 376,902 $ 374,506 Changes in Accrued ARP Revenues (439 ) (1,971 ) 2,104 Total Retail Sales Revenue 388,251 374,931 376,610 Wholesale Revenues – Company Generation 7,735 5,173 4,584 Other Electric Revenues 54,269 54,433 46,189 Total Electric Segment Revenues 450,255 434,537 427,383 Manufacturing Segment: Metal Parts and Tooling 223,765 189,242 185,868 Plastic Products and Tooling 35,836 33,939 31,431 Other 8,808 6,557 3,990 Total Manufacturing Segment Revenues 268,409 229,738 221,289 Plastics Segment – Sale of PVC Pipe Products 197,840 185,132 154,901 Intersegment Eliminations (57 ) (57 ) (34 ) Total $ 916,447 $ 849,350 $ 803,539 Cost of Products Sold Manufacturing $ 205,699 $ 176,473 $ 171,732 Plastics 148,881 140,107 123,496 Intersegment Eliminations (21 ) (18 ) (6 ) Total $ 354,559 $ 316,562 $ 295,222 Other Nonelectric Expenses Manufacturing $ 29,650 $ 23,785 $ 21,994 Plastics 12,323 11,564 9,402 Corporate 9,607 6,182 7,315 Intersegment Eliminations (36 ) (39 ) (28 ) Total $ 51,544 $ 41,492 $ 38,683 Depreciation and Amortization Electric $ 55,935 $ 53,276 $ 53,743 Manufacturing 14,794 15,379 15,794 Plastics 3,719 3,817 3,861 Corporate 218 73 47 Total $ 74,666 $ 72,545 $ 73,445 Operating Income (Loss) Electric $ 88,031 $ 94,797 $ 94,082 Manufacturing 18,266 14,101 11,769 Plastics 32,917 29,644 18,142 Corporate (9,825 ) (6,255 ) (7,362 ) Total $ 129,389 $ 132,287 $ 116,631 Interest Charges Electric $ 26,365 $ 25,334 $ 25,069 Manufacturing 2,230 2,215 3,859 Plastics 609 633 1,034 Corporate and Intersegment Eliminations 1,204 1,422 1,924 Total $ 30,408 $ 29,604 $ 31,886 (in thousands) 2018 2017 2016 Income Tax Expense (Benefit) Electric $ 5,685 $ 17,013 $ 16,366 Manufacturing 3,393 989 2,276 Plastics 8,728 7,448 6,538 Corporate (3,218 ) 1,806 (4,961 ) Total $ 14,588 $ 27,256 $ 20,219 Net Income (Loss) Electric $ 54,431 $ 49,446 $ 49,829 Manufacturing 12,839 11,050 5,694 Plastics 23,819 21,696 10,628 Corporate (8,744 ) (9,753 ) (3,830 ) Total $ 82,345 $ 72,439 $ 62,321 Capital Expenditures Electric $ 87,287 $ 118,444 $ 149,648 Manufacturing 13,316 9,916 8,429 Plastics 4,199 4,432 3,085 Corporate 623 121 97 Total $ 105,425 $ 132,913 $ 161,259 Identifiable Assets Electric $ 1,728,534 $ 1,690,224 $ 1,622,231 Manufacturing 187,556 167,023 166,525 Plastics 91,630 87,230 84,592 Corporate 44,797 59,801 39,037 Total $ 2,052,517 $ 2,004,278 $ 1,912,385

Note 3 - Rate and Regulatory Ma

Note 3 - Rate and Regulatory Matters	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Public Utilities Disclosure [Text Block]	3. Below are descriptions of OTP’s major capital expenditure projects that have had, or are expected to have, a significant impact on OTP’s revenue requirements, rates and alternative revenue recovery mechanisms, followed by summaries of specific electric rate or rider proceedings with the Minnesota Public Utilities Commission (MPUC), the North Dakota Public Service Commission (NDPSC), the South Dakota Public Utilities Commission (SDPUC) and the FERC, impacting OTP’s revenues in 2018, 2017 2016. Major Capital Expenditure Projects Merricourt Project November 16, 2016 150 $34.7 November 16, 2016, 2020 $200.5 2019. December 31, 2018, $4.9 November 3, 2017. October 26, 2017 Astoria Station 250 2021. December 31, 2018, $8.3 August 3, 2018 November 3, 2017, December 2018 January 2019. September 26, 2018 Big Stone South–Ellendale Multi-Value Transmission Project (MVP) 345 163 December 2011. second 2016 February 6, 2019. December 31, 2018 $106 100% Big Stone South–Brookings 345 MVP $73 100% 50.0% 70 2017. Recovery of OTP’s major transmission investments is through the MISO Tariff (several as MVPs) and, currently, Minnesota, North Dakota and South Dakota base rates and Transmission Cost Recovery (TCR) Riders. Reagent Costs OTP’s systemwide costs for reagents are expected to increase to approximately $2.2 May 2021 50%, 40% 10%, 2015 Minnesota General Rates 2016 March 2017 May 1, 2017. 8.61% 7.5056% 10.74% 9.41%. The MPUC’s order also included: ( 1 2 November 1, 2017. OTP accrued interim and rider rate refunds until final rates became effective. The final interim rate refund, including interest, of $9.0 November 17, 2017. 1 2 9.41% 2016 April 16, 2016, $0.9 $1.4 12 November 2017 Minnesota Conservation Improvement Programs (MNCIP) 1.5% The Minnesota Department of Commerce (MNDOC) may may not On May 25, 2016 13.5% 2017 12% 2018 10% 2019 1.7% 40% 2017 35% 2018 30% 2019 50%   Based on results from the 2016 $5.1 2016, $0.1 2015 2016 18% 2015 March 31, 2017 2016 not $5.0 September 15, 2017 October 1, 2017. Based on results from the 2017 $2.6 2017. 2017 2016 10%. 2017 not March 30, 2018. $2.6 June 13, 2018 $2.9 October 4, 2018, $2.9 no $0.3 Based on results from the 2018 $3.0 $3.15 2018. 2018 not $3.15 April 1, 2019. Transmission Cost Recovery Rider may may In OTP’s 2016 May 1, 2017, two August 18, 2017 On June 11, 2018 July 11, 2018 second third 2019. On November 30, 2018 two two June 1, 2019 first second 2019. second 2019, December 31, 2018 $2.3 Environmental Cost Recovery Rider 2010 2016 November 2017. 2018 zero December 1, 2018. Reagent Costs and Emission Allowances 2016 2016 May 1, 2017 North Dakota General Rates November 2, 2017 $13.1 8.72%. $13.1 7.97% 10.3%. December 20, 2017 $12.8 January 1, 2018. February 27, 2018 $4.5 $8.3 March 1, 2018. On March 23, 2018 $13.1 $7.1 4.8% $6.0 $4.8 $1.2 In a September 26, 2018 $4.6 3.1% 9.77% 52.5% March 2018 $7.1 4.8% 10.3%. not February 1, 2019, April 2019 $3.0 December 31, 2018, $0.8 January February 2018. OTP’s previously approved general rate increase in North Dakota of $3.6 3.0%, November 2009 December 2009. 8.62%, 10.75%. Renewable Resource Adjustment Effective in February 2019 2017 Transmission Cost Recovery Rider 2017 26 26A 2017 Environmental Cost Recovery Rider 2017 February 1, 2019, February 1, 2019 South Dakota General Rates April 20, 2018 $3.3 10.1%, first two October 18, 2018. February 5, 2019 March 26- 28, 2019. second 1.7% February 15, 2019 OTP’s previously approved general rate increase in South Dakota of approximately $643,000 2.32% April 2011 June 2011. 8.50%. Transmission Cost Recovery Rider January 29, 2018 October 18, 2018, 2018 Environmental Cost Recovery Rider October 18, 2018 Reagent Costs and Emission Allowances R ate Rider Updates The following table provides summary information on the status of updates since January 1, 2015 Rate Rider R - Request Date A - Approval Date Effective Date Requested or Approved Annual Revenue ($000s) Rate Minnesota Conservation Improvement Program 2017 Incentive and Cost Recovery A – October 4, 2018 November 1, 2018 $ 10,283 $0.00600/kwh 2016 Incentive and Cost Recovery A – September 15, 2017 October 1, 2017 $ 9,868 $0.00536/kwh 2015 Incentive and Cost Recovery A – July 19, 2016 October 1, 2016 $ 8,590 $0.00275/kwh 2014 Incentive and Cost Recovery A – July 10, 2015 October 1, 2015 $ 8,689 $0.00287/kwh Transmission Cost Recovery 2018 Annual Update–Scenario A R – November 30, 2018 June 1, 2019 $ 6,475 Various –Scenario B $ 2,708 Various 2017 Rate Reset A – October 30, 2017 November 1, 2017 $ (3,311 ) Various 2016 Annual Update A – July 5, 2016 September 1, 2016 $ 4,736 Various 2015 Annual Update A – March 9, 2016 April 1, 2016 $ 7,203 Various 2014 Annual Update A – February 18, 2015 March 1, 2015 $ 8,388 Various Environmental Cost Recovery 2018 Annual Update A – November 29, 2018 December 1, 2018 $ -- 0% of base 2017 Rate Reset A – October 30, 2017 November 1, 2017 $ (1,943 ) -0.935% of base 2016 Annual Update A – July 5, 2016 September 1, 2016 $ 11,884 6.927% of base 2015 Annual Update A – March 9, 2016 October 1, 2015 $ 12,104 7.006% of base Renewable Resource Adjustment 2018 Annual Update A – August 29, 2018 November 1, 2018 $ 5,886 $.00244/kwh 2017 Rate Reset A – October 30, 2017 November 1, 2017 $ 1,279 $.00049/kwh North Dakota Renewable Resource Adjustment 2019 Annual Update R – December 31, 2018 April 1, 2019 $ (236 ) -0.224% of base 2018 Rate Reset for effect of TCJA A – February 27, 2018 March 1, 2018 $ 9,650 7.493% of base 2017 Rate Reset A – December 20, 2017 January 1, 2018 $ 9,989 7.756% of base 2016 Annual Update A – March 15, 2017 April 1, 2017 $ 9,156 7.005% of base 2015 Annual Update A – June 22, 2016 July 1, 2016 $ 9,262 7.573% of base 2014 Annual Update A – March 25, 2015 April 1, 2015 $ 5,441 4.069% of base Transmission Cost Recovery 2018 Supplemental Update A – December 6, 2018 February 1, 2019 $ 4,801 Various 2018 Rate Reset for effect of TCJA A – February 27, 2018 March 1, 2018 $ 7,469 Various 2017 Annual Update A – November 29, 2017 January 1, 2018 $ 7,959 Various 2016 Annual Update A – December 14, 2016 January 1, 2017 $ 6,916 Various 2015 Annual Update A – December 16, 2015 January 1, 2016 $ 9,985 Various Environmental Cost Recovery 2018 Update A – December 19, 2018 February 1, 2019 $ (378 ) -0.310% of base 2018 Rate Reset for effect of TCJA A – February 27, 2018 March 1, 2018 $ 7,718 5.593% of base 2017 Rate Reset A – December 20, 2017 January 1, 2018 $ 8,537 6.629% of base 2017 Annual Update A – July 12, 2017 August 1, 2017 $ 9,917 7.633% of base 2016 Annual Update A – June 22, 2016 July 1, 2016 $ 10,359 7.904% of base 2015 Annual Update A – June 17, 2015 July 1, 2015 $ 12,249 9.193% of base South Dakota Transmission Cost Recovery 2018 Interim Rate Reset A – October 18, 2018 October 18, 2018 $ 1,171 Various 2017 Annual Update A – February 28, 2018 March 1, 2018 $ 1,779 Various 2016 Annual Update A – February 17, 2017 March 1, 2017 $ 2,053 Various 2015 Annual Update A – February 12, 2016 March 1, 2016 $ 1,895 Various 2014 Annual Update A – February 13, 2015 March 1, 2015 $ 1,538 Various Environmental Cost Recovery 2018 Interim Rate Reset A – October 18, 2018 October 18, 2018 $ (189 ) -$0.00075/kwh 2017 Annual Update A – October 13, 2017 November 1, 2017 $ 2,082 $0.00483/kwh 2016 Annual Update A – October 26, 2016 November 1, 2016 $ 2,238 $0.00536/kwh 2015 Annual Update A – October 15, 2015 November 1, 2015 $ 2,728 $0.00643/kwh Revenues Recorded under Rate Riders The following table presents revenue recorded by OTP under rate riders in place in Minnesota, North Dakota and South Dakota for the years ended December 31: Rate Rider (in thousands) 2018 2017 2016 Minnesota Conservation Improvement Program Costs and Incentives 1 $ 12,028 $ 9,225 $ 12,920 Renewable Resource Adjustment 3,067 (196 ) -- Environmental Cost Recovery (24 ) 8,148 12,443 Transmission Cost Recovery (2,039 ) 2,973 5,795 North Dakota Renewable Resource Adjustment 8,529 7,620 7,800 Environmental Cost Recovery 7,318 9,782 11,089 Transmission Cost Recovery 7,016 8,729 7,694 South Dakota Environmental Cost Recovery 1,676 2,345 2,538 Transmission Cost Recovery 1,664 1,843 1,820 Conservation Improvement Program Costs and Incentives 628 598 468 1 Includes MNCIP costs recovered in base rates. TCJA The TCJA, passed in December 2017, 35% 21%, January 1, 2018. 35% The MPUC required regulated utilities providing service in Minnesota to make filings by February 15, 2018. August 9, 2018 December 5, 2018 one ten 2017 second 2019, January 3, 2019 As described above, OTP’s current general rate cases in North Dakota and South Dakota reflect the ongoing impact of the TCJA in interim rates. OTP has accrued refund liabilities for the time periods when revenues were collected under rates set to recover higher levels of federal income taxes than OTP incurs under the lower federal tax rates in the TCJA. As of December 31, 2018, $8.4 $0.8 January February 2018, $1.0 October 18, 2018, $0.2 As of March 15, 2018, 2018 F ERC Wholesale power sales and transmission rates are subject to the jurisdiction of the FERC under the Federal Power Act of 1935 MVPs   On November 12, 2013 may 12.38% 9.15%. 15 November 12, 2013 February 11, 2015. December 22, 2015 10.32%, September 28, 2016 10.32%. September 2016 On November 6, 2014 50 January 5, 2015 November 12, 2013 10.82% 10.32% 0.5% September 28, 2016. On February 12, 2015 may 12.38% 8.67%. second second 15 February 12, 2015 May 11, 2016. June 18, 2015 February 16, 2016. June 30, 2016 9.7%. second Based on the probable reduction by the FERC in the ROE component of the MISO Tariff, OTP had a $2.7 December 31, 2016, first 15 February June 2017 2016 $2.7 December 31, 2016 $1.6 December 31, 2018. In June 2014, two two April 2017 June 2014 not June 2014 April 2017 September 29, 2017 second On October 16, 2018 April 2017. 10.41%, 13.08% 206 November 15, 2018, two February 13, 2019 April 10, 2019. OTP believes its estimated accrued MISO Tariff ROE refund liability of $1.6 December 31, 2018 second

Note 4 - Regulatory Assets and

Note 4 - Regulatory Assets and Liabilities	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Schedule of Regulatory Assets and Liabilities [Text Block]	4. As a regulated entity, OTP accounts for the financial effects of regulation in accordance with ASC 980. 980 605 25 December 31, 2018 Remaining Recovery/ (in thousands) Current Long-Term Total Refund Period (months) Regulatory Assets: Prior Service Costs and Actuarial Losses on Pensions and Other Postretirement Benefits 1 $ 6,346 $ 118,433 $ 124,779 see below Conservation Improvement Program Costs and Incentives 2 5,995 3,285 9,280 21 Accumulated ARO Accretion/Depreciation Adjustment 1 -- 7,169 7,169 asset lives Deferred Income Taxes 1 -- 2,423 2,423 asset lives Deferred Marked-to-Market Losses 1 1,661 743 2,404 24 Big Stone II Unrecovered Project Costs – Minnesota 1 681 947 1,628 28 Nonservice Costs Components of Postretirement Benefits Capitalized for Ratemaking Purposes and Subject to Deferred Recovery 1 -- 986 986 asset lives Debt Reacquisition Premiums 1 207 753 960 165 North Dakota Deferred Rate Case Expenses Subject to Recovery 1 455 -- 455 12 Minnesota Renewable Resource Recovery Rider Accrued Revenues 2 452 -- 452 12 Minnesota Transmission Cost Recovery Rider Accrued Revenues 2 444 -- 444 12 Big Stone II Unrecovered Project Costs – South Dakota 1 100 342 442 53 Minnesota Energy Intensive Trade Exposed Rider Accrued Revenues 1 328 -- 328 4 MISO Schedule 26/26A Transmission Cost Recovery Rider True-up 1 240 -- 240 12 South Dakota Deferred Rate Case Expenses Subject to Recovery 1 178 -- 178 12 Minnesota SPP Transmission Cost Recovery Tracker 1 -- 176 176 see below Minnesota Environmental Cost Recovery Rider Accrued Revenues 2 121 -- 121 12 North Dakota Environmental Cost Recovery Rider Accrued Revenues 2 17 -- 17 12 Total Regulatory Assets $ 17,225 $ 135,257 $ 152,482 Regulatory Liabilities: Deferred Income Taxes $ -- $ 142,779 $ 142,779 asset lives Accumulated Reserve for Estimated Removal Costs – Net of Salvage -- 83,229 83,229 asset lives South Dakota Environmental Cost Recovery Rider Accrued Refund 207 -- 207 12 MISO Schedule 26/26A Transmission Cost Recovery Rider True-up -- 187 187 24 North Dakota Renewable Resource Recovery Rider Accrued Refund 177 -- 177 12 South Dakota Transmission Cost Recovery Rider Accrued Refund 168 -- 168 12 Revenue for Rate Case Expenses Subject to Refund – Minnesota -- 166 166 see below Refundable Fuel Clause Adjustment Revenues 121 -- 121 12 North Dakota Transmission Cost Recovery Rider Accrued Refund 60 -- 60 12 Other 5 108 113 180 Total Regulatory Liabilities $ 738 $ 226,469 $ 227,207 Net Regulatory Asset/(Liability) Position $ 16,487 $ (91,212 ) $ (74,725 ) 1 Costs subject to recovery without a rate of return. 2 Amount eligible for recovery under an alternative revenue program which includes an incentive or rate of return. December 31, 2017 Remaining Recovery/ (in thousands) Current Long-Term Total Refund Period (months) Regulatory Assets: Prior Service Costs and Actuarial Losses on Pensions and Other Postretirement Benefits 1 $ 9,090 $ 112,487 $ 121,577 see below Conservation Improvement Program Costs and Incentives 2 7,385 2,774 10,159 21 Accumulated ARO Accretion/Depreciation Adjustment 1 -- 6,651 6,651 asset lives Deferred Marked-to-Market Losses 1 4,063 2,405 6,468 36 Big Stone II Unrecovered Project Costs – Minnesota 1 650 1,636 2,286 40 Debt Reacquisition Premiums 1 254 960 1,214 177 North Dakota Deferred Rate Case Expenses Subject to Recovery 1 309 -- 309 12 Big Stone II Unrecovered Project Costs – South Dakota 1 100 442 542 65 Minnesota Energy Intensive Trade Exposed Rider Accrued Revenues 1 75 -- 75 12 MISO Schedule 26/26A Transmission Cost Recovery Rider True-up 1 -- 1,985 1,985 24 North Dakota Environmental Cost Recovery Rider Accrued Revenues 2 152 -- 152 12 Minnesota Deferred Rate Case Expenses Subject to Recovery 1 267 -- 267 4 North Dakota Renewable Resource Recovery Rider Accrued Revenues 2 206 236 442 15 Total Regulatory Assets $ 22,551 $ 129,576 $ 152,127 Regulatory Liabilities: Deferred Income Taxes $ -- $ 149,052 $ 149,052 asset lives Accumulated Reserve for Estimated Removal Costs – Net of Salvage -- 83,100 83,100 asset lives South Dakota Environmental Cost Recovery Rider Accrued Refund 187 -- 187 12 MISO Schedule 26/26A Transmission Cost Recovery Rider True-up 132 48 180 24 South Dakota Transmission Cost Recovery Rider Accrued Refund 151 -- 151 12 Revenue for Rate Case Expenses Subject to Refund – Minnesota 208 -- 208 4 Refundable Fuel Clause Adjustment Revenues 5,778 -- 5,778 12 North Dakota Transmission Cost Recovery Rider Accrued Refund 349 -- 349 12 Minnesota Environmental Cost Recovery Rider Accrued Refund 1,667 -- 1,667 11 Minnesota Transmission Cost Recovery Rider Accrued Refund 802 609 1,411 22 Minnesota Renewable Resource Recovery Rider Accrued Refund 409 -- 409 12 Other 5 84 89 192 Total Regulatory Liabilities $ 9,688 $ 232,893 $ 242,581 Net Regulatory Asset/(Liability) Position $ 12,863 $ (103,317 ) $ (90,454 ) 1 Costs subject to recovery without a rate of return. 2 Amount eligible for recovery under an alternative revenue program which includes an incentive or rate of return. The regulatory asset related to prior service costs and actuarial losses on pensions and other postretirement benefits represents benefit costs and actuarial losses subject to recovery through rates as they are expensed over the remaining service lives of active employees included in the plans. These unrecognized benefit costs and actuarial losses are required to be recognized as components of Accumulated Other Comprehensive Income in equity under ASC Topic 715, Compensation—Retirement Benefits Conservation Improvement Program Costs and Incentives represent mandated conservation expenditures and incentives recoverable through retail electric rates. The Accumulated Asset Retirement Obligation (ARO) Accretion/Depreciation Adjustment will accrete and be amortized over the lives of property with asset retirement obligations. The regulatory asset and liability related to Deferred Income Taxes results from changes in statutory tax rates accounted for in accordance with ASC Topic 740, Income Taxes All Deferred Marked-to-Market Losses recorded as of December 31, 2018 December 2020. Big Stone II Unrecovered Project Costs – Minnesota are the Minnesota share of generation and transmission plant-related costs incurred by OTP related to its participation in the abandoned Big Stone II project. The Nonservice Costs Components of Postretirement Benefits Capitalized for Ratemaking Purposes and Subject to Deferred Recovery are employee benefit-related costs that are required to be capitalized for ratemaking purposes and are recovered over the depreciable lives of the assets to which the related labor costs were applied. Debt Reacquisition Premiums are being recovered from OTP customers over the remaining original lives of the reacquired debt issues, the longest of which is 165 North Dakota Deferred Rate Case Expenses Subject to Recovery relate to costs incurred in conjunction with OTP’s current rate case in North Dakota currently being recovered beginning with the establishment of interim rates in January 2018. Minnesota Renewable Resource Recovery Rider Accrued Revenues relate to an increase in renewable revenue requirements resulting from the expiration of tax credits for certain wind turbines. The balance represents amounts subject to recovery from Minnesota customers that have not December 31, 2018. The Minnesota Transmission Cost Recovery Rider Accrued Revenues relate to revenues earned on qualifying transmission system facilities and operating costs incurred to serve Minnesota customers that are recoverable from Minnesota customers as of December 31, 2018. Big Stone II Unrecovered Project Costs – South Dakota are the South Dakota share of generation and transmission plant-related costs incurred by OTP related to its participation in the abandoned Big Stone II project. Minnesota Energy Intensive Trade Exposed Rider Accrued Revenues relate to revenues recorded for fuel and purchased power costs reductions provided to customers in energy intensive trade exposed industries that are subject to recovery from other Minnesota customers. MISO Schedule 26/26A 26/26A South Dakota Deferred Rate Case Expenses Subject to Recovery relate to costs incurred in conjunction with OTP’s current rate case in South Dakota and are currently being recovered beginning with the establishment of interim rates in October 2018. The Minnesota SPP Transmission Cost Recovery Tracker regulatory asset relates to costs incurred to serve Minnesota customers that are subject to recovery but that have not December 31, 2018. The Minnesota Environmental Cost Recovery Rider Accrued Revenues relate to revenues earned on the Minnesota share of OTP’s investment in the Big Stone Plant AQCS project that are recoverable from Minnesota customers as of December 31, 2018. North Dakota Environmental Cost Recovery Rider Accrued Revenues relate to revenues earned on the North Dakota share of OTP’s investments in the Big Stone Plant AQCS and Hoot Lake Plant MATS projects and for reagent and emission allowances costs that are recoverable from North Dakota customers as of December 31, 2018. Minnesota Deferred Rate Case Expenses Subject to Recovery relate to costs incurred in conjunction with OTP’s 2016 24 April 2016. North Dakota Renewable Resource Recovery Rider Accrued Revenues relate to qualifying renewable resource costs incurred to serve North Dakota customers that had not December 31, 2017. The Accumulated Reserve for Estimated Removal Costs – Net of Salvage is reduced as actual removal costs, net of salvage revenues, are incurred. The South Dakota Environmental Cost Recovery Rider Accrued Refund relates to amounts collected on the South Dakota share of OTP’s investments in the Big Stone Plant AQCS and Hoot Lake Plant MATS projects that are refundable to South Dakota customers as of December 31, 2018. North Dakota Renewable Resource Recovery Rider Accrued Refund relates to amounts collected for qualifying renewable resource costs incurred to serve North Dakota customers that are refundable to North Dakota customers as of December 31, 2018. The South Dakota Transmission Cost Recovery Rider Accrued Refund relates to amounts collected for qualifying transmission system facilities and operating costs incurred to serve South Dakota customers that are refundable to South Dakota customers as of December 31, 2018. Revenue for Rate Case Expenses Subject to Refund – Minnesota relates to revenues collected under general rates to recover costs related to prior rate case proceedings in excess of the actual costs incurred, which were subject to refund over a 24 April 2016. The North Dakota Transmission Cost Recovery Rider Accrued Refund relates to amounts collected for qualifying transmission system facilities and operating costs incurred to serve North Dakota customers that are refundable to North Dakota customers as of December 31, 2018. The Minnesota Environmental Cost Recovery Rider Accrued Refund relates to amounts collected on the Minnesota share of OTP’s investment in the Big Stone Plant AQCS project that were refundable to Minnesota customers as of December 31, 2017. The Minnesota Transmission Cost Recovery Rider Accrued Refund relates to amounts collected for qualifying transmission system facilities and operating costs incurred to serve Minnesota customers that were refundable to Minnesota customers as of December 31, 2017. The Minnesota Renewable Resource Recovery Rider Accrued Refund relates to amounts collected for qualifying renewable resource costs incurred to serve Minnesota customers that were refundable to Minnesota customers as of December 31, 2017. If for any reason OTP ceases to meet the criteria for application of guidance under ASC 980 no 980

Note 5 - Common Shares and Earn

Note 5 - Common Shares and Earnings Per Share	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Stockholders Equity and Earnings per Share [Text Block]	5 . Common Shares and Earnings p er Share Shelf Registration and Common Share Distribution Agreement On May 3, 2018 may May 3, 2021. May 1, 2018 201 8 Common Stock Activity Following is a reconciliation of the Company’s common shares outstanding from December 31, 2017 December 31, 2018: Common Shares Outstanding, December 31, 2017 39,557,491 Issuances: Executive Stock Performance Awards (2015 awards) 114,648 Executive Stock Performance Awards (2016 and 2017 awards) 18,600 Vesting of Restricted Stock Units 26,575 Restricted Stock Issued to Directors 18,200 Directors Deferred Compensation 578 Retirements: Shares Withheld for Individual Income Tax Requirements (71,208 ) Common Shares Outstanding, December 31, 2018 39,664,884 2014 Stock Incentive Plan The 2014 2014 April 2014, 1,900,000 2014 1,121,330 December 31, 2018. 2014 December 13, 2023. Employee Stock Purchase Plan The 1999 85% six December 31, 2016. January 1, 2017, 100% six April 16, 2012, 900,000 1,400,000 1,400,000 366,867 December 31, 2018. 7,757 2018, 4,202 5,284 2017 53,875 2016. Dividend Reinvestment and Share Purchase Plan On May 3, 2018, 1,500,000 May 3, 2021. 2018, 116,822 1,383,178 December 31, 2018. 1,500,000 53,853 2018 87,634 2017. 97,698 2017 278,811 2016. Earnings Per Share The numerator used in the calculation of both basic and diluted earnings per common share is net income with no 2018, 2017 2016. not 2018 2017 2016 Weighted Average Common Shares Outstanding – Basic 39,599,944 39,457,261 38,546,459 Plus Outstanding Share Awards net of Share Reductions for Unrecognized Stock-Based Compensation Expense and Excess Tax Benefits: Shares Expected to be Awarded for Stock Performance Awards Granted to Executive Officers based on Measurement Period-to-Date Performance 212,043 210,784 118,644 Underlying Shares Related to Nonvested Restricted Stock Units Granted to Employees 59,980 56,952 45,712 Nonvested Restricted Shares 17,751 20,380 16,778 Shares Expected to be Issued Under the Deferred Compensation Program for Directors 2,478 2,970 3,417 Total Dilutive Shares 292,252 291,086 184,551 Weighted Average Common Shares Outstanding – Diluted 39,892,196 39,748,347 38,731,010 The effect of dilutive shares on earnings per share for the years ended December 31, 2018, 2017 2016, no $0.016

Note 6 - Share-based Payments

Note 6 - Share-based Payments	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Disclosure of Compensation Related Costs, Share-based Payments [Text Block]	6 . Share-Based Payments Purchase Plan Through December 31, 2016, 15% six 718, Compensation—Stock Compensation 718 , 15% 15% $173,000 2016. January 1, 2017, 100% six Restricted Stock Granted to Directors Under the 1999 2014 1999 April 8, 2017. 718 April 9, 2018, 18,200 April 9, 2018 $43.40 2018 33.3% April 8 2019 2021 not Presented below is a summary of the status of directors’ restricted stock awards for the years ended December 31: Directors’ Restricted Stock Awards 2018 2017 2016 Shares Weighted Average Grant-Date Fair Value Shares Weighted Average Grant-Date Fair Value Shares Weighted Average Grant-Date Fair Value Nonvested, Beginning of Year 46,800 $ 32.65 46,334 $ 29.71 38,217 $ 29.78 Granted 18,200 43.40 17,600 37.75 23,200 28.66 Vested 21,775 31.94 17,134 29.93 15,083 28.28 Forfeited -- -- -- Nonvested, End of Year 43,225 37.53 46,800 32.65 46,334 29.71 Compensation Expense Recognized $ 661,000 $ 658,000 $ 491,000 Fair Value of Shares Vested in Year $ 696,000 $ 513,000 $ 427,000 Restricted Stock Granted to Employees Under the 1999 2014 1999 April 8, 2017. 718 No 2014. Presented below is a summary of the status of employees’ restricted stock awards for the years ended December 31: Employees’ Restricted Stock Awards 2018 2017 2016 Shares Weighted Average Grant-Date Fair Value Shares Weighted Average Grant-Date Fair Value Shares Weighted Average Grant-Date Fair Value Nonvested, Beginning of Year 2,895 $ 29.41 7,180 $ 29.72 13,581 $ 28.56 Granted -- -- -- Vested 2,895 29.41 4,285 29.94 6,401 27.25 Forfeited -- -- -- Nonvested, End of Year -- -- 2,895 29.41 7,180 29.72 Compensation Expense Recognized $ 16,000 $ 70,000 $ 96,000 Fair Value of Awards Vested $ 85,000 $ 128,000 $ 174,000 Restricted Stock Units Granted to Executive Officers On February 5, 2018, 15,200 2014 $41.325 2018 25% February 6 2019 2022 Presented below is a summary of the status of restricted stock unit awards granted to executive officers for the years ended December 31: Executives’ Restricted Stock Unit Awards 2018 2017 2016 Shares Weighted Average Grant-Date Fair Value Shares Weighted Average Grant-Date Fair Value Shares Weighted Average Grant-Date Fair Value Nonvested, Beginning of Year 47,750 $ 32.71 41,825 $ 30.23 24,300 $ 31.682 Granted 15,200 41.325 15,900 37.65 22,000 28.915 Vested 17,650 32.462 9,975 30.16 4,475 31.69 Forfeited -- -- -- Nonvested, End of Year 45,300 35.70 47,750 32.71 41,825 30.23 Compensation Expense Recognized $ 769,000 $ 576,000 $ 446,000 Fair Value of Awards Vested $ 573,000 $ 301,000 $ 142,000 Restricted Stock Units Granted to Employees In 2018 2014 not Grant Date Units Granted Grant-Date Fair Value per Award Restricted Stock Units Vesting 100% on April 8, 2022 April 9, 2018 12,945 $ 38.45 Restricted Stock Units Vesting 100% on April 8, 2022 June 20, 2018 1,000 $ 42.46 Restricted Stock Units Vesting 100% on April 8, 2022 September 25, 2018 835 $ 43.25 The grant-date fair value of each restricted stock unit was based on the average of the high and low market price of the Company’s common stock on the date of grant, discounted for the value of the dividend exclusion over the four Presented below is a summary of the status of employees’ restricted stock unit awards for the years ended December 31: Employees’ Restricted Stock Unit Awards 2018 2017 2016 Restricted Stock Units Weighted Average Grant-Date Fair Value Restricted Stock Units Weighted Average Grant-Date Fair Value Restricted Stock Units Weighted Average Grant-Date Fair Value Nonvested, Beginning of Year 46,440 $ 27.07 47,370 $ 25.19 46,600 $ 23.75 Granted 14,780 38.99 10,995 33.28 17,220 24.54 Vested 8,925 25.23 11,550 25.30 12,250 19.03 Forfeited 2,825 25.86 375 26.92 4,200 24.51 Nonvested, End of Year 49,470 31.03 46,440 27.07 47,370 25.19 Compensation Expense Recognized $ 351,000 $ 331,000 $ 307,000 Fair Value of Awards Vested $ 225,000 $ 292,000 $ 233,000 Stock Performance Awards G ranted to Executive Officers Agreements for stock performance awards have been granted under the 2014 three January 1 3 3 three no On February 5, 2018 2014 2018 2020 2018 54,000 27,000 3 27,000 January 1, 2018 December 31, 2020, 20 January 1, 2018 20 January 1, 2021. may zero 150% 81,000 no 718, The table below provides a summary of stock performance awards granted and amounts expensed related to the stock performance awards: Performance Period Maximum Shares Subject To Award Target Shares Expense Recognized in the Year Ended December 31, Earned Shares 2018 2017 2016 2018-2020 81,000 54,000 $ 1,121,000 2017-2019 89,250 59,500 729,000 $ 854,000 7,500 2016-2018 122,250 81,500 772,000 580,000 $ 798,000 113,298 2015-2017 126,450 84,300 23,000 573,000 535,000 114,648 2014-2016 159,450 106,300 -- -- 332,000 121,491 Total $ 2,645,000 $ 2,007,000 $ 1,665,000 356,937 Stock-based payment expense recognized in 2018, 2017 2016 2018 2020, 2017 2019 2016 2018 The earned shares shown in the table above for the 2016 2018 2017 2019 2018 December 31, 2017 62 The earned shares shown in the table above for the 2016 2018 2019 1 41 3 3 10.00% 145.17% The earned shares shown in the table above for the 2015 2017 2018 2 42 3 3 10.00% 136.00% The earned shares shown in the table above for the 2014 2016 2017 19 43 114.29% July 1, 2015 2016 180 $26.35 $848,000. In connection with the resignation of an executive officer in May 2014, 8,900 2014. As of December 31, 2018, $4.3 1.9

Note 7 - Retained Earnings and

Note 7 - Retained Earnings and Dividend Restriction	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Retained Earnings Restrictions [Text Block]	7 . Retained Earnings and Dividend Restriction The Company is a holding company with no Both the Company and OTP credit agreements contain restrictions on the payment of cash dividends upon a default or event of default. An event of default would be considered to have occurred if the Company did not December 31, 2018, Under the Federal Power Act, a public utility may not 1 2 not 3 no The MPUC indirectly limits the amount of dividends OTP can pay to the Company by requiring an equity-to-total-capitalization ratio between 47.9% 58.5% 2018 October 18, 2018. December 31, 2018, 53.2% $477 $1.2

Note 8 - Leases

Note 8 - Leases	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Leases of Lessee Disclosure [Text Block]	8. The Company leases rail cars for transporting coal, warehouse and office space, land and certain office, manufacturing and material handling equipment, and currently has no OTP has obligations to make future operating lease payments primarily related to coal rail-car leases. OTP’s rail car lease payments are charged to fuel inventory and then expensed to production fuel – electric as a component of fuel cost when fuel is burned. OTP also leases office and operating equipment with lease payments charged to rent expense and reported in electric operation and maintenance expenses on the Company’s consolidated statements of income. From time to time, OTP will lease construction equipment or land for lay-down yards for materials used on capital projects. These leases are generally short term in nature with the lease payments being charged to the related construction project and included in construction work in progress or plant in service after the project is completed and placed in service. The Company’s nonelectric companies have obligations to make future operating lease payments primarily related to leases of buildings and manufacturing equipment. These payments are charged to rent expense accounts and reported in costs of goods sold or other nonelectric expenses, as appropriate, on the Company’s consolidated statements of income. Lease payment expenses including payments for rail car leases totaled $6,273,000, $6,237,000 $6,711,000 2018, 2017 2016, The amounts of the Company’s future operating leases obligations of December 31, 2018 Operating Leases (in thousands) OTP Nonelectric Total 2019 $ 1,099 $ 5,086 $ 6,185 2020 1,077 4,800 5,877 2021 1,047 3,971 5,018 2022 214 3,740 3,954 2023 196 3,385 3,581 Beyond 2023 448 6,295 6,743 Total $ 4,081 $ 27,277 $ 31,358 In February 2016, No. 2016 02. 12 The Company adopted the new standard on January 1, 2019 840. On implementation of the new lease accounting standard, ASC Topic 842, Leases January 2019, one not not Leases in place at the time of adoption will be capitalized on the basis of their remaining payment obligation balances, discounted to present value based on an explicit or implicit borrowing rate or on the Company’s incremental borrowing rate appropriate to the leased asset and lease terms. The remaining payments for operating lease right-of use assets will be charged to expense on a straight-line basis over the life of the lease beginning in January 2019. The Company estimates adoption of the standard will result in recognition of net lease assets and lease liabilities of approximately $20 January 1, 2019. not not Because the leases to be capitalized as right of use assets under ASC Topic 842 840, no

Note 9 - Commitments and Contin

Note 9 - Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Commitments and Contingencies Disclosure [Text Block]	9 . Commitments and Contingencies Construction and Other Purchase Commitments At December 31, 2018 2021 $64.5 $252,000, $280,000 $272,000 2018, 2017 2016, December 31, 2018 December 31, 2021 $5.0 Electric Utility Capacity and Energy Requirements and Coal Purchase and Delivery Contracts OTP has commitments for the purchase of capacity and energy requirements under agreements extending into 2041. 2040. 2020. May 14, 2018 December 31, 2020. no 2019 Cloud Peak Energy Resources LLC for the purchase of subbituminous coal for Hoot Lake Plant through December 31, 2023. no OTP Land Easements OTP has commitments to make future payments for land easements not $605,000, $593,000 $582,000 2018, 2017 2016, The amounts of the Company’s construction program and other commitments and commitments under capacity and energy agreements, coal purchase and coal delivery contracts and land easements as of December 31, 2018, (in thousands) Construction Program and Other Commitments Capacity and Energy Requirements Coal Purchase Commitments OTP Land Easement Payments 2019 $ 43,887 $ 24,925 $ 23,397 $ 617 2020 23,939 24,844 22,645 630 2021 1,681 12,988 22,935 642 2022 -- 11,827 22,793 655 2023 -- 11,827 23,955 668 Beyond 2023 -- 143,099 503,492 7,612 Total $ 69,507 $ 229,510 $ 619,217 $ 10,824 Contingencies OTP had a $1.6 December 31, 2018 3 Contingencies, by their nature, relate to uncertainties that require the Company’s management to exercise judgment both in assessing the likelihood a liability has been incurred as well as in estimating the amount of potential loss. In addition to the potential ROE refund described above, the most significant contingencies impacting the Company’s consolidated financial statements are those related to environmental remediation, risks associated with warranty claims relating to divested businesses that could exceed the established reserve amounts and litigation matters. Should all of these known items, excluding the ROE refund liability already recognized, result in liabilities being incurred, the loss could be as high as $1.0 In 2015 111 not 2016. 2017, April 2018. On August 21, 2018 not three Other The Company is a party to litigation and regulatory enforcement matters arising in the normal course of business. The Company regularly analyzes current information and, as necessary, provides accruals for liabilities that are probable of occurring and that can be reasonably estimated. The Company believes the effect on its consolidated results of operations, financial position and cash flows, if any, for the disposition of all matters pending as of December 31, 2018 not

Note 10 - Short-term and Long-t

Note 10 - Short-term and Long-term Borrowings	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Debt Disclosure [Text Block]	10 . Shor t-Term and Long-Term Borrowings Short-Term Debt The following table presents the status of the Company’s lines of credit as of December 31, 2018 December 31, 2017: (in thousands) Line Limit In Use on December 31, 2018 Restricted due to Outstanding Letters of Credit Available on December 31, 2018 Available on December 31, 2017 Otter Tail Corporation Credit Agreement $ 130,000 $ 9,215 $ -- $ 120,785 $ 130,000 OTP Credit Agreement 170,000 9,384 300 160,316 57,329 Total $ 300,000 $ 18,599 $ 300 $ 281,101 $ 187,329 Under the Otter Tail Corporation Credit Agreement (as defined below), the maximum amount of debt outstanding in 2018 $17.7 September 17, 2018 2018 $5.5 2018 3.8% 2.8% 2017. 2018 $122.0 January 16, 2018 2018 $21.6 2018 3.0% 2.4% 2017. 2018 $122.0 January 16, 2018 2018 $27.1 December 31, 2018 3.9%. On October 29, 2012 $130 may $250 October 31, 2018 one October 31, 2022 October 31, 2023. 1.50%, not not $40 On October 29, 2012 $170 may $250 October 31, 2018 one October 31, 2022 October 31, 2023. not $50 LIBOR 1.25%, not not not Both the Otter Tail Corporation Credit Agreement and the OTP Credit Agreement currently expire on October 31, 2023. January 1, 2022. not may zero Long-Term Debt Issuances and Retirements 2018 On November 14, 2017, 2018 $100 4.07% 2018A February 7, 2048 ( 2018 2018 February 7, 2018. 2018 OTP may 2018 not 10% 100% no 2018 2018 August 7, 2047 2018 2018 100% 2018 The 2018 2018 2018 not 2018 not 2018 2018 2018 2018 2018 2018 no 2016 On September 23, 2016 2016 $80 3.55% December 15, 2026 ( 2026 2026 December 13, 2016. 2016 2026 2016 2026 $52,330,000 9.000% December 15, 2016, $50 February 2016 The Company may 2026 not 10% 2026 100% no 2016 2026 September 15, 2026 2026 100% 2016 2016 2016 2016 2026 2026 The 2016 2016 2016 2016 not 2013 On August 14, 2013 2013 $60 4.68% February 27, 2029 ( $90 5.47% February 27, 2044 ( February 27, 2014. The 2013 may not 10% 100% no 2013 November 27, 2028 November 27, 2043, 100% 2013 100% 2013 The 2013 2013 2013 not 2013 not 2013 2013 2013 2013 2013 no 2007 2011 On December 1, 2011, $140 4.63% December 1, 2021 July 29, 2011 ( 2011 $122 three $30 6.15% 2022; $42 6.37% 2027; $50 6.47% 2037 2007 2007 August 20, 2007 ( 2007 August 21, 2017 $33 5.95%, 2007 August 20, 2017. The 2011 2007 may not 10% 100% 2011 2011 2011 2007 100% Shelf Registration On May 3, 2018 may May 3, 2021. The following tables provide a breakdown of the assignment of the Company’s consolidated short-term and long-term debt outstanding as of December 31, 2018 December 31, 2017: December 31, 201 8 (in thousands) OTP Otter Tail Corporation Otter Tail Corporation Consolidated Short-Term Debt $ 9,384 $ 9,215 $ 18,599 Long-Term Debt: 3.55% Guaranteed Senior Notes, due December 15, 2026 $ 80,000 $ 80,000 Senior Unsecured Notes 4.63%, due December 1, 2021 $ 140,000 140,000 Senior Unsecured Notes 6.15%, Series B, due August 20, 2022 30,000 30,000 Senior Unsecured Notes 6.37%, Series C, due August 20, 2027 42,000 42,000 Senior Unsecured Notes 4.68%, Series A, due February 27, 2029 60,000 60,000 Senior Unsecured Notes 6.47%, Series D, due August 20, 2037 50,000 50,000 Senior Unsecured Notes 5.47%, Series B, due February 27, 2044 90,000 90,000 Senior Unsecured Notes 4.07%, Series 2018A, due February 7, 2048 100,000 100,000 PACE Note, 2.54%, due March 18, 2021 523 523 Total $ 512,000 $ 80,523 $ 592,523 Less: Current Maturities net of Unamortized Debt Issuance Costs -- 172 172 Unamortized Long-Term Debt Issuance Costs 1,942 407 2,349 Total Long-Term Debt net of Unamortized Debt Issuance Costs $ 510,058 $ 79,944 $ 590,002 Total Short-Term and Long-Term Debt (with current maturities) $ 519,442 $ 89,331 $ 608,773 December 31, 2017 (in thousands) OTP Otter Tail Corporation Otter Tail Corporation Consolidated Short-Term Debt $ 112,371 $ -- $ 112,371 Long-Term Debt: 3.55% Guaranteed Senior Notes, due December 15, 2026 $ 80,000 $ 80,000 Senior Unsecured Notes 4.63%, due December 1, 2021 $ 140,000 140,000 Senior Unsecured Notes 6.15%, Series B, due August 20, 2022 30,000 30,000 Senior Unsecured Notes 6.37%, Series C, due August 20, 2027 42,000 42,000 Senior Unsecured Notes 4.68%, Series A, due February 27, 2029 60,000 60,000 Senior Unsecured Notes 6.47%, Series D, due August 20, 2037 50,000 50,000 Senior Unsecured Notes 5.47%, Series B, due February 27, 2044 90,000 90,000 North Dakota Development Note, 3.95%, due April 1, 2018 27 27 PACE Note, 2.54%, due March 18, 2021 684 684 Total $ 412,000 $ 80,711 $ 492,711 Less: Current Maturities net of Unamortized Debt Issuance Costs -- 186 186 Unamortized Long-Term Debt Issuance Costs 1,684 461 2,145 Total Long-Term Debt net of Unamortized Debt Issuance Costs $ 410,316 $ 80,064 $ 490,380 Total Short-Term and Long-Term Debt (with current maturities) $ 522,687 $ 80,250 $ 602,937 The aggregate amounts of maturities on bonds outstanding and other long-term obligations at December 31, 2018 five (in thousands) 2019 2020 2021 2022 2023 Aggregate Amounts of Debt Maturities $ 172 $ 185 $ 140,166 $ 30,000 $ -- Financial Covenants The Company and OTP were in compliance with the financial covenants in these debt agreements as of December 31, 2018. No The Company’s and OTP’s borrowing agreements are subject to certain financial covenants. Specifically: ● Under the Otter Tail Corporation Credit Agreement and the 2016 may not 0.60 1.00 1.50 1.00 ● Under the 2016 may not 10% ● Under the OTP Credit Agreement, OTP may not 0.60 1.00. ● Under the 2007 2011 may not 0.60 1.00 1.50 1.00, may not 20% ● Under the 2013 2018 may not 60% may not 20%

Note 11 - Pension Plan and Othe

Note 11 - Pension Plan and Other Postretirement Benefits	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Pension and Other Postretirement Benefits Disclosure [Text Block]	1 1 . Pension Plan and Other Postretirement Benefits Pension Plan The Company's noncontributory funded pension plan covers substantially all corporate employees and OTP nonunion employees hired prior to September 1, 2006, November 1, 2013, January 1, 2009 100% five 65, 62. no may The pension plan has a trustee who is responsible for pension payments to retirees and a separate pension fund manager responsible for managing the plan's assets. An independent actuary assists the Company in performing the necessary actuarial valuations for the plan. The plan assets consist of common stock and bonds of public companies, U.S. government securities, cash and cash equivalents and alternative investments. None The following table lists components of net periodic pension benefit cost for the year ended December 31: (in thousands) 2018 2017 2016 Service Cost–Benefit Earned During the Period $ 6,459 $ 5,629 $ 5,518 Interest Cost on Projected Benefit Obligation 13,452 14,139 14,195 Expected Return on Assets (21,199 ) (19,229 ) (19,454 ) Amortization of Prior Service Cost: From Regulatory Asset 16 120 189 From Other Comprehensive Income 1 -- 3 5 Amortization of Net Actuarial Loss: From Regulatory Asset 7,135 5,090 5,153 From Other Comprehensive Income 1 183 125 127 Net Periodic Pension Cost 2 $ 6,046 $ 5,877 $ 5,733 1 Corporate cost included in nonservice cost components of postretirement benefits. 2 Allocation of Costs: 2018 2017 2016 Service Costs included in OTP Capital Expenditures $ 1,542 $ 1,094 $ 1,009 Service costs included in electric operation and maintenance expenses 4,756 4,400 4,377 Service costs included in other nonelectric expenses 161 135 132 Nonservice costs capitalized (99 ) 48 39 Nonservice costs included in n onservice cost components of postretirement benefits (314 ) 200 176 Weighted average assumptions used to determine net periodic pension cost for the year ended December 31: 2018 2017 2016 Discount Rate 3.90 % 4.60 % 4.76 % Long-Term Rate of Return on Plan Assets 7.50 % 7.50 % 7.75 % Rate of Increase in Future Compensation Level See below 3.00 % 3.13 % Participants to Age 39 4.50 % Participants Age 40 to Age 49 3.50 % Participants Age 50 and Older 2.75 % The following table presents amounts recognized in the consolidated balance sheets as of December 31: (in thousands) 2018 2017 Regulatory Assets: Unrecognized Prior Service Cost $ 5 $ 21 Unrecognized Actuarial Loss 104,891 99,360 Total Regulatory Assets $ 104,896 $ 99,381 Accumulated Other Comprehensive Loss: Unrecognized Prior Service Cost $ 9 $ 9 Unrecognized Actuarial Loss 137 439 Total Accumulated Other Comprehensive Loss $ 146 $ 448 Noncurrent Liability $ 58,659 $ 67,399 Funded status as of December 31: (in thousands) 2018 2017 Accumulated Benefit Obligation $ (297,972 ) $ (316,095 ) Projected Benefit Obligation $ (328,442 ) $ (352,718 ) Fair Value of Plan Assets 269,783 285,319 Funded Status $ (58,659 ) $ (67,399 ) The following tables provide a reconciliation of the changes in the fair value of plan assets and the plan’s benefit obligations over the two December 31, 2018: (in thousands) 2018 2017 Reconciliation of Fair Value of Plan Assets: Fair Value of Plan Assets at January 1 $ 285,319 $ 254,346 Actual Return on Plan Assets (21,334 ) 44,181 Discretionary Company Contributions 20,000 -- Benefit Payments (14,202 ) (13,208 ) Fair Value of Plan Assets at December 31 $ 269,783 $ 285,319 Estimated Asset Return (7.3% ) 17.8 % Reconciliation of Projected Benefit Obligation: Projected Benefit Obligation at January 1 $ 352,718 $ 314,637 Service Cost 6,459 5,629 Interest Cost 13,452 14,139 Benefit Payments (14,202 ) (13,208 ) Actuarial (Gain) Loss (29,985 ) 31,521 Projected Benefit Obligation at December 31 $ 328,442 $ 352,718 Weighted average assumptions used to determine benefit obligations at December 31: 2018 2017 Discount Rate 4.50 % 3.90 % Rate of Increase in Future Compensation Level: Participants to Age 39 4.50 % 4.50 % Participants Age 40 to Age 49 3.50 % 3.50 % Participants Age 50 and Older 2.75 % 2.75 % The assumed rate of return on pension fund assets used for the determination of 2019 7.25%. Market-related value of plan assets The Company bases actuarial determination of pension plan expense or income on a market-related valuation of assets, which reduces year-to-year volatility. This market-related valuation calculation recognizes investment gains or losses over a five five Measurement Dates: 2018 2017 Net Periodic Pension Cost January 1, 2018 January 1, 2017 End of Year Benefit Obligations January 1, 2018 projected to December 31, 2018 January 1, 2017 projected to December 31, 2017 Market Value of Assets December 31, 2018 December 31, 2017 The estimated amounts of unrecognized net actuarial losses and prior service costs to be amortized from regulatory assets and accumulated other comprehensive loss into the net periodic pension cost in 2019 (in thousands) 2019 Decrease in Regulatory Assets: Amortization of Unrecognized Prior Service Cost $ 5 Amortization of Unrecognized Actuarial Loss 4,642 Decrease in Accumulated Other Comprehensive Loss: Amortization of Unrecognized Prior Service Cost 9 Amortization of Unrecognized Actuarial Loss 114 Total Estimated Amortization $ 4,770 Cash flows no December 31, 2018 $10 February 2019. The following benefit payments, which reflect expected future service, as appropriate, are expected to be paid out from plan assets: (in thousands) 2019 2020 2021 2022 2023 Years 2024-2028 $ 15,086 $ 15,689 $ 16,356 $ 17,017 $ 17,709 $ 96,186 The following objectives guide the investment strategy of the Company’s pension plan (the Plan): ● The assets of the Plan will be invested in accordance with all applicable laws in a manner consistent with fiduciary standards including Employee Retirement Income Security Act standards (if applicable). Specifically: o The safeguards and diversity that a prudent investor would adhere to must be present in the investment program. o All transactions undertaken on behalf of the Plan must be in the best interest of plan participants and their beneficiaries. ● The primary objective of the Plan is to provide a source of retirement income for its participants and beneficiaries. ● The near-term primary financial objective of the Plan is to improve the funded status of the Plan. ● A secondary financial objective is to minimize pension funding and expense volatility where possible. The asset allocation strategy developed by the Company’s Retirement Plans Administration Committee (the Committee) is based on the current needs of the Plan and the objectives listed above. An asset/liability review is conducted annually or as often as necessary to assess the impact of various asset allocations on funded status and other financial variables. The current needs of the Plan, the overall investment objectives above, the investment preferences and risk tolerance of the Committee and the desired degree of diversification suggest the need for an investment allocation including multiple asset classes. The asset allocation in the table below contains guideline percentages, at market value, of the total Plan invested in various asset classes. The Permitted Range is a guide and will at times not The policy of the Plan is to invest assets in accordance with the allocations shown below: Permitted Range Asset Class / PBO Funded Status < 85% PBO >=85% PBO >=90% PBO >=95% PBO >=100% PBO Equity 39% - 59% 34% - 54% 24% - 44% 14% - 34% 0% - 20% Investment Grade Fixed Income 22% - 42% 30% - 50% 40% - 60% 53% - 73% 70% - 100% Below Investment Grade Fixed Income* 0% - 15% 0% - 15% 0% - 15% 0% - 10% 0% - 10% Other** 5% - 20% 5% - 20% 5% - 20% 0% - 15% 0% - 15% * Includes (but not ** Other category may may The Company’s pension plan asset allocations at December 31, 2018 2017, Asset Allocation 2018 2017 Large Capitalization Equity Securities 17.5 % 23.5 % International Equity Securities 17.0 % 18.1 % Small and Mid-Capitalization Equity Securities 6.7 % 8.7 % SEI Dynamic Asset Allocation Fund 4.0 % 5.0 % Emerging Markets Equity Fund 3.4 % 5.5 % Equity Securities 48.6 % 60.8 % Fixed-Income Securities and Cash 47.1 % 35.2 % Other – SEI Energy Debt Collective Fund 4.3 % 4.0 % 100.0 % 100.0 % The following table presents the Company’s pension fund assets measured at fair value and included in Level 1 December 31: (in thousands) 2018 2017 Assets in Level 1 of the Fair Value Hierarchy $ 258,307 $ 273,999 SEI Energy Debt Collective Fund at NAV 11,476 11,320 Total Assets $ 269,783 $ 285,319 Fair Value Measurements of Pension Fund Assets ASC 715, Compensation – Retirement Benefits, three 820 10 35. The following table presents, the Company’s pension fund assets measured at fair value and included in Level 1 December 31: (in thousands) 2018 2017 Large Capitalization Equity Securities Mutual Fund $ 47,198 $ 66,946 International Equity Securities Mutual Funds 45,912 51,636 Small and Mid-Capitalization Equity Securities Mutual Fund 17,971 24,848 SEI Dynamic Asset Allocation Mutual Fund 10,929 14,371 Emerging Markets Equity Fund 9,197 15,824 Fixed Income Securities Mutual Funds 127,098 100,373 Cash Management – Money Market Fund 2 1 Total Assets $ 258,307 $ 273,999 The investments held by the SEI Energy Debt Collective Fund on December 31, 2018 2017 95 three $10.0 July 2015. third may 30 Executive Survivor and Supplemental Retirement Plan (ESSRP) The ESSRP is an unfunded, nonqualified benefit plan for executive officers and certain key management employees. The ESSRP provides defined benefit payments to these employees on their retirements for life or to their beneficiaries on their deaths for a 15 no The following table lists components of net periodic pension benefit cost for the year ended December 31: (in thousands) 2018 2017 2016 Service Cost–Benefit Earned During the Period $ 408 $ 290 $ 252 Interest Cost on Projected Benefit Obligation 1,589 1,686 1,667 Amortization of Prior Service Cost: From Regulatory Asset 20 16 16 From Other Comprehensive Income 1 34 38 38 Amortization of Net Actuarial Loss: From Regulatory Asset 206 285 293 From Other Comprehensive Income 1 722 440 446 Net Periodic Pension Cost 2 $ 2,979 $ 2,755 $ 2,712 1 Amortization of prior service costs and net actuarial losses from other comprehensive income are included in n onservice cost components of postretirement benefits on the face of the Company’s consolidated statements of income. 2 2018 2017 2016 Service costs included in electric operation and maintenance expenses $ 99 $ 94 $ 87 Service costs included in other nonelectric expenses 309 196 165 Nonservice costs included in n onservice cost components of postretirement benefits 2,571 2,465 2,460 Weighted average assumptions used to determine net periodic pension cost for the year ended December 31: 2018 2017 2016 Discount Rate 3.85 % 4.60 % 4.76 % Rate of Increase in Future Compensation Level 2.92 % 3.00 % 3.13 % The following table presents amounts recognized in the consolidated balance sheets as of December 31: (in thousands) 2018 2017 Regulatory Assets: Unrecognized Prior Service Cost $ 20 $ 40 Unrecognized Actuarial Loss 1,768 3,229 Total Regulatory Assets $ 1,788 $ 3,269 Projected Benefit Obligation Liability – Net Amount Recognized $ (39,699 ) $ (42,308 ) Accumulated Other Comprehensive Loss: Unrecognized Prior Service Cost $ 64 $ 98 Unrecognized Actuarial Loss 6,455 9,024 Total Accumulated Other Comprehensive Loss $ 6,519 $ 9,122 The following tables provide a reconciliation of the changes in the fair value of plan assets and the plan’s projected benefit obligations over the two December 31, 2018 December 31 (in thousands) 2018 2017 Reconciliation of Fair Value of Plan Assets: Fair Value of Plan Assets at January 1 $ -- $ -- Actual Return on Plan Assets -- -- Employer Contributions 1,505 1,175 Benefit Payments (1,505 ) (1,175 ) Fair Value of Plan Assets at December 31 $ -- $ -- Reconciliation of Projected Benefit Obligation: Projected Benefit Obligation at January 1 $ 42,308 $ 37,335 Service Cost 408 290 Interest Cost 1,589 1,686 Benefit Payments (1,505 ) (1,175 ) Plan Amendments -- -- Actuarial (Gain) Loss (3,101 ) 4,172 Projected Benefit Obligation at December 31 $ 39,699 $ 42,308 Weighted average assumptions used to determine benefit obligations at December 31: 2018 2017 Discount Rate 4.46 % 3.85 % Rate of Increase in Future Compensation Level: 3.40 % 2.92 % The estimated amounts of unrecognized net actuarial losses and prior service costs to be amortized from regulatory assets and accumulated other comprehensive loss into the net periodic pension cost for the ESSRP in 2019 (in thousands) 2019 Decrease in Regulatory Assets: Amortization of Unrecognized Prior Service Cost $ 5 Amortization of Unrecognized Actuarial Loss 124 Decrease in Accumulated Other Comprehensive Loss: Amortization of Unrecognized Prior Service Cost 17 Amortization of Unrecognized Actuarial Loss 348 Total Estimated Amortization $ 494 Cash flows no Years (in thousands) 2019 2020 2021 2022 2023 2024-2028 $ 1,468 $ 1,527 $ 1,618 $ 2,214 $ 2,635 $ 14,215 Other Postretirement Benefits The Company provides a portion of health insurance benefits for retired OTP and corporate employees. The retiree health insurance benefits will be available for all corporate employees and OTP nonunion employees hired prior to September 1, 2006, November 1, 2010, January 1, 2009 55 10 no December 31: (in thousands) 2018 2017 2016 Service Cost–Benefit Earned During the Period $ 1,526 $ 1,425 $ 1,301 Interest Cost on Projected Benefit Obligation 2,583 2,712 2,503 Amortization of Prior Service Cost From Regulatory Asset -- (4 ) 134 From Other Comprehensive Income 1 -- 4 3 Amortization of Net Actuarial Loss From Regulatory Asset 1,648 936 379 From Other Comprehensive Income 1 42 19 9 Net Periodic Postretirement Benefit Cost 2 $ 5,799 $ 5,092 $ 4,329 Effect of Medicare Part D Subsidy $ (470 ) $ (561 ) $ (923 ) 1 Corporate cost included in nonservice cost components of postretirement benefits. 2 Allocation of Cost: 201 8 201 7 201 6 Service Costs included in OTP capital expenditures $ 364 $ 277 $ 238 Service costs included in electric operation and maintenance expenses 1,124 1,114 1,032 Service costs included in other nonelectric expenses 38 34 31 Nonservice costs capitalized 1,020 712 554 Nonservice costs included in n onservice cost components of postretirement benefits 3,253 2,955 2,474 Weighted average assumptions used to determine net periodic postretirement benefit cost for the year ended December 31: 2018 2017 2016 Discount Rate 3.81 % 4.46 % 4.57 % The following table presents amounts recognized in the consolidated balance sheets as of December 31: (in thousands) 2018 2017 Projected Benefit Obligation Liability – Net Amount Recognized $ (71,561 ) $ (69,774 ) Unrecognized Net Actuarial Loss (Gain): Regulatory Asset $ 18,094 $ 18,927 Accumulated Other Comprehensive Income: (107 ) (111 ) Unrecognized Net Actuarial Loss $ 17,987 $ 18,816 The following tables provide a reconciliation of the changes in the fair value of plan assets and the plan’s projected benefit obligations and accrued postretirement benefit cost over the two December 31, 2018: (in thousands) 2018 2017 Reconciliation of Fair Value of Plan Assets: Fair Value of Plan Assets at January 1 $ -- $ -- Actual Return on Plan Assets -- -- Company Contributions 3,183 3,290 Benefit Payments (Net of Medicare Part D Subsidy) (6,684 ) (6,534 ) Participant Premium Payments 3,501 3,244 Fair Value of Plan Assets at December 31 $ -- $ -- Reconciliation of Projected Benefit Obligation: Projected Benefit Obligation at January 1 $ 69,774 $ 62,571 Service Cost (Net of Medicare Part D Subsidy) 1,526 1,425 Interest Cost (Net of Medicare Part D Subsidy) 2,583 2,712 Benefit Payments (Net of Medicare Part D Subsidy) (6,684 ) (6,534 ) Participant Premium Payments 3,501 3,244 Actuarial Loss 861 6,356 Projected Benefit Obligation at December 31 $ 71,561 $ 69,774 Reconciliation of Accrued Postretirement Cost: Accrued Postretirement Cost at January 1 $ (50,958 ) $ (49,156 ) Expense (5,799 ) (5,092 ) Net Company Contribution 3,183 3,290 Accrued Postretirement Cost at December 31 $ (53,574 ) $ (50,958 ) Weighted average assumptions used to determine benefit obligations at December 31: 2018 2017 Discount Rate 4.44 % 3.81 % Assumed healthcare cost-trend rates as of December 31: 2018 2017 Healthcare Cost-Trend Rate Assumed for Next Year Pre-65 7.00 % 5.85 % Healthcare Cost-Trend Rate Assumed for Next Year Post-65 7.00 % 6.03 % Rate to Which the Cost-Trend Rate is Assumed to Decline 4.50 % 4.50 % Year the Rate Reaches the Ultimate Trend Rate 2038 2038 Assumed healthcare cost-trend rates have a significant effect on the amounts reported for healthcare plans. A one 2018 (in thousands) 1 Point Increase 1 Point Decrease Effect on the Postretirement Benefit Obligation $ 9,095 $ (7,586 ) Effect on Total of Service and Interest Cost $ 758 $ (600 ) Effect on Expense $ 1,953 $ (1,589 ) Measurement Dates: 2018 2017 Net Periodic Postretirement Benefit Cost January 1, 2018 January 1, 2017 End of Year Benefit Obligations January 1, 2018 projected to December 31, 2018 January 1, 2017 projected to December 31, 2017 The estimated net amounts of unrecognized prior service cost to be amortized from regulatory assets and accumulated other comprehensive loss into the net periodic postretirement benefit cost in 2019 (in thousands) 2019 Decrease in Regulatory Assets: Amortization of Unrecognized Actuarial Loss $ 1,570 Decrease in Accumulated Other Comprehensive Loss: Amortization of Unrecognized Actuarial Loss 39 Total Estimated Amortization $ 1,609 Cash flows $4.2 2019. $0.4 2019. Years (in thousands) 2019 2020 2021 2022 2023 2024-2028 $ 4,247 $ 4,322 $ 4,483 $ 4,698 $ 4,781 $ 24,183 401K The Company sponsors a 401K $4,532,000 2018, $4,211,000 2017 $3,877,000 2016. Employee Stock Ownership Plan The Company has a stock ownership plan for the benefit of all its electric utility employees. Contributions made by the Company were $398,000 2018, $612,000 2017 $647,000 2016.

Note 12 - Fair Value of Financi

Note 12 - Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Fair Value Disclosures [Text Block]	1 2 . Fair Value of Financial Instruments The following methods and assumptions were used to estimate the fair value of each class of financial instruments for which it is practicable to estimate that value: Cash Equivalents Short-Term Debt December 31, 2018 December 31, 2017 1.50% 1.25% Long-Term Debt including Current Maturities 2 820. December 31, 2018 December 31, 2017 (in thousands) Carrying Amount Fair Value Carrying Amount Fair Value Cash and Cash Equivalents $ 861 $ 861 $ 16,216 $ 16,216 Short-Term Debt (18,599 ) (18,599 ) (112,371 ) (112,371 ) Long-Term Debt including Current Maturities (590,174 ) (601,513 ) (490,566 ) (543,691 )

Note 13 - Property, Plant and E

Note 13 - Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Property, Plant and Equipment Disclosure [Text Block]	1 3 . Property, Plant and Equipment (in thousands) December 31, 2018 December 31, 2017 Electric Plant in Service Production $ 905,224 $ 897,732 Transmission 512,832 500,352 Distribution 502,261 482,867 General 99,404 100,067 Electric Plant in Service 2,019,721 1,981,018 Construction Work in Progress 170,090 132,556 Total Gross Electric Plant 2,189,811 2,113,574 Less Accumulated Depreciation and Amortization 699,642 662,431 Net Electric Plant $ 1,490,169 $ 1,451,143 Nonelectric Operations Plant Equipment $ 170,634 $ 160,263 Buildings and Leasehold Improvements 53,011 52,280 Land 4,475 4,394 Nonelectric Operations Plant 228,120 216,937 Construction Work in Progress 11,536 8,511 Total Gross Nonelectric Plant 239,656 225,448 Less Accumulated Depreciation and Amortization 148,727 136,988 Net Nonelectric Operations Plant $ 90,929 $ 88,460 Net Plant $ 1,581,098 $ 1,539,603 The estimated service lives for rate-regulated properties is 5 82 2 40 Service Life Range (years) Low High Electric Fixed Assets: Production Plant 9 82 Transmission Plant 42 70 Distribution Plant 5 68 General Plant 5 50 Nonelectric Fixed Assets: Equipment 2 12 Buildings and Leasehold Improvements 5 40

Note 14 - Income Taxes

Note 14 - Income Taxes	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Income Tax Disclosure [Text Block]	1 4 . Income Taxes The total income tax expense differs from the amount computed by applying the federal income tax rate ( 21% 2018, 35% 2017 2016 (in thousands) 2018 2017 2016 Tax Computed at Federal Statutory Rate $ 20,356 $ 34,893 $ 28,889 Increases (Decreases) in Tax from: State Income Taxes Net of Federal Income Tax Expense 5,210 4,368 2,869 Differences Reversing in Excess of Federal Rates (3,432 ) 551 77 Federal Production Tax Credits (PTCs) (3,111 ) (7,527 ) (7,175 ) Permanent Differences, R&D Tax Credits, Unitary Tax and Other Adjustments (1,864 ) (1,873 ) (1,262 ) North Dakota Wind Tax Credit Amortization – Net of Federal Taxes (1,033 ) (850 ) (850 ) Excess Tax deduction - Equity Method Stock Awards (708 ) (751 ) -- Allowance for Funds Used During Construction – Equity (431 ) (322 ) (280 ) Employee Stock Ownership Plan Dividend Deduction (298 ) (509 ) (537 ) Investment Tax Credit Amortization (98 ) (164 ) (350 ) Corporate-owned Life Insurance (3 ) (845 ) (680 ) Section 199 Domestic Production Activities Deduction -- (1,471 ) (482 ) Effect of TCJA Tax Rate Reduction on Value of Net Deferred Tax Assets -- 1,756 -- Income Tax Expense $ 14,588 $ 27,256 $ 20,219 Overall Effective Federal, State and Foreign Income Tax Rate 15.0 % 27.3 % 24.5 % Income Tax Expense Includes the Following: Current Federal Income Taxes $ 4,960 $ 4,434 $ 989 Current State Income Taxes 1,395 1,128 1,208 Deferred Federal Income Taxes 8,065 25,648 23,774 Deferred State Income Taxes 4,410 4,587 2,623 Federal PTCs (3,111 ) (7,527 ) (7,175 ) North Dakota Wind Tax Credit Amortization – Net of Federal Taxes (1,033 ) (850 ) (850 ) Investment Tax Credit Amortization (98 ) (164 ) (350 ) Total $ 14,588 $ 27,256 $ 20,219 Total Income Before Income Taxes $ 96,933 $ 99,695 $ 82,540 The Company's deferred tax assets and liabilities were composed of the following on December 31: (in thousands) 2018 2017 Deferred Tax Assets Benefit Liabilities $ 33,967 $ 32,328 Regulatory Tax Liability 33,228 39,465 Retirement Benefits Liabilities 32,664 31,894 North Dakota Wind Tax Credits 32,570 32,962 Federal PTCs 32,101 40,614 Cost of Removal 21,787 21,800 Differences Related to Property 6,842 6,499 Net Operating Loss Carryforward 2,489 3,203 Vacation Accrual 1,919 1,844 Investment Tax Credits 449 515 Other 3,218 668 Valuation Allowance (600 ) -- Total Deferred Tax Assets $ 200,634 $ 211,792 Deferred Tax Liabilities Differences Related to Property $ (261,396 ) $ (257,906 ) Retirement Benefits Regulatory Asset (32,664 ) (31,894 ) Excess Tax over Book Pension (15,145 ) (14,077 ) North Dakota Wind Tax Credits (4,386 ) (4,112 ) Impact of State Net Operating Losses on Federal Taxes (523 ) (673 ) Other (7,496 ) (3,631 ) Total Deferred Tax Liabilities $ (321,610 ) $ (312,293 ) Deferred Income Taxes $ (120,976 ) $ (100,501 ) Federal PTCs are recognized as wind energy is generated based on a per kwh rate prescribed in applicable federal statutes. OTP’s kwh generation from its wind turbines eligible for PTCs decreased 53.0% 2018 2017 one 4.4% 2017 2016. 25 Schedule of expiration of tax credits and tax net operating losses available as of December 31, 2018: (in thousands) Amount 2022-2032 2033-2038 2039-2043 United States Federal Tax Credits $ 34,586 $ -- $ 34,586 $ -- State Net Operating Losses 2,489 2,396 93 -- State Tax Credits 33,106 273 3,326 29,507 The following table summarizes the activity related to the Company’s unrecognized tax benefits: (in thousands) 2018 2017 2016 Balance on January 1 $ 684 $ 891 $ 468 Increases Related to Tax Positions for Prior Years 6 28 406 Decreases Related to Tax Positions for Prior Years -- (172 ) -- Increases Related to Tax Positions for Current Year 778 143 114 Uncertain Positions Resolved During Year (186 ) (206 ) (97 ) Balance on December 31 $ 1,282 $ 684 $ 891 The balance of unrecognized tax benefits as of December 31, 2018 December 31, 2018 not 12 no December 31, 2018. The Company and its subsidiaries file a consolidated U.S. federal income tax return and various state income tax returns. As of December 31, 2018, no 2015 2014 TCJA In December 2017 35% 21% December 31, 2017. The Company measures deferred tax assets and liabilities using enacted tax rates that will apply in the years in which the temporary differences are expected to be recovered or paid. Accordingly, the Company’s deferred tax assets and liabilities were remeasured to reflect the reduction in the U.S. corporate income tax rate from 35% 21% 2017. one $1.8 2017. (in thousands) Deferred Tax Liability Deferred Tax Regulatory Liability Balance on January 1, 2017 $ 226,591 $ 818 Change due to 2017Accruals and Amortizations 20,012 376 TCJA Deferred Tax Valuation Adjustment (109,072 ) 109,072 Tax Effect on TCJA Deferred Tax Valuation Adjustment (38,786 ) 38,786 TCJA Adjustment to Income Tax Expense 1,756 -- Balance on December 31, 2017 $ 100,501 $ 149,052 The Company recognized the income tax effects of the TCJA in its 2017 No. 118, 740, Income Taxes one December 31, 2017 September 30, 2018. not 2018 December 31, 2017.

Note 15 - Asset Retirement Obli

Note 15 - Asset Retirement Obligations (AROs)	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Asset Retirement Obligation Disclosure [Text Block]	1 5 . Asset Retirement Obligations (AROs) The Company’s AROs are related to OTP’s coal-fired generation plants and its 92 no OTP recorded no 2018. Reconciliations of carrying amounts of the present value of the Company’s legal AROs, capitalized asset retirement costs and related accumulated depreciation and a summary of settlement activity for the years ended December 31, 2018 2017 (in thousands) 2018 2017 Asset Retirement Obligations Beginning Balance $ 8,719 $ 8,341 New Obligations Recognized -- -- Adjustments Due to Revisions in Cash Flow Estimates -- -- Accrued Accretion 398 378 Settlements -- -- Ending Balance $ 9,117 $ 8,719 Asset Retirement Costs Capitalized Beginning Balance $ 2,983 $ 2,983 New Obligations Recognized -- -- Adjustments Due to Revisions in Cash Flow Estimates -- -- Settlements -- -- Ending Balance $ 2,983 $ 2,983 Accumulated Depreciation – Asset Retirement Costs Capitalized Beginning Balance $ 915 $ 795 New Obligations Recognized -- -- Adjustments Due to Revisions in Cash Flow Estimates -- -- Depreciation Expense 119 120 Settlements -- -- Ending Balance $ 1,034 $ 915 Settlements None None Original Capitalized Asset Retirement Cost – Retired $ -- $ -- Accumulated Depreciation -- -- Asset Retirement Obligation $ -- $ -- Settlement Cost -- -- Gain on Settlement – Deferred Under Regulatory Accounting $ -- $ --

Note 16 - Subsequent Events

Note 16 - Subsequent Events	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Subsequent Events [Text Block]	1 6 . Subsequent Events Stock Incentive Awards On February 13, 2019 2014 Award Shares/Units Granted Weighted Average Grant- Date Fair Value per Award Vesting Restricted Stock Units Granted 15,600 $ 49.6225 25% per year through February 6, 2023 Stock Performance Awards Granted: Under Executive Agreement 47,800 $ 42.875 December 31, 2021 Under Legacy Agreement 7,800 $ 45.885 December 31, 2021 The vesting of restricted stock units is accelerated in the event of a change in control, disability, death or retirement, subject to proration in certain cases. All restricted stock units granted to executive officers are eligible to receive dividend equivalent payments on all unvested awards over the awards respective vesting periods, subject to forfeiture under the terms of the restricted stock unit award agreements. The grant-date fair value of each restricted stock unit was the average of the high and low market price per share on the date of grant. Under the performance share awards the aggregate award for performance at target is 55,600 27,800 3 27,800 January 1, 2019 December 31, 2021, 20 January 1, 2019 20 January 1, 2022. may zero 150% 83,400 no 718, Under the 2019 The end of the period over which compensation expense is recognized for the above share-based awards for the individual grantees is the earlier of the indicated vesting period for the respective awards or the date the grantee becomes eligible for retirement as defined in their award agreement.

Schedule 1 - Condensed Financia

Schedule 1 - Condensed Financial Information of Registrant	12 Months Ended
	Dec. 31, 2018
Notes to Financial Statements
Condensed Financial Information of Parent Company Only Disclosure [Text Block]	SCHEDULE 1 - Condensed financial information of registrant Otter Tail Corporation (PARENT COMPANY) Condensed Balance Sheets, December 31 (in thousands) 201 8 2017 Assets Current Assets Cash and Cash Equivalents $ -- $ 16,371 Accounts Receivable -- -- Accounts Receivable from Subsidiaries 1,931 2,098 Interest Receivable from Subsidiaries 117 117 Notes Receivable from Subsidiaries 1,167 1,752 Other 3,482 1,130 Total Current Assets 6,697 21,468 Investments in Subsidiaries 787,869 724,613 Notes Receivable from Subsidiaries 79,422 79,611 Deferred Income Taxes 21,100 27,923 Other Assets 31,547 31,559 Total Assets $ 926,635 $ 885,174 Liabilities and Equity Current Liabilities Short-Term Debt $ 9,215 $ -- Current Maturities of Long-Term Debt 172 186 Accounts Payable to Subsidiaries 7 6 Notes Payable to Subsidiaries 60,626 61,908 Other 9,994 7,799 Total Current Liabilities 80,014 69,899 Other Noncurrent Liabilities 37,814 38,319 Commitments and Contingencies Capitalization Long-Term Debt, Net of Current Maturities 79,944 80,064 Common Shareholder Equity 728,863 696,892 Total Capitalization 808,807 776,956 Total Liabilities and Equity $ 926,635 $ 885,174 See accompanying notes to condensed financial statements. Otter Tail Corporation (PARENT COMPANY) Condensed Statements of Income--For the Years Ended December 31 (in thousands) 201 8 2017 2016 Operating Loss Revenue from Contracts with Customers $ -- $ -- $ -- Operating Expenses 9,916 7,138 8,530 Operating Loss (9,916 ) (7,138 ) (8,530 ) Other Income (Expense) Equity Income in Earnings of Subsidiaries 91,446 82,715 67,047 Interest Charges (4,043 ) (4,270 ) (6,817 ) Interest Charges to Subsidiaries (387 ) (244 ) (173 ) Interest Income from Subsidiaries 2,839 2,848 4,897 Nonservice Cost Components of Postretirement Benefits (1,422 ) (1,215 ) (1,159 ) Other Income 550 1,054 1,621 Total Other Income 88,983 80,888 65,416 Income Before Income Taxes 79,067 73,750 56,886 Income Tax (Benefit) Expense (3,278 ) 1,311 (5,435 ) Net Income $ 82,345 $ 72,439 $ 62,321 See accompanying notes to condensed financial statements. Otter Tail Corporation (PARENT COMPANY) Condensed Statements of Cash Flows--For the Years Ended December 31 (in thousands) 201 8 2017 2016 Cash Flows from Operating Activities Net Cash Provided by Operating Activities $ 56,947 $ 50,205 $ 83,296 Cash Flows from Investing Activities Return of Capital (Investment in Subsidiaries) (24,764 ) -- 9,912 Debt Repaid by (Issued to) Subsidiaries 774 151 (3,309 ) Cash (Used in) Provided by Investing Activities (623 ) (121 ) 106 Net Cash (Used in) Provided by Investing Activities (24,613 ) 30 6,709 Cash Flows from Financing Activities Change in Checks Written in Excess of Cash 31 -- (428 ) Net Short-Term Borrowings (Repayments) 9,215 -- (59,666 ) (Repayments to) Borrowings from Subsidiaries (1,281 ) 23,389 (60,948 ) Proceeds from Issuance of Common Stock -- 4,349 44,435 Common Stock Issuance Expenses (108 ) -- (562 ) Payments for Retirement of Capital Stock (3,011 ) (1,799 ) (104 ) Proceeds from the Issuance of Long-Term Debt -- -- 130,000 Short-Term and Long-Term Debt Issuance Expenses (164 ) (158 ) (723 ) Payments for Retirement of Long-Term Debt (189 ) (15,231 ) (87,547 ) Dividends Paid and Other Distributions (53,198 ) (50,632 ) (48,244 ) Net Cash Used in Financing Activities (48,705 ) (40,082 ) (83,787 ) Net Change in Cash and Cash Equivalents (16,371 ) 10,153 6,218 Cash and Cash Equivalents at Beginning of Period 16,371 6,218 -- Cash and Cash Equivalents at End of Period $ -- $ 16,371 $ 6,218 See accompanying notes to condensed financial statements. Incorporated by reference are Otter Tail Corporation’s consolidated statements of comprehensive income and common shareholders’ equity in Part II, Item 8. Basis of Presentation The condensed financial information of Otter Tail Corporation is presented to comply with Rule 12 04 X. not 10 Otter Tail Corporation’s investments in subsidiaries are presented under the equity method of accounting. Under this method, the assets and liabilities of subsidiaries are not Related Party Transactions As of December 31, 2018: (in thousands) Accounts Receivable Interest Receivable Current Notes Receivable Long-Term Notes Receivable Accounts Payable Current Notes Payable Otter Tail Power Company $ 1,877 $ -- $ -- $ -- $ 7 $ -- Vinyltech Corporation 4 17 -- 11,500 -- 15,305 Northern Pipe Products, Inc. -- 8 -- 5,522 -- 5,623 BTD Manufacturing, Inc. -- 77 415 52,000 -- -- T.O. Plastics, Inc. -- 15 -- 10,400 -- 14,308 Varistar Corporation -- -- 752 -- -- 25,390 Otter Tail Assurance Limited 50 -- -- -- -- -- $ 1,931 $ 117 $ 1,167 $ 79,422 $ 7 $ 60,626 As of December 31, 2017: (in thousands) Accounts Receivable Interest Receivable Current Notes Receivable Long-Term Notes Receivable Accounts Payable Current Notes Payable Otter Tail Power Company $ 2,067 $ -- $ -- $ -- $ 6 $ -- Vinyltech Corporation 2 17 -- 11,500 -- 20,603 Northern Pipe Products, Inc. 4 8 -- 5,711 -- 8,186 BTD Manufacturing, Inc. -- 77 -- 52,000 -- 7,260 Wind Tower Business -- -- 1,461 -- -- -- Dock and Boatlift Business -- -- 291 -- -- -- T.O. Plastics, Inc. -- 15 -- 10,400 -- 13,446 Varistar Corporation -- -- -- -- -- 12,413 Otter Tail Assurance Limited 25 -- -- -- -- -- $ 2,098 $ 117 $ 1,752 $ 79,611 $ 6 $ 61,908 Dividends Dividends paid to Otter Tail Corporation (the Parent) from its subsidiaries were as follows: (in thousands) 2018 2017 2016 Cash Dividends Paid to Parent by Subsidiaries $ 53,134 $ 50,571 $ 77,779 See Otter Tail Corporation’s notes to consolidated financial statements in Part II, Item 8 Other schedules are omitted because of the absence of the conditions under which they are required, because the amounts are insignificant or because the information required is included in the financial statements or the notes thereto.

Significant Accounting Policies

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Consolidation, Policy [Policy Text Block]	Principles of Consolidation The consolidated financial statements of Otter Tail Corporation and its wholly owned subsidiaries (the Company) include the accounts of the following segments: Electric, Manufacturing and Plastics. See note 2 980, Regulated Operations 980
Reclassification, Policy [Policy Text Block]	Prior Period Re classificatio ns In 2018, 2014 09, Revenues from Contracts with Customers (Topic 606 2014 09 2017 07, Compensation—Retirement Benefits (Topic 715 2017 07 The updates in ASU 2014 09, not The updates in ASU 2017 07 $4,405,000 2017 $3,951,000 2016, $1,215,000 2017 $1,159,000 2016 $5,620,000 2017 $5,110,000 2016. Additionally, in 2018 no December 31, 2018, 2017 2016. The above reclassifications resulted in no December 31, 2017 2016.
Regulation, Policy [Policy Text Block]	Regulation and ASC 980 The Company’s regulated electric utility company, Otter Tail Power Company (OTP), accounts for the financial effects of regulation in accordance with ASC 980. 4 OTP is subject to various state and federal agency regulations. The accounting policies followed by this business are subject to the Uniform System of Accounts of the Federal Energy Regulatory Commission (FERC). These accounting policies differ in some respects from those used by the Company’s nonelectric businesses.
Plant Retirement and Abandonment, Policy [Policy Text Block]	Plant, Retirements and Depreciation Utility plant is stated at original cost. The cost of additions includes contracted work, direct labor and materials, allocable overheads and allowance for funds used during construction. The amount of interest capitalized on electric utility plant was $1,206,000 2018, $741,000 2017 $495,000 2016. 5 82 2.76% 2018, 2.74% 2017 2.88% 2016. Property and equipment of nonelectric operations are carried at historical cost or at fair value if acquired in a business combination and are depreciated on a straight-line basis over the assets’ estimated useful lives ( 2 40 No 2018, 2017 2016.
Long-lived Assets, Policy [Policy Text Block]	Recoverability of Long-Lived Assets The Company reviews its long-lived assets whenever events or changes in circumstances indicate the carrying amount of the assets may not
Joint Ownership, Policy [Policy Text Block]	Jointly Owned Facilities OTP is a joint owner in two four one December 31, 2018 2017 Jointly Owned Facilities ( dollars in thousands) OTP Ownership Percentage Electric Plant in Service Construction Work in Progress Accumulated Depreciation Net Plant December 31, 201 8 Big Stone Plant 53.9 % $ 336,051 $ 361 $ (92,007 ) $ 244,405 Coyote Station 35.0 % 177,713 2,588 (100,997 ) 79,304 Fargo–Monticello 345 kV line 14.2 % 78,184 -- (5,891 ) 72,293 Brookings–Southeast Twin Cities 345 kV line 4.8 % 26,281 -- (1,713 ) 24,568 Bemidji–Grand Rapids 230 kV line 14.8 % 16,331 -- (2,091 ) 14,240 Big Stone South–Brookings 345 kV line 50.0 % 53,235 (150 ) (1,264 ) 51,821 Big Stone South–Ellendale 345 kV line 1 50.0 % -- 106,490 -- 106,490 December 31, 2017 Big Stone Plant 53.9 % $ 329,942 $ 1,074 $ (74,165 ) $ 256,851 Coyote Station 35.0 % 177,721 158 (103,944 ) 73,935 Fargo–Monticello 345 kV line 14.2 % 78,192 -- (4,667 ) 73,525 Brookings–Southeast Twin Cities 345 kV line 4.8 % 26,269 -- (1,293 ) 24,976 Bemidji–Grand Rapids 230 kV line 14.8 % 16,331 -- (1,753 ) 14,578 Big Stone South–Brookings 345 kV line 50.0 % 53,225 -- (434 ) 52,791 Big Stone South–Ellendale 345 kV line 1 50.0 % -- 89,980 -- 89,980 1 Midcontinent Independent System Operator, Inc. (MISO) Multi-Value Project (MVP) designation provides for a return on invested funds while under construction under the MISO Open Access Transmission, Energy and Operating Reserve Markets Tariff (MISO Tariff). The Company’s share of direct revenue and expenses of the jointly owned facilities is included in operating revenue and expenses in the consolidated statements of income.
Consolidation, Variable Interest Entity, Policy [Policy Text Block]	Coyote Station Lignite Supply Agreement – Variable Interest Entity October 2012 May 2016 December 2040. May 2016 December 2040 No none, none not If the LSA terminates prior to the expiration of its term or the production period terminates prior to December 31, 2040 December 31, 2018 $53.9 35%
Income Tax, Policy [Policy Text Block]	Income Taxes Comprehensive interperiod income tax allocation is used for substantially all book and tax temporary differences. Deferred income taxes arise for all temporary differences between the book and tax basis of assets and liabilities. Deferred taxes are recorded using the tax rates scheduled by tax law to be in effect in the periods when the temporary differences reverse. The Company amortizes investment tax credits over the estimated lives of related property. The Company records income taxes in accordance with ASC Topic 740, Income Taxes, not” not” 50%. 14 The Company also is required to assess the realizability of its deferred tax assets, taking into consideration the Company’s forecast of future taxable income, the reversal of other existing temporary differences, available net operating loss carryforwards and available tax planning strategies that could be implemented to realize the deferred tax assets. Based on this assessment, management must evaluate the need for, and amount of, valuation allowances against the Company’s deferred tax assets. To the extent facts and circumstances change in the future, adjustments to the valuation allowance may On December 22, 2017, 2017 14
Revenue Recognition, Policy [Policy Text Block]	Revenue Recognition In May 2014 2014 09, Revenue from Contracts with Customers (Topic 606 606 606 January 1, 2018 not 606 no 606 606 Due to the diverse business operations of the Company, recognition of revenue from contracts with customers depends on the product produced and sold or service performed. The Company recognizes revenue from contracts with customers, at prices that are fixed or determinable as evidenced by an agreement with the customer, when the Company has met its performance obligation under the contract and it is probable that the Company will collect the amount to which it is entitled in exchange for the goods or services transferred or to be transferred to the customer. Depending on the product produced and sold or service performed and the terms of the agreement with the customer, the Company recognizes revenue either over time, in the case of delivery or transmission of electricity or related services or the production and storage of certain custom-made products, or at a point in time for the delivery of standardized products and other products made to the customers specifications where the terms of the contract require transfer of the completed product. Based on review of the Company’s revenue streams, the Company has not 606. In addition to recognizing revenue from contracts with customers under ASC 606, 980. not Electric Segment Revenues two 1 2 Most Electric segment revenues are earned from the generation, transmission and sale of electricity to retail customers at rates approved by regulatory commissions in the states where OTP provides service. OTP also earns revenue from the transmission of electricity for others over the transmission assets it owns separately, or jointly with other transmission service providers, under rate tariffs established by the independent transmission system operator and approved by the FERC. These revenues account for over 80% 2. third ARPs provide for adjustments to rates outside of a general rate case proceeding, usually as a surcharge applied to future billings typically through the use of rate riders subject to periodic adjustments, to encourage or incentivize investments in certain areas such as conservation, renewable energy, pollution reduction or control, improved infrastructure of the transmission grid or other programs that provide benefits to the general public under public policy, laws or regulations. ARP riders generally provide for the recovery of specified costs and investments and include an incentive component to provide the regulated utility with a return on amounts invested. OTP has recovered costs and earned incentives or returns on investments subject to recovery under several ARP rate riders, including: ● In Minnesota: Transmission Cost Recovery (TCR), Environmental Cost Recovery (ECR), Renewable Resource Adjustment (RRA) and Conservation Improvement Program riders. ● In North Dakota: TCR, ECR and RRA riders. ● In South Dakota: TCR, ECR and Energy Efficiency Plan (conservation) riders. OTP accrues ARP revenue on the basis of costs incurred, investments made and returns on those investments that qualify for recovery through established riders. Amounts billed under riders in effect at the time of the billing are included in revenues from contracts with customers net of amounts billed that are subject to refund through future rider adjustments. Amounts accrued and subject to recovery through future rider rate updates and adjustments are reported as ARP revenue adjustments on a separate line in the revenue section of the Company’s consolidated statement of income. See table in note 3 December 31, 2018, 2017 2016. Manufacturing Segment Revenues company expects the customer will take. For revenue recognized on products when shipped, the operating companies have no Plastics Segment Revenues no one See operating revenue table in note 2 December 31, 2018, 2017 2016.
Agreements Subject to Legally Enforceable Netting Arrangements [Policy Text Block]	Agreements Subject to Legally Enforceable Netting Arrangements OTP has certain derivative contracts that are designated as normal purchases. Individual counterparty exposures for these contracts can be offset according to legally enforceable netting arrangements. The Company does not
Standard Product Warranty, Policy [Policy Text Block]	Warranty Reserves Certain products sold by the Company’s manufacturing and plastics companies carry product warranties for one no not three
Shipping and Handling Expenses [Policy Text Block]	Shipping and Handling Costs The Company includes revenues received for shipping and handling in operating revenues. Expenses paid for shipping and handling are recorded as part of cost of goods sold.
Use of Estimates, Policy [Policy Text Block]	U se of Estimates The Company uses estimates based on the best information available in recording transactions and balances resulting from business operations. As better information becomes available (or actual amounts are known), the recorded estimates are revised. Consequently, operating results can be affected by revisions to prior accounting estimates.
Cash and Cash Equivalents, Policy [Policy Text Block]	Cash Equivalents The Company considers all highly liquid debt instruments purchased with maturity of 90
Investment, Policy [Policy Text Block]	Investments The following table provides a breakdown of the Company’s investments at December 31: (in thousands) 2018 2017 Cost Method: Economic Development Loan Pools $ 34 $ 45 Other 123 115 Equity Method Partnerships 26 24 Marketable Debt Securities Classified as Available-for-Sale 7,484 7,160 Marketable Equity Securities Classified as Available-for-Sale 1,294 1,285 Total Investments $ 8,961 $ 8,629 The Company’s marketable securities classified as available-for-sale are held for insurance purposes and are reflected at their fair values on December 31, 2018.
Fair Value of Financial Instruments, Policy [Policy Text Block]	Fair Value Measurements The Company follows ASC Topic 820, Fair Value Measurements and Disclosures 820 820 three Level 1 1 Level 2 2 Level 3 no 3 may The following tables present, for each of the hierarchy levels, the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2018 December 31, 2017: Dec ember 3 1 , 201 8 (in thousands) Level 1 Level 2 Level 3 Assets: Investments: Equity Funds – Held by Captive Insurance Company $ 1,294 Corporate Debt Securities – Held by Captive Insurance Company $ 5,898 Government-Backed and Government-Sponsored Enterprises’ Debt Securities – Held by Captive Insurance Company 1,586 Other Assets: Money Market and Mutual Funds – Nonqualified Retirement Savings Plan 838 Total Assets $ 2,132 $ 7,484 December 31, 201 7 (in thousands) Level 1 Level 2 Level 3 Assets: Investments: Equity Funds – Held by Captive Insurance Company $ 1,285 Corporate Debt Securities – Held by Captive Insurance Company $ 5,373 Government-Backed and Government-Sponsored Enterprises’ Debt Securities – Held by Captive Insurance Company 1,787 Other Assets: Money Market and Mutual Funds – Nonqualified Retirement Savings Plan 823 Total Assets $ 2,108 $ 7,160 The valuation techniques and inputs used for the Level 2 Government-Backed and Government-Sponsored Enterprises’ and Corporate Debt Securities Held by the Company’s Captive Insurance Company third may
Inventory, Policy [Policy Text Block]	Inventories Inventories, valued at the lower of cost or net realizable value, consist of the following: December 31, December 31, (in thousands) 2018 2017 Finished Goods $ 37,130 $ 26,605 Work in Process 20,393 14,222 Raw Material, Fuel and Supplies 48,747 47,207 Total Inventories $ 106,270 $ 88,034
Goodwill and Intangible Assets, Policy [Policy Text Block]	Goodwill and Other Intangible Assets The Company accounts for goodwill and other intangible assets in accordance with the requirements of ASC Topic 350, Intangibles—Goodwill and Other, fourth may not The following tables summarize changes to goodwill by business segment during 2018 2017: (in thousands) Gross Balance December 31, 2017 Accumulated Impairments Balance (net of impairments) December 31, 2017 Adjustments to Goodwill in 2018 Balance (net of impairments) December 31, 2018 Manufacturing $ 18,270 $ -- $ 18,270 $ -- $ 18,270 Plastics 19,302 -- 19,302 -- 19,302 Total $ 37,572 $ -- $ 37,572 $ -- $ 37,572 (in thousands) Gross Balance December 31, 2016 Accumulated Impairments Balance (net of impairments) December 31, 2016 Adjustments to Goodwill in 2017 Balance (net of impairments) December 31, 2017 Manufacturing $ 18,270 $ -- $ 18,270 $ -- $ 18,270 Plastics 19,302 -- 19,302 -- 19,302 Total $ 37,572 $ -- $ 37,572 $ -- $ 37,572 Intangible assets with finite lives are amortized over their estimated useful lives and reviewed for impairment in accordance with requirements under ASC Topic 360 10 35, Property, Plant, and Equipment—Overall—Subsequent Measurement The following table summarizes the components of the Company’s intangible assets at December 31, 2018 December 31, 2017: December 31, 201 8 (in thousands) Gross Carrying Amount Accumulated Amortization Net Carrying Amount Remaining Amortization Periods (months) Amortizable Intangible Assets: Customer Relationships $ 22,491 $ 10,127 $ 12,364 12-200 Other 154 68 86 20 Total $ 22,645 $ 10,195 $ 12,450 December 31, 201 7 (in thousands) Amortizable Intangible Assets: Customer Relationships $ 22,491 $ 8,994 $ 13,497 24-212 Covenant not to Compete 590 459 131 8 Other 154 17 137 32 Total $ 23,235 $ 9,470 $ 13,765 The amortization expense for these intangible assets was: (in thousands) 2018 2017 2016 Amortization Expense – Intangible Assets $ 1,315 $ 1,347 $ 1,436 The estimated annual amortization expense for these intangible assets for the next five (in thousands) 2019 2020 2021 2022 2023 Estimated Amortization Expense – Intangible Assets $ 1,184 $ 1,133 $ 1,099 $ 1,099 $ 1,099
Cash Flow Supplemental [Policy Text Block]	Supplemental Disclosures of Cash Flow Information As of December 31, (in thousands) 2018 2017 Noncash Investing Activities: Transactions Related to Capital Additions not Settled in Cash $ 13,757 $ 13,887 (in thousands) 2018 2017 2016 Cash Paid (Received) During the Year for: Interest (net of amount capitalized) $ 28,109 $ 29,791 $ 31,269 Income Taxes $ 6,109 $ 5,064 $ (1,291 )
New Accounting Pronouncements, Policy [Policy Text Block]	New Accounting Standards Adopted ASU 2014 09 May 2014 2014 09, Revenue from Contracts with Customers (Topic 606 606 January 1, 2018 ASU 2016 01 January 2016 No. 2016 01, Financial Instruments—Overall (Subtopic 825 10 2016 01 2016 01 2016 01 December 15, 2017, 2016 01 first 2018, ASU 2017 07 March 2017 2017 07 715, Compensation—Retirement Benefits 715 , not not 2017 07 715 2017 07 2017 07 December 15, 2017, January 1, 2018. The majority of the Company’s benefit costs to which the amendments in ASU 2017 07 2017 07 2017 07. The Company’s nonservice cost components of net periodic postretirement benefit costs that were capitalized to plant in service in 2017 2017 07 2017 $0.8 2017 2016 December 31, 2018, 2017 2016 11 New Accounting Standards Pending Adoption ASU 2016 02 February 2016 No. 2016 02, Leases (Topic 842 2016 02 2016 02 842, 840 842 842 842 842 2016 02 December 15, 2018, 2016 02 2016 02 first 2019. first not 8 ASU 2017 04 January 2017 No. 2017 04, Intangibles—Goodwill and Other (Topic 350 2017 04 2 2 2, 2017 04, not The amendments in ASU 2017 04 no 2 2017 04 December 15, 2019. January 1, 2017. 2017 04 2019. 2018, no no 2 December 31, 2018. ASU 2018 02 February 2018 No. 2018 02, Income Statement—Reporting Comprehensive Income (Topic 220 2018 02 2018 02, 2018 02 December 15, 2018, 2018 02 2018 02 first 2019 not $784,000

Note 1 - Summary of Significa_2

Note 1 - Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Public Utility Property, Plant, and Equipment [Table Text Block]	Jointly Owned Facilities ( dollars in thousands) OTP Ownership Percentage Electric Plant in Service Construction Work in Progress Accumulated Depreciation Net Plant December 31, 201 8 Big Stone Plant 53.9 % $ 336,051 $ 361 $ (92,007 ) $ 244,405 Coyote Station 35.0 % 177,713 2,588 (100,997 ) 79,304 Fargo–Monticello 345 kV line 14.2 % 78,184 -- (5,891 ) 72,293 Brookings–Southeast Twin Cities 345 kV line 4.8 % 26,281 -- (1,713 ) 24,568 Bemidji–Grand Rapids 230 kV line 14.8 % 16,331 -- (2,091 ) 14,240 Big Stone South–Brookings 345 kV line 50.0 % 53,235 (150 ) (1,264 ) 51,821 Big Stone South–Ellendale 345 kV line 1 50.0 % -- 106,490 -- 106,490 December 31, 2017 Big Stone Plant 53.9 % $ 329,942 $ 1,074 $ (74,165 ) $ 256,851 Coyote Station 35.0 % 177,721 158 (103,944 ) 73,935 Fargo–Monticello 345 kV line 14.2 % 78,192 -- (4,667 ) 73,525 Brookings–Southeast Twin Cities 345 kV line 4.8 % 26,269 -- (1,293 ) 24,976 Bemidji–Grand Rapids 230 kV line 14.8 % 16,331 -- (1,753 ) 14,578 Big Stone South–Brookings 345 kV line 50.0 % 53,225 -- (434 ) 52,791 Big Stone South–Ellendale 345 kV line 1 50.0 % -- 89,980 -- 89,980
Investment [Table Text Block]	(in thousands) 2018 2017 Cost Method: Economic Development Loan Pools $ 34 $ 45 Other 123 115 Equity Method Partnerships 26 24 Marketable Debt Securities Classified as Available-for-Sale 7,484 7,160 Marketable Equity Securities Classified as Available-for-Sale 1,294 1,285 Total Investments $ 8,961 $ 8,629
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]	Dec ember 3 1 , 201 8 (in thousands) Level 1 Level 2 Level 3 Assets: Investments: Equity Funds – Held by Captive Insurance Company $ 1,294 Corporate Debt Securities – Held by Captive Insurance Company $ 5,898 Government-Backed and Government-Sponsored Enterprises’ Debt Securities – Held by Captive Insurance Company 1,586 Other Assets: Money Market and Mutual Funds – Nonqualified Retirement Savings Plan 838 Total Assets $ 2,132 $ 7,484 December 31, 201 7 (in thousands) Level 1 Level 2 Level 3 Assets: Investments: Equity Funds – Held by Captive Insurance Company $ 1,285 Corporate Debt Securities – Held by Captive Insurance Company $ 5,373 Government-Backed and Government-Sponsored Enterprises’ Debt Securities – Held by Captive Insurance Company 1,787 Other Assets: Money Market and Mutual Funds – Nonqualified Retirement Savings Plan 823 Total Assets $ 2,108 $ 7,160
Schedule of Inventory, Current [Table Text Block]	December 31, December 31, (in thousands) 2018 2017 Finished Goods $ 37,130 $ 26,605 Work in Process 20,393 14,222 Raw Material, Fuel and Supplies 48,747 47,207 Total Inventories $ 106,270 $ 88,034
Schedule of Goodwill [Table Text Block]	(in thousands) Gross Balance December 31, 2017 Accumulated Impairments Balance (net of impairments) December 31, 2017 Adjustments to Goodwill in 2018 Balance (net of impairments) December 31, 2018 Manufacturing $ 18,270 $ -- $ 18,270 $ -- $ 18,270 Plastics 19,302 -- 19,302 -- 19,302 Total $ 37,572 $ -- $ 37,572 $ -- $ 37,572 (in thousands) Gross Balance December 31, 2016 Accumulated Impairments Balance (net of impairments) December 31, 2016 Adjustments to Goodwill in 2017 Balance (net of impairments) December 31, 2017 Manufacturing $ 18,270 $ -- $ 18,270 $ -- $ 18,270 Plastics 19,302 -- 19,302 -- 19,302 Total $ 37,572 $ -- $ 37,572 $ -- $ 37,572
Schedule of Finite-Lived Intangible Assets [Table Text Block]	December 31, 201 8 (in thousands) Gross Carrying Amount Accumulated Amortization Net Carrying Amount Remaining Amortization Periods (months) Amortizable Intangible Assets: Customer Relationships $ 22,491 $ 10,127 $ 12,364 12-200 Other 154 68 86 20 Total $ 22,645 $ 10,195 $ 12,450 December 31, 201 7 (in thousands) Amortizable Intangible Assets: Customer Relationships $ 22,491 $ 8,994 $ 13,497 24-212 Covenant not to Compete 590 459 131 8 Other 154 17 137 32 Total $ 23,235 $ 9,470 $ 13,765
Finite-lived Intangible Assets Amortization Expense [Table Text Block]	(in thousands) 2018 2017 2016 Amortization Expense – Intangible Assets $ 1,315 $ 1,347 $ 1,436
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block]	(in thousands) 2019 2020 2021 2022 2023 Estimated Amortization Expense – Intangible Assets $ 1,184 $ 1,133 $ 1,099 $ 1,099 $ 1,099
Schedule of Cash Flow, Supplemental Disclosures [Table Text Block]	As of December 31, (in thousands) 2018 2017 Noncash Investing Activities: Transactions Related to Capital Additions not Settled in Cash $ 13,757 $ 13,887 (in thousands) 2018 2017 2016 Cash Paid (Received) During the Year for: Interest (net of amount capitalized) $ 28,109 $ 29,791 $ 31,269 Income Taxes $ 6,109 $ 5,064 $ (1,291 )

Note 2 - Business Segment Inf_2

Note 2 - Business Segment Information (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Schedule of Segment Reporting Information, by Segment [Table Text Block]	(in thousands) 2018 2017 2016 Operating Revenue Electric Segment: Retail Sales Revenue from Contracts with Customers $ 388,690 $ 376,902 $ 374,506 Changes in Accrued ARP Revenues (439 ) (1,971 ) 2,104 Total Retail Sales Revenue 388,251 374,931 376,610 Wholesale Revenues – Company Generation 7,735 5,173 4,584 Other Electric Revenues 54,269 54,433 46,189 Total Electric Segment Revenues 450,255 434,537 427,383 Manufacturing Segment: Metal Parts and Tooling 223,765 189,242 185,868 Plastic Products and Tooling 35,836 33,939 31,431 Other 8,808 6,557 3,990 Total Manufacturing Segment Revenues 268,409 229,738 221,289 Plastics Segment – Sale of PVC Pipe Products 197,840 185,132 154,901 Intersegment Eliminations (57 ) (57 ) (34 ) Total $ 916,447 $ 849,350 $ 803,539 Cost of Products Sold Manufacturing $ 205,699 $ 176,473 $ 171,732 Plastics 148,881 140,107 123,496 Intersegment Eliminations (21 ) (18 ) (6 ) Total $ 354,559 $ 316,562 $ 295,222 Other Nonelectric Expenses Manufacturing $ 29,650 $ 23,785 $ 21,994 Plastics 12,323 11,564 9,402 Corporate 9,607 6,182 7,315 Intersegment Eliminations (36 ) (39 ) (28 ) Total $ 51,544 $ 41,492 $ 38,683 Depreciation and Amortization Electric $ 55,935 $ 53,276 $ 53,743 Manufacturing 14,794 15,379 15,794 Plastics 3,719 3,817 3,861 Corporate 218 73 47 Total $ 74,666 $ 72,545 $ 73,445 Operating Income (Loss) Electric $ 88,031 $ 94,797 $ 94,082 Manufacturing 18,266 14,101 11,769 Plastics 32,917 29,644 18,142 Corporate (9,825 ) (6,255 ) (7,362 ) Total $ 129,389 $ 132,287 $ 116,631 Interest Charges Electric $ 26,365 $ 25,334 $ 25,069 Manufacturing 2,230 2,215 3,859 Plastics 609 633 1,034 Corporate and Intersegment Eliminations 1,204 1,422 1,924 Total $ 30,408 $ 29,604 $ 31,886 (in thousands) 2018 2017 2016 Income Tax Expense (Benefit) Electric $ 5,685 $ 17,013 $ 16,366 Manufacturing 3,393 989 2,276 Plastics 8,728 7,448 6,538 Corporate (3,218 ) 1,806 (4,961 ) Total $ 14,588 $ 27,256 $ 20,219 Net Income (Loss) Electric $ 54,431 $ 49,446 $ 49,829 Manufacturing 12,839 11,050 5,694 Plastics 23,819 21,696 10,628 Corporate (8,744 ) (9,753 ) (3,830 ) Total $ 82,345 $ 72,439 $ 62,321 Capital Expenditures Electric $ 87,287 $ 118,444 $ 149,648 Manufacturing 13,316 9,916 8,429 Plastics 4,199 4,432 3,085 Corporate 623 121 97 Total $ 105,425 $ 132,913 $ 161,259 Identifiable Assets Electric $ 1,728,534 $ 1,690,224 $ 1,622,231 Manufacturing 187,556 167,023 166,525 Plastics 91,630 87,230 84,592 Corporate 44,797 59,801 39,037 Total $ 2,052,517 $ 2,004,278 $ 1,912,385

Note 3 - Rate and Regulatory _2

Note 3 - Rate and Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Schedule of Information on Status of Updates for Previous Periods [Table Text Block]	Rate Rider R - Request Date A - Approval Date Effective Date Requested or Approved Annual Revenue ($000s) Rate Minnesota Conservation Improvement Program 2017 Incentive and Cost Recovery A – October 4, 2018 November 1, 2018 $ 10,283 $0.00600/kwh 2016 Incentive and Cost Recovery A – September 15, 2017 October 1, 2017 $ 9,868 $0.00536/kwh 2015 Incentive and Cost Recovery A – July 19, 2016 October 1, 2016 $ 8,590 $0.00275/kwh 2014 Incentive and Cost Recovery A – July 10, 2015 October 1, 2015 $ 8,689 $0.00287/kwh Transmission Cost Recovery 2018 Annual Update–Scenario A R – November 30, 2018 June 1, 2019 $ 6,475 Various –Scenario B $ 2,708 Various 2017 Rate Reset A – October 30, 2017 November 1, 2017 $ (3,311 ) Various 2016 Annual Update A – July 5, 2016 September 1, 2016 $ 4,736 Various 2015 Annual Update A – March 9, 2016 April 1, 2016 $ 7,203 Various 2014 Annual Update A – February 18, 2015 March 1, 2015 $ 8,388 Various Environmental Cost Recovery 2018 Annual Update A – November 29, 2018 December 1, 2018 $ -- 0% of base 2017 Rate Reset A – October 30, 2017 November 1, 2017 $ (1,943 ) -0.935% of base 2016 Annual Update A – July 5, 2016 September 1, 2016 $ 11,884 6.927% of base 2015 Annual Update A – March 9, 2016 October 1, 2015 $ 12,104 7.006% of base Renewable Resource Adjustment 2018 Annual Update A – August 29, 2018 November 1, 2018 $ 5,886 $.00244/kwh 2017 Rate Reset A – October 30, 2017 November 1, 2017 $ 1,279 $.00049/kwh North Dakota Renewable Resource Adjustment 2019 Annual Update R – December 31, 2018 April 1, 2019 $ (236 ) -0.224% of base 2018 Rate Reset for effect of TCJA A – February 27, 2018 March 1, 2018 $ 9,650 7.493% of base 2017 Rate Reset A – December 20, 2017 January 1, 2018 $ 9,989 7.756% of base 2016 Annual Update A – March 15, 2017 April 1, 2017 $ 9,156 7.005% of base 2015 Annual Update A – June 22, 2016 July 1, 2016 $ 9,262 7.573% of base 2014 Annual Update A – March 25, 2015 April 1, 2015 $ 5,441 4.069% of base Transmission Cost Recovery 2018 Supplemental Update A – December 6, 2018 February 1, 2019 $ 4,801 Various 2018 Rate Reset for effect of TCJA A – February 27, 2018 March 1, 2018 $ 7,469 Various 2017 Annual Update A – November 29, 2017 January 1, 2018 $ 7,959 Various 2016 Annual Update A – December 14, 2016 January 1, 2017 $ 6,916 Various 2015 Annual Update A – December 16, 2015 January 1, 2016 $ 9,985 Various Environmental Cost Recovery 2018 Update A – December 19, 2018 February 1, 2019 $ (378 ) -0.310% of base 2018 Rate Reset for effect of TCJA A – February 27, 2018 March 1, 2018 $ 7,718 5.593% of base 2017 Rate Reset A – December 20, 2017 January 1, 2018 $ 8,537 6.629% of base 2017 Annual Update A – July 12, 2017 August 1, 2017 $ 9,917 7.633% of base 2016 Annual Update A – June 22, 2016 July 1, 2016 $ 10,359 7.904% of base 2015 Annual Update A – June 17, 2015 July 1, 2015 $ 12,249 9.193% of base South Dakota Transmission Cost Recovery 2018 Interim Rate Reset A – October 18, 2018 October 18, 2018 $ 1,171 Various 2017 Annual Update A – February 28, 2018 March 1, 2018 $ 1,779 Various 2016 Annual Update A – February 17, 2017 March 1, 2017 $ 2,053 Various 2015 Annual Update A – February 12, 2016 March 1, 2016 $ 1,895 Various 2014 Annual Update A – February 13, 2015 March 1, 2015 $ 1,538 Various Environmental Cost Recovery 2018 Interim Rate Reset A – October 18, 2018 October 18, 2018 $ (189 ) -$0.00075/kwh 2017 Annual Update A – October 13, 2017 November 1, 2017 $ 2,082 $0.00483/kwh 2016 Annual Update A – October 26, 2016 November 1, 2016 $ 2,238 $0.00536/kwh 2015 Annual Update A – October 15, 2015 November 1, 2015 $ 2,728 $0.00643/kwh
Schedule of Revenues Recorded under Rate Riders [Table Text Block]	Rate Rider (in thousands) 2018 2017 2016 Minnesota Conservation Improvement Program Costs and Incentives 1 $ 12,028 $ 9,225 $ 12,920 Renewable Resource Adjustment 3,067 (196 ) -- Environmental Cost Recovery (24 ) 8,148 12,443 Transmission Cost Recovery (2,039 ) 2,973 5,795 North Dakota Renewable Resource Adjustment 8,529 7,620 7,800 Environmental Cost Recovery 7,318 9,782 11,089 Transmission Cost Recovery 7,016 8,729 7,694 South Dakota Environmental Cost Recovery 1,676 2,345 2,538 Transmission Cost Recovery 1,664 1,843 1,820 Conservation Improvement Program Costs and Incentives 628 598 468 1 Includes MNCIP costs recovered in base rates.

Note 4 - Regulatory Assets an_2

Note 4 - Regulatory Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Schedule of Regulatory Assets and Liabilities [Table Text Block]	December 31, 2018 Remaining Recovery/ (in thousands) Current Long-Term Total Refund Period (months) Regulatory Assets: Prior Service Costs and Actuarial Losses on Pensions and Other Postretirement Benefits 1 $ 6,346 $ 118,433 $ 124,779 see below Conservation Improvement Program Costs and Incentives 2 5,995 3,285 9,280 21 Accumulated ARO Accretion/Depreciation Adjustment 1 -- 7,169 7,169 asset lives Deferred Income Taxes 1 -- 2,423 2,423 asset lives Deferred Marked-to-Market Losses 1 1,661 743 2,404 24 Big Stone II Unrecovered Project Costs – Minnesota 1 681 947 1,628 28 Nonservice Costs Components of Postretirement Benefits Capitalized for Ratemaking Purposes and Subject to Deferred Recovery 1 -- 986 986 asset lives Debt Reacquisition Premiums 1 207 753 960 165 North Dakota Deferred Rate Case Expenses Subject to Recovery 1 455 -- 455 12 Minnesota Renewable Resource Recovery Rider Accrued Revenues 2 452 -- 452 12 Minnesota Transmission Cost Recovery Rider Accrued Revenues 2 444 -- 444 12 Big Stone II Unrecovered Project Costs – South Dakota 1 100 342 442 53 Minnesota Energy Intensive Trade Exposed Rider Accrued Revenues 1 328 -- 328 4 MISO Schedule 26/26A Transmission Cost Recovery Rider True-up 1 240 -- 240 12 South Dakota Deferred Rate Case Expenses Subject to Recovery 1 178 -- 178 12 Minnesota SPP Transmission Cost Recovery Tracker 1 -- 176 176 see below Minnesota Environmental Cost Recovery Rider Accrued Revenues 2 121 -- 121 12 North Dakota Environmental Cost Recovery Rider Accrued Revenues 2 17 -- 17 12 Total Regulatory Assets $ 17,225 $ 135,257 $ 152,482 Regulatory Liabilities: Deferred Income Taxes $ -- $ 142,779 $ 142,779 asset lives Accumulated Reserve for Estimated Removal Costs – Net of Salvage -- 83,229 83,229 asset lives South Dakota Environmental Cost Recovery Rider Accrued Refund 207 -- 207 12 MISO Schedule 26/26A Transmission Cost Recovery Rider True-up -- 187 187 24 North Dakota Renewable Resource Recovery Rider Accrued Refund 177 -- 177 12 South Dakota Transmission Cost Recovery Rider Accrued Refund 168 -- 168 12 Revenue for Rate Case Expenses Subject to Refund – Minnesota -- 166 166 see below Refundable Fuel Clause Adjustment Revenues 121 -- 121 12 North Dakota Transmission Cost Recovery Rider Accrued Refund 60 -- 60 12 Other 5 108 113 180 Total Regulatory Liabilities $ 738 $ 226,469 $ 227,207 Net Regulatory Asset/(Liability) Position $ 16,487 $ (91,212 ) $ (74,725 ) December 31, 2017 Remaining Recovery/ (in thousands) Current Long-Term Total Refund Period (months) Regulatory Assets: Prior Service Costs and Actuarial Losses on Pensions and Other Postretirement Benefits 1 $ 9,090 $ 112,487 $ 121,577 see below Conservation Improvement Program Costs and Incentives 2 7,385 2,774 10,159 21 Accumulated ARO Accretion/Depreciation Adjustment 1 -- 6,651 6,651 asset lives Deferred Marked-to-Market Losses 1 4,063 2,405 6,468 36 Big Stone II Unrecovered Project Costs – Minnesota 1 650 1,636 2,286 40 Debt Reacquisition Premiums 1 254 960 1,214 177 North Dakota Deferred Rate Case Expenses Subject to Recovery 1 309 -- 309 12 Big Stone II Unrecovered Project Costs – South Dakota 1 100 442 542 65 Minnesota Energy Intensive Trade Exposed Rider Accrued Revenues 1 75 -- 75 12 MISO Schedule 26/26A Transmission Cost Recovery Rider True-up 1 -- 1,985 1,985 24 North Dakota Environmental Cost Recovery Rider Accrued Revenues 2 152 -- 152 12 Minnesota Deferred Rate Case Expenses Subject to Recovery 1 267 -- 267 4 North Dakota Renewable Resource Recovery Rider Accrued Revenues 2 206 236 442 15 Total Regulatory Assets $ 22,551 $ 129,576 $ 152,127 Regulatory Liabilities: Deferred Income Taxes $ -- $ 149,052 $ 149,052 asset lives Accumulated Reserve for Estimated Removal Costs – Net of Salvage -- 83,100 83,100 asset lives South Dakota Environmental Cost Recovery Rider Accrued Refund 187 -- 187 12 MISO Schedule 26/26A Transmission Cost Recovery Rider True-up 132 48 180 24 South Dakota Transmission Cost Recovery Rider Accrued Refund 151 -- 151 12 Revenue for Rate Case Expenses Subject to Refund – Minnesota 208 -- 208 4 Refundable Fuel Clause Adjustment Revenues 5,778 -- 5,778 12 North Dakota Transmission Cost Recovery Rider Accrued Refund 349 -- 349 12 Minnesota Environmental Cost Recovery Rider Accrued Refund 1,667 -- 1,667 11 Minnesota Transmission Cost Recovery Rider Accrued Refund 802 609 1,411 22 Minnesota Renewable Resource Recovery Rider Accrued Refund 409 -- 409 12 Other 5 84 89 192 Total Regulatory Liabilities $ 9,688 $ 232,893 $ 242,581 Net Regulatory Asset/(Liability) Position $ 12,863 $ (103,317 ) $ (90,454 )

Note 5 - Common Shares and Ea_2

Note 5 - Common Shares and Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Schedule of Common Stock Outstanding Roll Forward [Table Text Block]	Common Shares Outstanding, December 31, 2017 39,557,491 Issuances: Executive Stock Performance Awards (2015 awards) 114,648 Executive Stock Performance Awards (2016 and 2017 awards) 18,600 Vesting of Restricted Stock Units 26,575 Restricted Stock Issued to Directors 18,200 Directors Deferred Compensation 578 Retirements: Shares Withheld for Individual Income Tax Requirements (71,208 ) Common Shares Outstanding, December 31, 2018 39,664,884
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]	2018 2017 2016 Weighted Average Common Shares Outstanding – Basic 39,599,944 39,457,261 38,546,459 Plus Outstanding Share Awards net of Share Reductions for Unrecognized Stock-Based Compensation Expense and Excess Tax Benefits: Shares Expected to be Awarded for Stock Performance Awards Granted to Executive Officers based on Measurement Period-to-Date Performance 212,043 210,784 118,644 Underlying Shares Related to Nonvested Restricted Stock Units Granted to Employees 59,980 56,952 45,712 Nonvested Restricted Shares 17,751 20,380 16,778 Shares Expected to be Issued Under the Deferred Compensation Program for Directors 2,478 2,970 3,417 Total Dilutive Shares 292,252 291,086 184,551 Weighted Average Common Shares Outstanding – Diluted 39,892,196 39,748,347 38,731,010

Note 6 - Share-based Payments (

Note 6 - Share-based Payments (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Schedule of Share-based Compensation, Nonemployee Director Stock Award Plan, Activity [Table Text Block]	Directors’ Restricted Stock Awards 2018 2017 2016 Shares Weighted Average Grant-Date Fair Value Shares Weighted Average Grant-Date Fair Value Shares Weighted Average Grant-Date Fair Value Nonvested, Beginning of Year 46,800 $ 32.65 46,334 $ 29.71 38,217 $ 29.78 Granted 18,200 43.40 17,600 37.75 23,200 28.66 Vested 21,775 31.94 17,134 29.93 15,083 28.28 Forfeited -- -- -- Nonvested, End of Year 43,225 37.53 46,800 32.65 46,334 29.71 Compensation Expense Recognized $ 661,000 $ 658,000 $ 491,000 Fair Value of Shares Vested in Year $ 696,000 $ 513,000 $ 427,000
Schedule of Share-based Compensation, Restricted Stock Activity [Table Text Block]	Employees’ Restricted Stock Awards 2018 2017 2016 Shares Weighted Average Grant-Date Fair Value Shares Weighted Average Grant-Date Fair Value Shares Weighted Average Grant-Date Fair Value Nonvested, Beginning of Year 2,895 $ 29.41 7,180 $ 29.72 13,581 $ 28.56 Granted -- -- -- Vested 2,895 29.41 4,285 29.94 6,401 27.25 Forfeited -- -- -- Nonvested, End of Year -- -- 2,895 29.41 7,180 29.72 Compensation Expense Recognized $ 16,000 $ 70,000 $ 96,000 Fair Value of Awards Vested $ 85,000 $ 128,000 $ 174,000
Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block]	Employees’ Restricted Stock Unit Awards 2018 2017 2016 Restricted Stock Units Weighted Average Grant-Date Fair Value Restricted Stock Units Weighted Average Grant-Date Fair Value Restricted Stock Units Weighted Average Grant-Date Fair Value Nonvested, Beginning of Year 46,440 $ 27.07 47,370 $ 25.19 46,600 $ 23.75 Granted 14,780 38.99 10,995 33.28 17,220 24.54 Vested 8,925 25.23 11,550 25.30 12,250 19.03 Forfeited 2,825 25.86 375 26.92 4,200 24.51 Nonvested, End of Year 49,470 31.03 46,440 27.07 47,370 25.19 Compensation Expense Recognized $ 351,000 $ 331,000 $ 307,000 Fair Value of Awards Vested $ 225,000 $ 292,000 $ 233,000
Key Employees [Member]
Notes Tables
Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block]	Grant Date Units Granted Grant-Date Fair Value per Award Restricted Stock Units Vesting 100% on April 8, 2022 April 9, 2018 12,945 $ 38.45 Restricted Stock Units Vesting 100% on April 8, 2022 June 20, 2018 1,000 $ 42.46 Restricted Stock Units Vesting 100% on April 8, 2022 September 25, 2018 835 $ 43.25
Executive Officers [Member]
Notes Tables
Schedule of Share-based Compensation, Restricted Stock Activity [Table Text Block]	Executives’ Restricted Stock Unit Awards 2018 2017 2016 Shares Weighted Average Grant-Date Fair Value Shares Weighted Average Grant-Date Fair Value Shares Weighted Average Grant-Date Fair Value Nonvested, Beginning of Year 47,750 $ 32.71 41,825 $ 30.23 24,300 $ 31.682 Granted 15,200 41.325 15,900 37.65 22,000 28.915 Vested 17,650 32.462 9,975 30.16 4,475 31.69 Forfeited -- -- -- Nonvested, End of Year 45,300 35.70 47,750 32.71 41,825 30.23 Compensation Expense Recognized $ 769,000 $ 576,000 $ 446,000 Fair Value of Awards Vested $ 573,000 $ 301,000 $ 142,000
Stock Performance Awards [Member]
Notes Tables
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award [Table Text Block]	Performance Period Maximum Shares Subject To Award Target Shares Expense Recognized in the Year Ended December 31, Earned Shares 2018 2017 2016 2018-2020 81,000 54,000 $ 1,121,000 2017-2019 89,250 59,500 729,000 $ 854,000 7,500 2016-2018 122,250 81,500 772,000 580,000 $ 798,000 113,298 2015-2017 126,450 84,300 23,000 573,000 535,000 114,648 2014-2016 159,450 106,300 -- -- 332,000 121,491 Total $ 2,645,000 $ 2,007,000 $ 1,665,000 356,937

Note 8 - Leases (Tables)

Note 8 - Leases (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Lessee, Operating Lease, Liability, Maturity [Table Text Block]	Operating Leases (in thousands) OTP Nonelectric Total 2019 $ 1,099 $ 5,086 $ 6,185 2020 1,077 4,800 5,877 2021 1,047 3,971 5,018 2022 214 3,740 3,954 2023 196 3,385 3,581 Beyond 2023 448 6,295 6,743 Total $ 4,081 $ 27,277 $ 31,358

Note 9 - Commitments and Cont_2

Note 9 - Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Contractual Obligation, Fiscal Year Maturity Schedule [Table Text Block]	(in thousands) Construction Program and Other Commitments Capacity and Energy Requirements Coal Purchase Commitments OTP Land Easement Payments 2019 $ 43,887 $ 24,925 $ 23,397 $ 617 2020 23,939 24,844 22,645 630 2021 1,681 12,988 22,935 642 2022 -- 11,827 22,793 655 2023 -- 11,827 23,955 668 Beyond 2023 -- 143,099 503,492 7,612 Total $ 69,507 $ 229,510 $ 619,217 $ 10,824

Note 10 - Short-term and Long_2

Note 10 - Short-term and Long-term Borrowings (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Schedule of Line of Credit Facilities [Table Text Block]	(in thousands) Line Limit In Use on December 31, 2018 Restricted due to Outstanding Letters of Credit Available on December 31, 2018 Available on December 31, 2017 Otter Tail Corporation Credit Agreement $ 130,000 $ 9,215 $ -- $ 120,785 $ 130,000 OTP Credit Agreement 170,000 9,384 300 160,316 57,329 Total $ 300,000 $ 18,599 $ 300 $ 281,101 $ 187,329
Schedule of Debt [Table Text Block]	December 31, 201 8 (in thousands) OTP Otter Tail Corporation Otter Tail Corporation Consolidated Short-Term Debt $ 9,384 $ 9,215 $ 18,599 Long-Term Debt: 3.55% Guaranteed Senior Notes, due December 15, 2026 $ 80,000 $ 80,000 Senior Unsecured Notes 4.63%, due December 1, 2021 $ 140,000 140,000 Senior Unsecured Notes 6.15%, Series B, due August 20, 2022 30,000 30,000 Senior Unsecured Notes 6.37%, Series C, due August 20, 2027 42,000 42,000 Senior Unsecured Notes 4.68%, Series A, due February 27, 2029 60,000 60,000 Senior Unsecured Notes 6.47%, Series D, due August 20, 2037 50,000 50,000 Senior Unsecured Notes 5.47%, Series B, due February 27, 2044 90,000 90,000 Senior Unsecured Notes 4.07%, Series 2018A, due February 7, 2048 100,000 100,000 PACE Note, 2.54%, due March 18, 2021 523 523 Total $ 512,000 $ 80,523 $ 592,523 Less: Current Maturities net of Unamortized Debt Issuance Costs -- 172 172 Unamortized Long-Term Debt Issuance Costs 1,942 407 2,349 Total Long-Term Debt net of Unamortized Debt Issuance Costs $ 510,058 $ 79,944 $ 590,002 Total Short-Term and Long-Term Debt (with current maturities) $ 519,442 $ 89,331 $ 608,773 December 31, 2017 (in thousands) OTP Otter Tail Corporation Otter Tail Corporation Consolidated Short-Term Debt $ 112,371 $ -- $ 112,371 Long-Term Debt: 3.55% Guaranteed Senior Notes, due December 15, 2026 $ 80,000 $ 80,000 Senior Unsecured Notes 4.63%, due December 1, 2021 $ 140,000 140,000 Senior Unsecured Notes 6.15%, Series B, due August 20, 2022 30,000 30,000 Senior Unsecured Notes 6.37%, Series C, due August 20, 2027 42,000 42,000 Senior Unsecured Notes 4.68%, Series A, due February 27, 2029 60,000 60,000 Senior Unsecured Notes 6.47%, Series D, due August 20, 2037 50,000 50,000 Senior Unsecured Notes 5.47%, Series B, due February 27, 2044 90,000 90,000 North Dakota Development Note, 3.95%, due April 1, 2018 27 27 PACE Note, 2.54%, due March 18, 2021 684 684 Total $ 412,000 $ 80,711 $ 492,711 Less: Current Maturities net of Unamortized Debt Issuance Costs -- 186 186 Unamortized Long-Term Debt Issuance Costs 1,684 461 2,145 Total Long-Term Debt net of Unamortized Debt Issuance Costs $ 410,316 $ 80,064 $ 490,380 Total Short-Term and Long-Term Debt (with current maturities) $ 522,687 $ 80,250 $ 602,937
Schedule of Maturities of Long-term Debt [Table Text Block]	(in thousands) 2019 2020 2021 2022 2023 Aggregate Amounts of Debt Maturities $ 172 $ 185 $ 140,166 $ 30,000 $ --

Note 11 - Pension Plan and Ot_2

Note 11 - Pension Plan and Other Postretirement Benefits (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Schedule of Net Benefit Costs [Table Text Block]	(in thousands) 2018 2017 2016 Service Cost–Benefit Earned During the Period $ 6,459 $ 5,629 $ 5,518 Interest Cost on Projected Benefit Obligation 13,452 14,139 14,195 Expected Return on Assets (21,199 ) (19,229 ) (19,454 ) Amortization of Prior Service Cost: From Regulatory Asset 16 120 189 From Other Comprehensive Income 1 -- 3 5 Amortization of Net Actuarial Loss: From Regulatory Asset 7,135 5,090 5,153 From Other Comprehensive Income 1 183 125 127 Net Periodic Pension Cost 2 $ 6,046 $ 5,877 $ 5,733 1 Corporate cost included in nonservice cost components of postretirement benefits. 2 Allocation of Costs: 2018 2017 2016 Service Costs included in OTP Capital Expenditures $ 1,542 $ 1,094 $ 1,009 Service costs included in electric operation and maintenance expenses 4,756 4,400 4,377 Service costs included in other nonelectric expenses 161 135 132 Nonservice costs capitalized (99 ) 48 39 Nonservice costs included in n onservice cost components of postretirement benefits (314 ) 200 176 (in thousands) 2018 2017 2016 Service Cost–Benefit Earned During the Period $ 408 $ 290 $ 252 Interest Cost on Projected Benefit Obligation 1,589 1,686 1,667 Amortization of Prior Service Cost: From Regulatory Asset 20 16 16 From Other Comprehensive Income 1 34 38 38 Amortization of Net Actuarial Loss: From Regulatory Asset 206 285 293 From Other Comprehensive Income 1 722 440 446 Net Periodic Pension Cost 2 $ 2,979 $ 2,755 $ 2,712 2 2018 2017 2016 Service costs included in electric operation and maintenance expenses $ 99 $ 94 $ 87 Service costs included in other nonelectric expenses 309 196 165 Nonservice costs included in n onservice cost components of postretirement benefits 2,571 2,465 2,460 (in thousands) 2018 2017 2016 Service Cost–Benefit Earned During the Period $ 1,526 $ 1,425 $ 1,301 Interest Cost on Projected Benefit Obligation 2,583 2,712 2,503 Amortization of Prior Service Cost From Regulatory Asset -- (4 ) 134 From Other Comprehensive Income 1 -- 4 3 Amortization of Net Actuarial Loss From Regulatory Asset 1,648 936 379 From Other Comprehensive Income 1 42 19 9 Net Periodic Postretirement Benefit Cost 2 $ 5,799 $ 5,092 $ 4,329 Effect of Medicare Part D Subsidy $ (470 ) $ (561 ) $ (923 ) 2 Allocation of Cost: 201 8 201 7 201 6 Service Costs included in OTP capital expenditures $ 364 $ 277 $ 238 Service costs included in electric operation and maintenance expenses 1,124 1,114 1,032 Service costs included in other nonelectric expenses 38 34 31 Nonservice costs capitalized 1,020 712 554 Nonservice costs included in n onservice cost components of postretirement benefits 3,253 2,955 2,474
Schedule of Amounts Recognized in Balance Sheet [Table Text Block]	(in thousands) 2018 2017 Regulatory Assets: Unrecognized Prior Service Cost $ 5 $ 21 Unrecognized Actuarial Loss 104,891 99,360 Total Regulatory Assets $ 104,896 $ 99,381 Accumulated Other Comprehensive Loss: Unrecognized Prior Service Cost $ 9 $ 9 Unrecognized Actuarial Loss 137 439 Total Accumulated Other Comprehensive Loss $ 146 $ 448 Noncurrent Liability $ 58,659 $ 67,399 (in thousands) 2018 2017 Regulatory Assets: Unrecognized Prior Service Cost $ 20 $ 40 Unrecognized Actuarial Loss 1,768 3,229 Total Regulatory Assets $ 1,788 $ 3,269 Projected Benefit Obligation Liability – Net Amount Recognized $ (39,699 ) $ (42,308 ) Accumulated Other Comprehensive Loss: Unrecognized Prior Service Cost $ 64 $ 98 Unrecognized Actuarial Loss 6,455 9,024 Total Accumulated Other Comprehensive Loss $ 6,519 $ 9,122 (in thousands) 2018 2017 Projected Benefit Obligation Liability – Net Amount Recognized $ (71,561 ) $ (69,774 ) Unrecognized Net Actuarial Loss (Gain): Regulatory Asset $ 18,094 $ 18,927 Accumulated Other Comprehensive Income: (107 ) (111 ) Unrecognized Net Actuarial Loss $ 17,987 $ 18,816
Schedule of Net Funded Status [Table Text Block]	(in thousands) 2018 2017 Accumulated Benefit Obligation $ (297,972 ) $ (316,095 ) Projected Benefit Obligation $ (328,442 ) $ (352,718 ) Fair Value of Plan Assets 269,783 285,319 Funded Status $ (58,659 ) $ (67,399 )
Schedule of Changes in Projected Benefit Obligations and Changes in Plan Assets [Table Text Block]	(in thousands) 2018 2017 Reconciliation of Fair Value of Plan Assets: Fair Value of Plan Assets at January 1 $ 285,319 $ 254,346 Actual Return on Plan Assets (21,334 ) 44,181 Discretionary Company Contributions 20,000 -- Benefit Payments (14,202 ) (13,208 ) Fair Value of Plan Assets at December 31 $ 269,783 $ 285,319 Estimated Asset Return (7.3% ) 17.8 % Reconciliation of Projected Benefit Obligation: Projected Benefit Obligation at January 1 $ 352,718 $ 314,637 Service Cost 6,459 5,629 Interest Cost 13,452 14,139 Benefit Payments (14,202 ) (13,208 ) Actuarial (Gain) Loss (29,985 ) 31,521 Projected Benefit Obligation at December 31 $ 328,442 $ 352,718 (in thousands) 2018 2017 Reconciliation of Fair Value of Plan Assets: Fair Value of Plan Assets at January 1 $ -- $ -- Actual Return on Plan Assets -- -- Employer Contributions 1,505 1,175 Benefit Payments (1,505 ) (1,175 ) Fair Value of Plan Assets at December 31 $ -- $ -- Reconciliation of Projected Benefit Obligation: Projected Benefit Obligation at January 1 $ 42,308 $ 37,335 Service Cost 408 290 Interest Cost 1,589 1,686 Benefit Payments (1,505 ) (1,175 ) Plan Amendments -- -- Actuarial (Gain) Loss (3,101 ) 4,172 Projected Benefit Obligation at December 31 $ 39,699 $ 42,308 (in thousands) 2018 2017 Reconciliation of Fair Value of Plan Assets: Fair Value of Plan Assets at January 1 $ -- $ -- Actual Return on Plan Assets -- -- Company Contributions 3,183 3,290 Benefit Payments (Net of Medicare Part D Subsidy) (6,684 ) (6,534 ) Participant Premium Payments 3,501 3,244 Fair Value of Plan Assets at December 31 $ -- $ -- Reconciliation of Projected Benefit Obligation: Projected Benefit Obligation at January 1 $ 69,774 $ 62,571 Service Cost (Net of Medicare Part D Subsidy) 1,526 1,425 Interest Cost (Net of Medicare Part D Subsidy) 2,583 2,712 Benefit Payments (Net of Medicare Part D Subsidy) (6,684 ) (6,534 ) Participant Premium Payments 3,501 3,244 Actuarial Loss 861 6,356 Projected Benefit Obligation at December 31 $ 71,561 $ 69,774 Reconciliation of Accrued Postretirement Cost: Accrued Postretirement Cost at January 1 $ (50,958 ) $ (49,156 ) Expense (5,799 ) (5,092 ) Net Company Contribution 3,183 3,290 Accrued Postretirement Cost at December 31 $ (53,574 ) $ (50,958 )
Schedule of Assumptions Used [Table Text Block]	2018 2017 Discount Rate 4.50 % 3.90 % Rate of Increase in Future Compensation Level: Participants to Age 39 4.50 % 4.50 % Participants Age 40 to Age 49 3.50 % 3.50 % Participants Age 50 and Older 2.75 % 2.75 % 2018 2017 Discount Rate 4.46 % 3.85 % Rate of Increase in Future Compensation Level: 3.40 % 2.92 % 2018 2017 Discount Rate 4.44 % 3.81 %
Schedule of Measurement Dates [Table Text Block]	<table style="margin: 0pt auto 0pt 18pt; text-indent: 0px; font-family: "Times New Roman", Times, serif; font-size: 10pt; min-; min-width: 700px;" cellspacing="0pt" cellpadding="0pt"> <tr style="font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <td style="vertical-align: bottom; border-bottom: 1px solid rgb(0, 0, 0); width: 32%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">Measurement Dates:</div> </td> <td style="vertical-align: bottom; border-bottom: 1px solid rgb(0, 0, 0); width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">2018</div> </td> <td style="vertical-align: bottom; border-bottom: 1px solid rgb(0, 0, 0); width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">2017</div> </td> </tr> <tr style="font-family: "Times New Roman", Times, serif; font-size: 10pt; background-color: rgb(204, 238, 255);"> <td style="vertical-align: top; border-bottom: 1px rgb(0, 0, 0); width: 32%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">Net Periodic Pension Cost</div> </td> <td style="vertical-align: top; border-bottom: 1px rgb(0, 0, 0); width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2018</div> </td> <td style="vertical-align: top; border-bottom: 1px rgb(0, 0, 0); width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2017</div> </td> </tr> <tr style="font-family: "Times New Roman", Times, serif; font-size: 10pt; background-color: rgb(255, 255, 255);"> <td style="vertical-align: top; width: 32%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">End of Year Benefit Obligations</div> </td> <td style="vertical-align: bottom; width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2018 projected to December 31, 2018</div> </td> <td style="vertical-align: bottom; width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2017 projected to December 31, 2017</div> </td> </tr> <tr style="font-family: "Times New Roman", Times, serif; font-size: 10pt; background-color: rgb(204, 238, 255);"> <td style="vertical-align: bottom; width: 32%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">Market Value of Assets</div> </td> <td style="vertical-align: bottom; width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">December 31, 2018</div> </td> <td style="vertical-align: bottom; width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">December 31, 2017</div> </td> </tr> </table></div><div style="display: inline; font-family: times new roman; font-size: 10pt"><table cellpadding="0pt" cellspacing="0pt" style="margin: 0pt auto 0pt 18pt; text-indent: 0px; font-family: "Times New Roman", Times, serif; font-size: 10pt; min-; min-width: 700px;"> <tr style="font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <td style="vertical-align: bottom; border-bottom: 1px solid rgb(0, 0, 0); width: 32%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">Measurement Dates:</div> </td> <td style="vertical-align: bottom; border-bottom: 1px solid rgb(0, 0, 0); width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">2018</div> </td> <td style="vertical-align: bottom; border-bottom: 1px solid rgb(0, 0, 0); width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">2017</div> </td> </tr> <tr style="background-color: rgb(204, 238, 255); font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <td style="vertical-align: bottom; border-bottom: 1px rgb(0, 0, 0); width: 32%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">Net Periodic Postretirement Benefit Cost</div> </td> <td style="vertical-align: bottom; border-bottom: 1px rgb(0, 0, 0); width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2018</div> </td> <td style="vertical-align: bottom; border-bottom: 1px rgb(0, 0, 0); width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2017</div> </td> </tr> <tr style="background-color: rgb(255, 255, 255); font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <td style="vertical-align: top; width: 32%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> </td> <td style="vertical-align: bottom; width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> </td> <td style="vertical-align: bottom; width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> </td> </tr> <tr style="background-color: rgb(204, 238, 255); font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <td style="vertical-align: top; width: 32%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">End of Year Benefit Obligations</div> </td> <td style="vertical-align: top; width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2018 projected to December 31, 2018</div> </td> <td style="vertical-align: top; width: 34%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2017 projected to December 31, 2017</div> </td> </tr> </table></div>" id="sjs-B9">&lt;div style="display: inline; font-family: times new roman; font-size: 10pt">&lt;table style="margin: 0pt auto 0pt 18pt; text-indent: 0px; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt; min-; min-width: 700px;" cellspacing="0pt" cellpadding="0pt"> &lt;tr style="font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;td style="vertical-align: bottom; border-bottom: 1px solid rgb(0, 0, 0); width: 32%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">Measurement Dates:&lt;/div> &lt;/td> &lt;td style="vertical-align: bottom; border-bottom: 1px solid rgb(0, 0, 0); width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">2018&lt;/div> &lt;/td> &lt;td style="vertical-align: bottom; border-bottom: 1px solid rgb(0, 0, 0); width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">2017&lt;/div> &lt;/td> &lt;/tr> &lt;tr style="font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt; background-color: rgb(204, 238, 255);"> &lt;td style="vertical-align: top; border-bottom: 1px rgb(0, 0, 0); width: 32%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">Net Periodic Pension Cost&lt;/div> &lt;/td> &lt;td style="vertical-align: top; border-bottom: 1px rgb(0, 0, 0); width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;">&lt;div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> &lt;/div>&lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2018&lt;/div> &lt;/td> &lt;td style="vertical-align: top; border-bottom: 1px rgb(0, 0, 0); width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;">&lt;div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> &lt;/div>&lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2017&lt;/div> &lt;/td> &lt;/tr> &lt;tr style="font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt; background-color: rgb(255, 255, 255);"> &lt;td style="vertical-align: top; width: 32%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">End of Year Benefit Obligations&lt;/div> &lt;/td> &lt;td style="vertical-align: bottom; width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;">&lt;div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> &lt;/div>&lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2018 projected to December 31, 2018&lt;/div> &lt;/td> &lt;td style="vertical-align: bottom; width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;">&lt;div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> &lt;/div>&lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2017 projected to December 31, 2017&lt;/div> &lt;/td> &lt;/tr> &lt;tr style="font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt; background-color: rgb(204, 238, 255);"> &lt;td style="vertical-align: bottom; width: 32%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">Market Value of Assets&lt;/div> &lt;/td> &lt;td style="vertical-align: bottom; width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;">&lt;div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> &lt;/div>&lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">December 31, 2018&lt;/div> &lt;/td> &lt;td style="vertical-align: bottom; width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;">&lt;div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> &lt;/div>&lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">December 31, 2017&lt;/div> &lt;/td> &lt;/tr> &lt;/table>&lt;/div>&lt;div style="display: inline; font-family: times new roman; font-size: 10pt">&lt;table cellpadding="0pt" cellspacing="0pt" style="margin: 0pt auto 0pt 18pt; text-indent: 0px; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt; min-; min-width: 700px;"> &lt;tr style="font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;td style="vertical-align: bottom; border-bottom: 1px solid rgb(0, 0, 0); width: 32%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">Measurement Dates:&lt;/div> &lt;/td> &lt;td style="vertical-align: bottom; border-bottom: 1px solid rgb(0, 0, 0); width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">2018&lt;/div> &lt;/td> &lt;td style="vertical-align: bottom; border-bottom: 1px solid rgb(0, 0, 0); width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">2017&lt;/div> &lt;/td> &lt;/tr> &lt;tr style="background-color: rgb(204, 238, 255); font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;td style="vertical-align: bottom; border-bottom: 1px rgb(0, 0, 0); width: 32%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">Net Periodic Postretirement Benefit Cost&lt;/div> &lt;/td> &lt;td style="vertical-align: bottom; border-bottom: 1px rgb(0, 0, 0); width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;">&lt;div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> &lt;/div>&lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2018&lt;/div> &lt;/td> &lt;td style="vertical-align: bottom; border-bottom: 1px rgb(0, 0, 0); width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;">&lt;div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> &lt;/div>&lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2017&lt;/div> &lt;/td> &lt;/tr> &lt;tr style="background-color: rgb(255, 255, 255); font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;td style="vertical-align: top; width: 32%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;">&nbsp;&lt;/td> &lt;td style="vertical-align: bottom; width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;">&nbsp;&lt;/td> &lt;td style="vertical-align: bottom; width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;">&nbsp;&lt;/td> &lt;/tr> &lt;tr style="background-color: rgb(204, 238, 255); font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;td style="vertical-align: top; width: 32%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;"> &lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:left;">End of Year Benefit Obligations&lt;/div> &lt;/td> &lt;td style="vertical-align: top; width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;">&lt;div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> &lt;/div>&lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2018 projected to December 31, 2018&lt;/div> &lt;/td> &lt;td style="vertical-align: top; width: 34%; font-family: &quot;Times New Roman&quot;, Times, serif; font-size: 10pt;">&lt;div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> &lt;/div>&lt;div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 1, 2017 projected to December 31, 2017&lt;/div> &lt;/td> &lt;/tr> &lt;/table>&lt;/div>
Schedule of Expected Benefit Payments [Table Text Block]	(in thousands) 2019 2020 2021 2022 2023 Years 2024-2028 $ 15,086 $ 15,689 $ 16,356 $ 17,017 $ 17,709 $ 96,186 Years (in thousands) 2019 2020 2021 2022 2023 2024-2028 $ 1,468 $ 1,527 $ 1,618 $ 2,214 $ 2,635 $ 14,215 Years (in thousands) 2019 2020 2021 2022 2023 2024-2028 $ 4,247 $ 4,322 $ 4,483 $ 4,698 $ 4,781 $ 24,183
Schedule of Weighted Average Asset Allocation [Table Text Block]	Asset Allocation 2018 2017 Large Capitalization Equity Securities 17.5 % 23.5 % International Equity Securities 17.0 % 18.1 % Small and Mid-Capitalization Equity Securities 6.7 % 8.7 % SEI Dynamic Asset Allocation Fund 4.0 % 5.0 % Emerging Markets Equity Fund 3.4 % 5.5 % Equity Securities 48.6 % 60.8 % Fixed-Income Securities and Cash 47.1 % 35.2 % Other – SEI Energy Debt Collective Fund 4.3 % 4.0 % 100.0 % 100.0 %
Schedule of Health Care Cost Trend Rates [Table Text Block]	2018 2017 Healthcare Cost-Trend Rate Assumed for Next Year Pre-65 7.00 % 5.85 % Healthcare Cost-Trend Rate Assumed for Next Year Post-65 7.00 % 6.03 % Rate to Which the Cost-Trend Rate is Assumed to Decline 4.50 % 4.50 % Year the Rate Reaches the Ultimate Trend Rate 2038 2038
Schedule of Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates [Table Text Block]	(in thousands) 1 Point Increase 1 Point Decrease Effect on the Postretirement Benefit Obligation $ 9,095 $ (7,586 ) Effect on Total of Service and Interest Cost $ 758 $ (600 ) Effect on Expense $ 1,953 $ (1,589 )
Pension Plan [Member]
Notes Tables
Schedule of Assumptions Used in Calculating Net Periodic Benefit Cost [Table Text Block]	2018 2017 2016 Discount Rate 3.90 % 4.60 % 4.76 % Long-Term Rate of Return on Plan Assets 7.50 % 7.50 % 7.75 % Rate of Increase in Future Compensation Level See below 3.00 % 3.13 % Participants to Age 39 4.50 % Participants Age 40 to Age 49 3.50 % Participants Age 50 and Older 2.75 % 2018 2017 2016 Discount Rate 3.85 % 4.60 % 4.76 % Rate of Increase in Future Compensation Level 2.92 % 3.00 % 3.13 % 2018 2017 2016 Discount Rate 3.81 % 4.46 % 4.57 %
Schedule of Amounts in Regulatory Assets and Accumulated Other Comprehensive Income (Loss) to Be Recognized Over Next Fiscal Year [Table Text Block]	(in thousands) 2019 Decrease in Regulatory Assets: Amortization of Unrecognized Prior Service Cost $ 5 Amortization of Unrecognized Actuarial Loss 4,642 Decrease in Accumulated Other Comprehensive Loss: Amortization of Unrecognized Prior Service Cost 9 Amortization of Unrecognized Actuarial Loss 114 Total Estimated Amortization $ 4,770 (in thousands) 2019 Decrease in Regulatory Assets: Amortization of Unrecognized Prior Service Cost $ 5 Amortization of Unrecognized Actuarial Loss 124 Decrease in Accumulated Other Comprehensive Loss: Amortization of Unrecognized Prior Service Cost 17 Amortization of Unrecognized Actuarial Loss 348 Total Estimated Amortization $ 494 (in thousands) 2019 Decrease in Regulatory Assets: Amortization of Unrecognized Actuarial Loss $ 1,570 Decrease in Accumulated Other Comprehensive Loss: Amortization of Unrecognized Actuarial Loss 39 Total Estimated Amortization $ 1,609
Schedule of Target and Tactical Allocation of Plan Assets [Table Text Block]	Permitted Range Asset Class / PBO Funded Status < 85% PBO >=85% PBO >=90% PBO >=95% PBO >=100% PBO Equity 39% - 59% 34% - 54% 24% - 44% 14% - 34% 0% - 20% Investment Grade Fixed Income 22% - 42% 30% - 50% 40% - 60% 53% - 73% 70% - 100% Below Investment Grade Fixed Income* 0% - 15% 0% - 15% 0% - 15% 0% - 10% 0% - 10% Other** 5% - 20% 5% - 20% 5% - 20% 0% - 15% 0% - 15%
Schedule of Allocation of Plan Assets [Table Text Block]	(in thousands) 2018 2017 Assets in Level 1 of the Fair Value Hierarchy $ 258,307 $ 273,999 SEI Energy Debt Collective Fund at NAV 11,476 11,320 Total Assets $ 269,783 $ 285,319
Fair Value Measurement Inputs and Valuation Techniques [Table Text Block]	(in thousands) 2018 2017 Large Capitalization Equity Securities Mutual Fund $ 47,198 $ 66,946 International Equity Securities Mutual Funds 45,912 51,636 Small and Mid-Capitalization Equity Securities Mutual Fund 17,971 24,848 SEI Dynamic Asset Allocation Mutual Fund 10,929 14,371 Emerging Markets Equity Fund 9,197 15,824 Fixed Income Securities Mutual Funds 127,098 100,373 Cash Management – Money Market Fund 2 1 Total Assets $ 258,307 $ 273,999

Note 12 - Fair Value of Finan_2

Note 12 - Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Fair Value, by Balance Sheet Grouping [Table Text Block]	December 31, 2018 December 31, 2017 (in thousands) Carrying Amount Fair Value Carrying Amount Fair Value Cash and Cash Equivalents $ 861 $ 861 $ 16,216 $ 16,216 Short-Term Debt (18,599 ) (18,599 ) (112,371 ) (112,371 ) Long-Term Debt including Current Maturities (590,174 ) (601,513 ) (490,566 ) (543,691 )

Note 13 - Property, Plant and_2

Note 13 - Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Property, Plant and Equipment [Table Text Block]	(in thousands) December 31, 2018 December 31, 2017 Electric Plant in Service Production $ 905,224 $ 897,732 Transmission 512,832 500,352 Distribution 502,261 482,867 General 99,404 100,067 Electric Plant in Service 2,019,721 1,981,018 Construction Work in Progress 170,090 132,556 Total Gross Electric Plant 2,189,811 2,113,574 Less Accumulated Depreciation and Amortization 699,642 662,431 Net Electric Plant $ 1,490,169 $ 1,451,143 Nonelectric Operations Plant Equipment $ 170,634 $ 160,263 Buildings and Leasehold Improvements 53,011 52,280 Land 4,475 4,394 Nonelectric Operations Plant 228,120 216,937 Construction Work in Progress 11,536 8,511 Total Gross Nonelectric Plant 239,656 225,448 Less Accumulated Depreciation and Amortization 148,727 136,988 Net Nonelectric Operations Plant $ 90,929 $ 88,460 Net Plant $ 1,581,098 $ 1,539,603
Schedule of Useful Lives for Property, Plant Equipment [Table Text Block]	Service Life Range (years) Low High Electric Fixed Assets: Production Plant 9 82 Transmission Plant 42 70 Distribution Plant 5 68 General Plant 5 50 Nonelectric Fixed Assets: Equipment 2 12 Buildings and Leasehold Improvements 5 40

Note 14 - Income Taxes (Tables)

Note 14 - Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]	(in thousands) 2018 2017 2016 Tax Computed at Federal Statutory Rate $ 20,356 $ 34,893 $ 28,889 Increases (Decreases) in Tax from: State Income Taxes Net of Federal Income Tax Expense 5,210 4,368 2,869 Differences Reversing in Excess of Federal Rates (3,432 ) 551 77 Federal Production Tax Credits (PTCs) (3,111 ) (7,527 ) (7,175 ) Permanent Differences, R&D Tax Credits, Unitary Tax and Other Adjustments (1,864 ) (1,873 ) (1,262 ) North Dakota Wind Tax Credit Amortization – Net of Federal Taxes (1,033 ) (850 ) (850 ) Excess Tax deduction - Equity Method Stock Awards (708 ) (751 ) -- Allowance for Funds Used During Construction – Equity (431 ) (322 ) (280 ) Employee Stock Ownership Plan Dividend Deduction (298 ) (509 ) (537 ) Investment Tax Credit Amortization (98 ) (164 ) (350 ) Corporate-owned Life Insurance (3 ) (845 ) (680 ) Section 199 Domestic Production Activities Deduction -- (1,471 ) (482 ) Effect of TCJA Tax Rate Reduction on Value of Net Deferred Tax Assets -- 1,756 -- Income Tax Expense $ 14,588 $ 27,256 $ 20,219 Overall Effective Federal, State and Foreign Income Tax Rate 15.0 % 27.3 % 24.5 % Income Tax Expense Includes the Following: Current Federal Income Taxes $ 4,960 $ 4,434 $ 989 Current State Income Taxes 1,395 1,128 1,208 Deferred Federal Income Taxes 8,065 25,648 23,774 Deferred State Income Taxes 4,410 4,587 2,623 Federal PTCs (3,111 ) (7,527 ) (7,175 ) North Dakota Wind Tax Credit Amortization – Net of Federal Taxes (1,033 ) (850 ) (850 ) Investment Tax Credit Amortization (98 ) (164 ) (350 ) Total $ 14,588 $ 27,256 $ 20,219 Total Income Before Income Taxes $ 96,933 $ 99,695 $ 82,540
Schedule of Deferred Tax Assets and Liabilities [Table Text Block]	(in thousands) 2018 2017 Deferred Tax Assets Benefit Liabilities $ 33,967 $ 32,328 Regulatory Tax Liability 33,228 39,465 Retirement Benefits Liabilities 32,664 31,894 North Dakota Wind Tax Credits 32,570 32,962 Federal PTCs 32,101 40,614 Cost of Removal 21,787 21,800 Differences Related to Property 6,842 6,499 Net Operating Loss Carryforward 2,489 3,203 Vacation Accrual 1,919 1,844 Investment Tax Credits 449 515 Other 3,218 668 Valuation Allowance (600 ) -- Total Deferred Tax Assets $ 200,634 $ 211,792 Deferred Tax Liabilities Differences Related to Property $ (261,396 ) $ (257,906 ) Retirement Benefits Regulatory Asset (32,664 ) (31,894 ) Excess Tax over Book Pension (15,145 ) (14,077 ) North Dakota Wind Tax Credits (4,386 ) (4,112 ) Impact of State Net Operating Losses on Federal Taxes (523 ) (673 ) Other (7,496 ) (3,631 ) Total Deferred Tax Liabilities $ (321,610 ) $ (312,293 ) Deferred Income Taxes $ (120,976 ) $ (100,501 )
Schedule of Expiration of Operating Loss Carryforwards and Tax Credit Carryforwards [Table Text Block]	(in thousands) Amount 2022-2032 2033-2038 2039-2043 United States Federal Tax Credits $ 34,586 $ -- $ 34,586 $ -- State Net Operating Losses 2,489 2,396 93 -- State Tax Credits 33,106 273 3,326 29,507
Summary of Income Tax Contingencies [Table Text Block]	(in thousands) 2018 2017 2016 Balance on January 1 $ 684 $ 891 $ 468 Increases Related to Tax Positions for Prior Years 6 28 406 Decreases Related to Tax Positions for Prior Years -- (172 ) -- Increases Related to Tax Positions for Current Year 778 143 114 Uncertain Positions Resolved During Year (186 ) (206 ) (97 ) Balance on December 31 $ 1,282 $ 684 $ 891
Impacts of Tax Cuts an Jobs Act on Deferred Taxes and Regulatory Liabilities [Table Text Block]	(in thousands) Deferred Tax Liability Deferred Tax Regulatory Liability Balance on January 1, 2017 $ 226,591 $ 818 Change due to 2017Accruals and Amortizations 20,012 376 TCJA Deferred Tax Valuation Adjustment (109,072 ) 109,072 Tax Effect on TCJA Deferred Tax Valuation Adjustment (38,786 ) 38,786 TCJA Adjustment to Income Tax Expense 1,756 -- Balance on December 31, 2017 $ 100,501 $ 149,052

Note 15 - Asset Retirement Ob_2

Note 15 - Asset Retirement Obligations (AROs) (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Schedule of Change in Asset Retirement Obligation [Table Text Block]	(in thousands) 2018 2017 Asset Retirement Obligations Beginning Balance $ 8,719 $ 8,341 New Obligations Recognized -- -- Adjustments Due to Revisions in Cash Flow Estimates -- -- Accrued Accretion 398 378 Settlements -- -- Ending Balance $ 9,117 $ 8,719 Asset Retirement Costs Capitalized Beginning Balance $ 2,983 $ 2,983 New Obligations Recognized -- -- Adjustments Due to Revisions in Cash Flow Estimates -- -- Settlements -- -- Ending Balance $ 2,983 $ 2,983 Accumulated Depreciation – Asset Retirement Costs Capitalized Beginning Balance $ 915 $ 795 New Obligations Recognized -- -- Adjustments Due to Revisions in Cash Flow Estimates -- -- Depreciation Expense 119 120 Settlements -- -- Ending Balance $ 1,034 $ 915 Settlements None None Original Capitalized Asset Retirement Cost – Retired $ -- $ -- Accumulated Depreciation -- -- Asset Retirement Obligation $ -- $ -- Settlement Cost -- -- Gain on Settlement – Deferred Under Regulatory Accounting $ -- $ --

Note 16 - Subsequent Events (Ta

Note 16 - Subsequent Events (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Schedule of Subsequent Events [Table Text Block]	Award Shares/Units Granted Weighted Average Grant- Date Fair Value per Award Vesting Restricted Stock Units Granted 15,600 $ 49.6225 25% per year through February 6, 2023 Stock Performance Awards Granted: Under Executive Agreement 47,800 $ 42.875 December 31, 2021 Under Legacy Agreement 7,800 $ 45.885 December 31, 2021

Schedule 1 - Condensed Financ_2

Schedule 1 - Condensed Financial Information of Registrant (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Schedule of Related Party Transactions [Table Text Block]	As of December 31, 2018: (in thousands) Accounts Receivable Interest Receivable Current Notes Receivable Long-Term Notes Receivable Accounts Payable Current Notes Payable Otter Tail Power Company $ 1,877 $ -- $ -- $ -- $ 7 $ -- Vinyltech Corporation 4 17 -- 11,500 -- 15,305 Northern Pipe Products, Inc. -- 8 -- 5,522 -- 5,623 BTD Manufacturing, Inc. -- 77 415 52,000 -- -- T.O. Plastics, Inc. -- 15 -- 10,400 -- 14,308 Varistar Corporation -- -- 752 -- -- 25,390 Otter Tail Assurance Limited 50 -- -- -- -- -- $ 1,931 $ 117 $ 1,167 $ 79,422 $ 7 $ 60,626 As of December 31, 2017: (in thousands) Accounts Receivable Interest Receivable Current Notes Receivable Long-Term Notes Receivable Accounts Payable Current Notes Payable Otter Tail Power Company $ 2,067 $ -- $ -- $ -- $ 6 $ -- Vinyltech Corporation 2 17 -- 11,500 -- 20,603 Northern Pipe Products, Inc. 4 8 -- 5,711 -- 8,186 BTD Manufacturing, Inc. -- 77 -- 52,000 -- 7,260 Wind Tower Business -- -- 1,461 -- -- -- Dock and Boatlift Business -- -- 291 -- -- -- T.O. Plastics, Inc. -- 15 -- 10,400 -- 13,446 Varistar Corporation -- -- -- -- -- 12,413 Otter Tail Assurance Limited 25 -- -- -- -- -- $ 2,098 $ 117 $ 1,752 $ 79,611 $ 6 $ 61,908
Cash Dividends Paid to Parent by Subsidiaries [Table Text Block]	(in thousands) 2018 2017 2016 Cash Dividends Paid to Parent by Subsidiaries $ 53,134 $ 50,571 $ 77,779
Parent Company [Member]
Notes Tables
Condensed Balance Sheet [Table Text Block]	SCHEDULE 1 - Condensed financial information of registrant Otter Tail Corporation (PARENT COMPANY) Condensed Balance Sheets, December 31 (in thousands) 201 8 2017 Assets Current Assets Cash and Cash Equivalents $ -- $ 16,371 Accounts Receivable -- -- Accounts Receivable from Subsidiaries 1,931 2,098 Interest Receivable from Subsidiaries 117 117 Notes Receivable from Subsidiaries 1,167 1,752 Other 3,482 1,130 Total Current Assets 6,697 21,468 Investments in Subsidiaries 787,869 724,613 Notes Receivable from Subsidiaries 79,422 79,611 Deferred Income Taxes 21,100 27,923 Other Assets 31,547 31,559 Total Assets $ 926,635 $ 885,174 Liabilities and Equity Current Liabilities Short-Term Debt $ 9,215 $ -- Current Maturities of Long-Term Debt 172 186 Accounts Payable to Subsidiaries 7 6 Notes Payable to Subsidiaries 60,626 61,908 Other 9,994 7,799 Total Current Liabilities 80,014 69,899 Other Noncurrent Liabilities 37,814 38,319 Commitments and Contingencies Capitalization Long-Term Debt, Net of Current Maturities 79,944 80,064 Common Shareholder Equity 728,863 696,892 Total Capitalization 808,807 776,956 Total Liabilities and Equity $ 926,635 $ 885,174
Condensed Income Statement [Table Text Block]	Otter Tail Corporation (PARENT COMPANY) Condensed Statements of Income--For the Years Ended December 31 (in thousands) 201 8 2017 2016 Operating Loss Revenue from Contracts with Customers $ -- $ -- $ -- Operating Expenses 9,916 7,138 8,530 Operating Loss (9,916 ) (7,138 ) (8,530 ) Other Income (Expense) Equity Income in Earnings of Subsidiaries 91,446 82,715 67,047 Interest Charges (4,043 ) (4,270 ) (6,817 ) Interest Charges to Subsidiaries (387 ) (244 ) (173 ) Interest Income from Subsidiaries 2,839 2,848 4,897 Nonservice Cost Components of Postretirement Benefits (1,422 ) (1,215 ) (1,159 ) Other Income 550 1,054 1,621 Total Other Income 88,983 80,888 65,416 Income Before Income Taxes 79,067 73,750 56,886 Income Tax (Benefit) Expense (3,278 ) 1,311 (5,435 ) Net Income $ 82,345 $ 72,439 $ 62,321
Condensed Cash Flow Statement [Table Text Block]	Otter Tail Corporation (PARENT COMPANY) Condensed Statements of Cash Flows--For the Years Ended December 31 (in thousands) 201 8 2017 2016 Cash Flows from Operating Activities Net Cash Provided by Operating Activities $ 56,947 $ 50,205 $ 83,296 Cash Flows from Investing Activities Return of Capital (Investment in Subsidiaries) (24,764 ) -- 9,912 Debt Repaid by (Issued to) Subsidiaries 774 151 (3,309 ) Cash (Used in) Provided by Investing Activities (623 ) (121 ) 106 Net Cash (Used in) Provided by Investing Activities (24,613 ) 30 6,709 Cash Flows from Financing Activities Change in Checks Written in Excess of Cash 31 -- (428 ) Net Short-Term Borrowings (Repayments) 9,215 -- (59,666 ) (Repayments to) Borrowings from Subsidiaries (1,281 ) 23,389 (60,948 ) Proceeds from Issuance of Common Stock -- 4,349 44,435 Common Stock Issuance Expenses (108 ) -- (562 ) Payments for Retirement of Capital Stock (3,011 ) (1,799 ) (104 ) Proceeds from the Issuance of Long-Term Debt -- -- 130,000 Short-Term and Long-Term Debt Issuance Expenses (164 ) (158 ) (723 ) Payments for Retirement of Long-Term Debt (189 ) (15,231 ) (87,547 ) Dividends Paid and Other Distributions (53,198 ) (50,632 ) (48,244 ) Net Cash Used in Financing Activities (48,705 ) (40,082 ) (83,787 ) Net Change in Cash and Cash Equivalents (16,371 ) 10,153 6,218 Cash and Cash Equivalents at Beginning of Period 16,371 6,218 -- Cash and Cash Equivalents at End of Period $ -- $ 16,371 $ 6,218

Note 1 - Summary of Significa_3

Note 1 - Summary of Significant Accounting Policies (Details Textual) - USD ($)	3 Months Ended	12 Months Ended			36 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Utilities Operating Expense, Maintenance and Operations, Total		$ 155,534,000	$ 146,914,000	$ 147,274,000
Other Cost and Expense, Operating, Total		51,544,000	41,492,000	38,683,000
Net Periodic Defined Benefits Expense (Reversal of Expense), Excluding Service Cost Component		$ 5,509,000	5,620,000	5,110,000
Standard Product Warranty Term		1 year
Standard Product Warranty Accrual, Decrease for Payments					$ 0
Coyote Creek Mining Company, L.L.C. (CCMC) [Member] | Otter Tail Power Company [Member] | Lignite Sales Agreement [Member]
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount		$ 53,900,000			$ 53,900,000
Variable Interest Entity Reporting Entity Involvement, Maximum Loss Exposure, Percentage		35.00%
Electric Plant [Member]
Interest Costs Capitalized		$ 1,206,000	$ 741,000	$ 495,000
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service		2.76%	2.74%	2.88%
Electric Plant [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life		5 years
Electric Plant [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life		82 years
Nonelectric Plant [Member]
Interest Costs Capitalized		$ 0	$ 0	$ 0
Nonelectric Plant [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life		2 years
Nonelectric Plant [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life		40 years
Accounting Standards Update 2017-07 [Member]
Utilities Operating Expense, Maintenance and Operations, Total			(4,405,000)	(3,951,000)
Other Cost and Expense, Operating, Total			(1,215,000)	(1,159,000)
Net Periodic Defined Benefits Expense (Reversal of Expense), Excluding Service Cost Component			5,620,000	$ 5,110,000
Accounting Standards Update 2017-07 [Member] | Pro Forma [Member]
Postretirement Benefit Plan, Nonservice Costs Capitalized to Plant in Service During Fiscal Year			$ 800,000
Accounting Standards Update 2018-02 [Member] | Scenario, Forecast [Member]
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	$ 784,000

Note 1 - Summary of Significa_4

Note 1 - Summary of Significant Accounting Policies - OTP's Ownership Interests in Jointly Owned Facilities (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Big Stone Plant [Member]
Electric Plant in Service		$ 336,051	$ 329,942
Construction Work in Progress		361	1,074
Accumulated Depreciation		(92,007)	(74,165)
Net Plant		$ 244,405	$ 256,851
Big Stone Plant [Member] | Otter Tail Power Company [Member]
Ownership Percentage		53.90%	53.90%
Coyote Station [Member]
Electric Plant in Service		$ 177,713	$ 177,721
Construction Work in Progress		2,588	158
Accumulated Depreciation		(100,997)	(103,944)
Net Plant		$ 79,304	$ 73,935
Coyote Station [Member] | Otter Tail Power Company [Member]
Ownership Percentage		35.00%	35.00%
Fargo Project [Member]
Electric Plant in Service		$ 78,184	$ 78,192
Construction Work in Progress
Accumulated Depreciation		(5,891)	(4,667)	[1]
Net Plant		$ 72,293	$ 73,525
Fargo Project [Member] | Otter Tail Power Company [Member]
Ownership Percentage		14.20%	14.20%
Brookings Project [Member]
Electric Plant in Service		$ 26,281	$ 26,269
Construction Work in Progress
Accumulated Depreciation		(1,713)	(1,293)
Net Plant		$ 24,568	$ 24,976
Brookings Project [Member] | Otter Tail Power Company [Member]
Ownership Percentage		4.80%	4.80%
Bemidji Project [Member]
Electric Plant in Service		$ 16,331	$ 16,331
Construction Work in Progress
Accumulated Depreciation		(2,091)	(1,753)
Net Plant		$ 14,240	$ 14,578
Bemidji Project [Member] | Otter Tail Power Company [Member]
Ownership Percentage		14.80%	14.80%
Big Stone South Brookings Multi Value Project [Member]
Electric Plant in Service		$ 53,235	$ 53,225
Construction Work in Progress		(150)
Accumulated Depreciation		(1,264)	(434)
Net Plant		$ 51,821	$ 52,791
Big Stone South Brookings Multi Value Project [Member] | Otter Tail Power Company [Member]
Ownership Percentage		50.00%	50.00%
Big Stone South Ellendale Multi Value Project [Member]
Electric Plant in Service	[1]
Construction Work in Progress	[1]	106,490	89,980
Accumulated Depreciation	[1]
Net Plant	[1]	$ 106,490	$ 89,980
Big Stone South Ellendale Multi Value Project [Member] | Otter Tail Power Company [Member]
Ownership Percentage	[1]	50.00%	50.00%
[1]	Midcontinent Independent System Operator, Inc. (MISO) Multi-Value Project (MVP) designation provides for a return on invested funds while under construction under the MISO Open Access Transmission, Energy and Operating Reserve Markets Tariff (MISO Tariff).

Note 1 - Summary of Significa_5

Note 1 - Summary of Significant Accounting Policies - Breakdown of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Total Investments	$ 8,961	$ 8,629
Economic Development Loan Pools [Member]
Cost Method	34	45
Other Cost Method [Member]
Cost Method	123	115
Partnerships [Member]
Equity Method Partnerships	26	24
Debt Securities [Member]
Marketable Securities Classified as Available-for-Sale	7,484	7,160
Equity Securities [Member]
Marketable Securities Classified as Available-for-Sale	$ 1,294	$ 1,285

Note 1 - Summary of Significa_6

Note 1 - Summary of Significant Accounting Policies - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Inputs, Level 1 [Member]
Assets:
Total Assets	$ 2,132	$ 2,108
Fair Value, Inputs, Level 1 [Member] | Equity Funds [Member]
Assets:
Investments	1,294	1,285
Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Assets:
Investments
Fair Value, Inputs, Level 1 [Member] | Government-backed and Government-sponsored Enterprises' Debt Securities [Member]
Assets:
Investments
Fair Value, Inputs, Level 1 [Member] | Money Market and Mutual Funds [Member]
Assets:
Other Assets	838	823
Fair Value, Inputs, Level 2 [Member]
Assets:
Total Assets	7,484	7,160
Fair Value, Inputs, Level 2 [Member] | Equity Funds [Member]
Assets:
Investments
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Assets:
Investments	5,898	5,373
Fair Value, Inputs, Level 2 [Member] | Government-backed and Government-sponsored Enterprises' Debt Securities [Member]
Assets:
Investments	1,586	$ 1,787
Fair Value, Inputs, Level 2 [Member] | Money Market and Mutual Funds [Member]
Assets:
Other Assets

Note 1 - Summary of Significa_7

Note 1 - Summary of Significant Accounting Policies - Inventories (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Finished Goods	$ 37,130	$ 26,605
Work in Process	20,393	14,222
Raw Material, Fuel and Supplies	48,747	47,207
Total Inventories	$ 106,270	$ 88,034

Note 1 - Summary of Significa_8

Note 1 - Summary of Significant Accounting Policies - Summary of Changes to Goodwill by Business Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Gross Balance		$ 37,572	$ 37,572
Accumulated Impairments
Balance	$ 37,572	37,572	37,572
Adjustments to Goodwill
Manufacturing [Member]
Gross Balance		18,270	18,270
Accumulated Impairments
Balance	18,270	18,270	18,270
Adjustments to Goodwill
Plastics [Member]
Gross Balance		19,302	19,302
Accumulated Impairments
Balance	19,302	19,302	$ 19,302
Adjustments to Goodwill

Note 1 - Summary of Significa_9

Note 1 - Summary of Significant Accounting Policies - Components of Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Amortizable Intangible Assets:
Gross Carrying Amount	$ 22,645	$ 23,235
Accumulated Amortization	10,195	9,470
Net Carrying Amount	12,450	13,765
Customer Relationships [Member]
Amortizable Intangible Assets:
Gross Carrying Amount	22,491	22,491
Accumulated Amortization	10,127	8,994
Net Carrying Amount	$ 12,364	$ 13,497
Customer Relationships [Member] | Minimum [Member]
Amortizable Intangible Assets:
Remaining Amortization Periods (Month)	1 year	2 years
Customer Relationships [Member] | Maximum [Member]
Amortizable Intangible Assets:
Remaining Amortization Periods (Month)	16 years 240 days	17 years 240 days
Other Intangible Assets [Member]
Amortizable Intangible Assets:
Gross Carrying Amount	$ 154	$ 154
Accumulated Amortization	68	17
Net Carrying Amount	$ 86	$ 137
Remaining Amortization Periods (Month)	1 year 240 days	2 years 240 days
Covenant Not to Compete [Member]
Amortizable Intangible Assets:
Gross Carrying Amount		$ 590
Accumulated Amortization		459
Net Carrying Amount		$ 131
Remaining Amortization Periods (Month)		240 days

Note 1 - Summary of Signific_10

Note 1 - Summary of Significant Accounting Policies - Amortization Expense for Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amortization Expense – Intangible Assets	$ 1,315	$ 1,347	$ 1,436

Note 1 - Summary of Signific_11

Note 1 - Summary of Significant Accounting Policies - Estimated Annual Amortization Expense for Intangible Assets (Details) $ in Thousands	Dec. 31, 2018USD ($)
2,019	$ 1,184
2,020	1,133
2,021	1,099
2,022	1,099
2,023	$ 1,099

Note 1 - Summary of Signific_12

Note 1 - Summary of Significant Accounting Policies - Supplemental Disclosure of Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Transactions Related to Capital Additions not Settled in Cash	$ 13,757	$ 13,887
Cash Paid (Received) During the Year for:
Interest (net of amount capitalized)	28,109	29,791	$ 31,269
Income Taxes	$ 6,109	$ 5,064	$ (1,291)

Note 2 - Business Segment Inf_3

Note 2 - Business Segment Information (Details Textual)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Reportable Segments	3
Sales Revenue, Segment [Member] | Customer Concentration Risk [Member]
Number of Customers	0	0	0
Sales Revenue, Segment [Member] | Customer Concentration Risk [Member] | Electric [Member]
Number of Customers	1
Concentration Risk, Percentage	11.20%
Sales Revenue, Segment [Member] | Customer Concentration Risk [Member] | Manufacturing [Member]
Number of Customers	5
Concentration Risk, Percentage	52.00%
Sales Revenue, Segment [Member] | Customer Concentration Risk [Member] | Manufacturing [Member] | Customer that Manufactures and Sells Recreational Vehicles [Member]
Number of Customers	1
Concentration Risk, Percentage	22.20%
Sales Revenue, Segment [Member] | Customer Concentration Risk [Member] | Manufacturing [Member] | Customer that Manufactures and Sells Lawn and Garden Equipment [Member]
Number of Customers	1
Concentration Risk, Percentage	11.20%
Sales Revenue, Segment [Member] | Customer Concentration Risk [Member] | Plastics [Member]
Number of Customers	2
Sales Revenue, Segment [Member] | Customer Concentration Risk [Member] | Plastics [Member] | Customer One [Member]
Concentration Risk, Percentage	22.10%
Sales Revenue, Segment [Member] | Customer Concentration Risk [Member] | Plastics [Member] | Customer Two [Member]
Concentration Risk, Percentage	17.00%
Sales Revenue, Net [Member] | UNITED STATES
Concentration Risk, Percentage	98.40%	98.20%	98.60%

Note 2 - Business Segment Inf_4

Note 2 - Business Segment Information - Business Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total	$ 916,447	$ 849,350	$ 803,539
Cost of Products Sold	354,559	316,562	295,222
Other Nonelectric Expenses	51,544	41,492	38,683
Depreciation and Amortization	74,666	72,545	73,445
Operating Income (Loss)	129,389	132,287	116,631
Interest Charges	30,408	29,604	31,886
Income Tax Expense (Benefit)	14,588	27,256	20,219
Net Income (Loss)	82,345	72,439	62,321
Capital Expenditures	105,425	132,913	161,259
Identifiable Assets	2,052,517	2,004,278	1,912,385
Corporate, Non-Segment [Member]
Other Nonelectric Expenses	9,607	6,182	7,315
Depreciation and Amortization	218	73	47
Operating Income (Loss)	(9,825)	(6,255)	(7,362)
Income Tax Expense (Benefit)	(3,218)	1,806	(4,961)
Net Income (Loss)	(8,744)	(9,753)	(3,830)
Capital Expenditures	623	121	97
Identifiable Assets	44,797	59,801	39,037
Intersegment Eliminations [Member]
Regulated operating revenues	(57)	(57)	(34)
Cost of Products Sold	(21)	(18)	(6)
Other Nonelectric Expenses	(36)	(39)	(28)
Electric [Member] | Operating Segments [Member]
Revenue from Contracts with Customers	54,269	54,433	46,189
Regulated operating revenues	450,255	434,537	427,383
Depreciation and Amortization	55,935	53,276	53,743
Operating Income (Loss)	88,031	94,797	94,082
Interest Charges	26,365	25,334	25,069
Income Tax Expense (Benefit)	5,685	17,013	16,366
Net Income (Loss)	54,431	49,446	49,829
Capital Expenditures	87,287	118,444	149,648
Identifiable Assets	1,728,534	1,690,224	1,622,231
Electric [Member] | Operating Segments [Member] | Retail [Member]
Revenue from Contracts with Customers	388,690	376,902	374,506
Changes in Accrued ARP Revenues	(439)	(1,971)	2,104
Regulated operating revenues	388,251	374,931	376,610
Electric [Member] | Operating Segments [Member] | Wholesale [Member]
Revenue from Contracts with Customers	7,735	5,173	4,584
Manufacturing [Member] | Operating Segments [Member]
Revenue from Contracts with Customers	268,409	229,738	221,289
Cost of Products Sold	205,699	176,473	171,732
Other Nonelectric Expenses	29,650	23,785	21,994
Depreciation and Amortization	14,794	15,379	15,794
Operating Income (Loss)	18,266	14,101	11,769
Interest Charges	2,230	2,215	3,859
Income Tax Expense (Benefit)	3,393	989	2,276
Net Income (Loss)	12,839	11,050	5,694
Capital Expenditures	13,316	9,916	8,429
Identifiable Assets	187,556	167,023	166,525
Manufacturing [Member] | Operating Segments [Member] | Metal Parts and Tooling [Member]
Revenue from Contracts with Customers	223,765	189,242	185,868
Manufacturing [Member] | Operating Segments [Member] | Plastic Products [Member]
Revenue from Contracts with Customers	35,836	33,939	31,431
Manufacturing [Member] | Operating Segments [Member] | Manufactured Product, Other [Member]
Revenue from Contracts with Customers	8,808	6,557	3,990
Plastics [Member] | Operating Segments [Member]
Revenue from Contracts with Customers	197,840	185,132	154,901
Cost of Products Sold	148,881	140,107	123,496
Other Nonelectric Expenses	12,323	11,564	9,402
Depreciation and Amortization	3,719	3,817	3,861
Operating Income (Loss)	32,917	29,644	18,142
Interest Charges	609	633	1,034
Income Tax Expense (Benefit)	8,728	7,448	6,538
Net Income (Loss)	23,819	21,696	10,628
Capital Expenditures	4,199	4,432	3,085
Identifiable Assets	91,630	87,230	84,592
Corporate and Eliminations [Member]
Interest Charges	$ 1,204	$ 1,422	$ 1,924

Note 3 - Rate and Regulatory _3

Note 3 - Rate and Regulatory Matters (Details Textual) - USD ($)	Oct. 16, 2018	Sep. 26, 2018	Apr. 20, 2018	Mar. 23, 2018	Mar. 22, 2018	Nov. 17, 2017	Mar. 31, 2017	Nov. 16, 2016	Sep. 28, 2016	May 25, 2016	Feb. 12, 2015	Nov. 06, 2014	Nov. 12, 2013	Jun. 01, 2011	Jun. 30, 2016	Dec. 22, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Oct. 04, 2018	Jun. 13, 2018	Mar. 01, 2018	Dec. 20, 2017	Nov. 02, 2017	Oct. 31, 2017	Apr. 16, 2016	Apr. 15, 2016	Nov. 25, 2009
Loss Contingency, Estimate of Possible Loss																	$ 1,000,000
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent																	21.00%	35.00%	35.00%
Regulatory Liabilities, Total																	$ 227,207,000	$ 242,581,000
Minnesota [Member]
Accrued Refund Liabilities Resulting from Tax Rate Reduction																	8,400,000
NORTH DAKOTA
Accrued Refund Liabilities Resulting from Tax Rate Reduction																	800,000
South Dakota [Member]
Accrued Refund Liabilities Resulting from Tax Rate Reduction																	1,000,000
Federal Energy Regulatory Commission [Member]
Accrued Refund Liabilities Resulting from Tax Rate Reduction																	200,000
Otter Tail Power Company [Member]
Increase in Reagent Costs and Emission Allowances																	$ 2,200,000
Environmental Cost Recovery Rider Rate																	0.00%
Otter Tail Power Company [Member] | Litigatio Case with Minnesota Customers [Member]
Loss Contingency, Estimate of Possible Loss																	$ 2,300,000
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member]
Contract with Customer, Refund Liability, Total																	1,600,000
Current Return on Equity Used in Transmission Rates									10.32%				12.38%			10.32%
Proposed Reduced Return on Equity Used in Transmission Rates											8.67%		9.15%		9.70%
Additional Incentive Basis Point												0.50%
Expected Percentage of Return on Equity	10.41%								10.82%
Regulatory Liabilities, Total																	$ 1,600,000		$ 2,700,000
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member] | Maximum [Member]
Proposed Reduced Return on Equity Used in Transmission Rates	13.08%
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member]
Percentage of Reagent Costs and Emission Allowances Shared																	50.00%
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | The 2016 General Rate Case [Member]
Percentage of Allowed Rate of Return on Rate Base																										7.5056%	8.61%
Percentage of Allowed Rate of Return on Equity																										9.41%	10.74%
Estimated Interim Rate Refund						$ 9,000,000
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Environmental Cost Recovery Rider [Member] | The 2016 General Rate Case [Member]
Revenues Collected Under Riders, Subject to Customer Refund																									$ 900,000
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Transmission Cost Recovery Rider [Member] | The 2016 General Rate Case [Member]
Revenues Collected Under Riders, Subject to Customer Refund																									$ 1,400,000
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | ECR and TCR Riders [Member] | The 2016 General Rate Case [Member]
Revenues Collected Under Riders, Subject to Refund, Period of Refund																		1 year
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Conservation Improvement Program [Member]
Utility Incentive Percentage in Next Rolling Twelve Months										13.50%
Utility Incentive Percentage in Next Rolling Year Two										12.00%
Utility Incentive Percentage in Next Rolling Year Three										10.00%
Assumed Savings of Utility										1.70%
Financial Incentive, Maximum Percentage of Spending, Year One										40.00%
Financial Incentive, Maximum Percentage of Spending, Year Two										35.00%
Financial Incentive, Maximum Percentage of Spending, Year Three										30.00%
Expected Rate of Financial Incentive Reduction										50.00%
Financial Incentives Recognized During Period																	$ 3,000,000	$ 2,600,000	5,100,000
Financial Incentive, True-up Amount																			$ 100,000
Percentage Increase in Energy Savings																			18.00%
Performance Incentive							$ 5,000,000
Percentage Decrease in Energy Savings																		10.00%
Amount Of Financial Incentive Requested																		$ 2,600,000		$ 2,900,000	$ 2,900,000
Amount Of Financial Incentive Requested, At Risk of Subsequent Refund																	300,000
Financial Incentive, Potential Amount to be Earned																	$ 3,150,000
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Conservation Improvement Program [Member] | The 2016 General Rate Case [Member] | Minimum [Member]
Percentage of Operating Revenue from Service to Be Invested in Energy Conservation																	1.50%
Otter Tail Power Company [Member] | North Dakota Public Service Commission [Member]
Percentage of Reagent Costs and Emission Allowances Shared																	40.00%
Otter Tail Power Company [Member] | North Dakota Public Service Commission [Member] | The 2010 General Rate Case [Member]
Percentage of Allowed Rate of Return on Rate Base																								7.97%				8.62%
Percentage of Allowed Rate of Return on Equity																								10.30%				10.75%
General Rate Revenue Increase Requested																							$ 12,800,000	$ 13,100,000				$ 3,600,000
Percentage of Increase in Base Rate Revenue Requested																								8.72%				3.00%
Public Utilities, Interim Rate Requirement, Decrease in Amount																						$ 4,500,000
Public Utilities, Interim Rate Requirement, Amount																						$ 8,300,000
Otter Tail Power Company [Member] | North Dakota Public Service Commission [Member] | The 2017 General Rate Case [Member]
Percentage of Allowed Rate of Return on Equity				10.30%
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 4,600,000		$ 7,100,000	$ 13,100,000
Public Utilities, Requested Rate Increase (Decrease), Percentage		3.10%		4.80%
General Rate Revenue Increase Requested, Decrease				$ 6,000,000
General Rate Revenue Increase Requested, Decrease, Amount Related to Tax Reform				4,800,000
General Rate Revenue Increase Requested, Decrease, Amount Related to Updates Other Than Tax Reform				$ 1,200,000
Percentage of Requested Allowed Rate of Return on Equity		9.77%
Equity to Total Capitalization Ratio Basis for Return on Equity		52.50%
Contract with Customer, Refund Liability, Total																	$ 3,000,000
Excess Revenue Collected for Income Taxes																	$ 800,000
Otter Tail Power Company [Member] | South Dakota Public Utilities Commission [Member]
Percentage of Reagent Costs and Emission Allowances Shared																	10.00%
Otter Tail Power Company [Member] | South Dakota Public Utilities Commission [Member] | The 2018 General Rate Case [Member]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 3,300,000
Increase in Annual Non-fuel Rates Requested, Step One, Percentage			10.10%
Increase in Annual Non-fuel Rates Requested, Step Two, Percentage			1.70%
Otter Tail Power Company [Member] | South Dakota Public Utilities Commission [Member] | The 2010 General Rate Case [Member]
Public Utilities General Rate Revenue Increase Approved														$ 643,000
Percentage of Increase in Base Rate Revenue Approved														2.32%
Public Utilities Allowed Rate of Return on Rate Base Subsequent to Approval of Increase in Base Rate														8.50%
Otter Tail Power Company [Member] | Astoria Station [Member]
Current Project Cost																	$ 8,300,000
Otter Tail Power Company [Member] | Big Stone South - Ellendale MVP [Member] | Federal Energy Regulatory Commission [Member]
Current Project Cost																	$ 106,000,000
Expanded Capacity of Projects																	345
Extended Distance of Transmission Line																	163
Percentage of Assets of Project																	100.00%
Otter Tail Power Company [Member] | Big Stone South - Brookings MVP [Member]
Current Project Cost																	$ 73,000,000
Expanded Capacity of Projects																	345
Extended Distance of Transmission Line																	70
Percentage of Assets of Project																	100.00%
Jointly Owned Utility Plant, Proportionate Ownership Share																	50.00%
Otter Tail Power Company [Member] | EDF [Member] | Merricourt Project [Member]
Asset Purchase Agreement, Purchase Price								$ 34,700,000
Turnkey Engineering, Procurement and Construction Services Agreement, Costs								$ 200,500,000
Current Project Cost																	$ 4,900,000

Note 3 - Rate and Regulatory _4

Note 3 - Rate and Regulatory Matters - Summary of Status of Updates for Previous Two Years for Various Rate Riders (Details) - Otter Tail Power Company [Member] $ in Thousands		12 Months Ended
		Dec. 31, 2018USD ($)kWh	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Minnesota [Member] | Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue	[1]	$ 12,028	$ 9,225	$ 12,920
Minnesota [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2017 Incentive and Cost Recovery [Member]
A - Approval Date		Oct. 4, 2018
Effective Date Requested or Approved		Nov. 1, 2018
Annual Revenue		$ 10,283
Rate rider rate (Kilowatt-Hour) | kWh		0.006
Minnesota [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2016 Incentive and Cost Recovery [Member]
A - Approval Date		Sep. 15, 2017
Effective Date Requested or Approved		Oct. 1, 2017
Annual Revenue		$ 9,868
Rate rider rate (Kilowatt-Hour) | kWh		0.00536
Minnesota [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2015 Incentive and Cost Recovery [Member]
A - Approval Date		Jul. 19, 2016
Effective Date Requested or Approved		Oct. 1, 2016
Annual Revenue		$ 8,590
Rate rider rate (Kilowatt-Hour) | kWh		0.00275
Minnesota [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2014 Incentive and Cost Recovery [Member]
A - Approval Date		Jul. 10, 2015
Effective Date Requested or Approved		Oct. 1, 2015
Annual Revenue		$ 8,689
Rate rider rate (Kilowatt-Hour) | kWh		0.00287
Minnesota [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue		$ (2,039)	2,973	5,795
Minnesota [Member] | Transmission Cost Recovery Rider [Member] | The 2018 Annual Update - Scenario A [Member]
Effective Date Requested or Approved		Jun. 1, 2019
Annual Revenue		$ 6,475
R - Request Date		Nov. 30, 2018
Rate		Various
Minnesota [Member] | Transmission Cost Recovery Rider [Member] | The 2018 Annual Update - Scenario B [Member]
Annual Revenue		$ 2,708
Rate		Various
Minnesota [Member] | Transmission Cost Recovery Rider [Member] | The 2017 Rate Reset [Member]
A - Approval Date		Oct. 30, 2017
Effective Date Requested or Approved		Nov. 1, 2017
Annual Revenue		$ (3,311)
Rate		Various
Minnesota [Member] | Transmission Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
A - Approval Date		Jul. 5, 2016
Effective Date Requested or Approved		Sep. 1, 2016
Annual Revenue		$ 4,736
Rate		Various
Minnesota [Member] | Transmission Cost Recovery Rider [Member] | The 2015 Annual Update [Member]
A - Approval Date		Mar. 9, 2016
Effective Date Requested or Approved		Apr. 1, 2016
Annual Revenue		$ 7,203
Rate		Various
Minnesota [Member] | Transmission Cost Recovery Rider [Member] | The 2014 Annual Update [Member]
A - Approval Date		Feb. 18, 2015
Effective Date Requested or Approved		Mar. 1, 2015
Annual Revenue		$ 8,388
Rate		Various
Minnesota [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue		$ (24)	8,148	12,443
Minnesota [Member] | Environmental Cost Recovery Rider [Member] | The 2017 Rate Reset [Member]
A - Approval Date		Oct. 30, 2017
Effective Date Requested or Approved		Nov. 1, 2017
Annual Revenue		$ (1,943)
Rate of base		(0.935%)
Minnesota [Member] | Environmental Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
A - Approval Date		Jul. 5, 2016
Effective Date Requested or Approved		Sep. 1, 2016
Annual Revenue		$ 11,884
Rate of base		6.927%
Minnesota [Member] | Environmental Cost Recovery Rider [Member] | The 2015 Annual Update [Member]
A - Approval Date		Mar. 9, 2016
Effective Date Requested or Approved		Oct. 1, 2015
Annual Revenue		$ 12,104
Rate of base		7.006%
Minnesota [Member] | Environmental Cost Recovery Rider [Member] | The 2018 Annual Update [Member]
A - Approval Date		Nov. 29, 2018
Effective Date Requested or Approved		Dec. 1, 2018
Annual Revenue
Rate of base		0.00%
Minnesota [Member] | Renewable Resource Adjustment [Member]
Annual Revenue		$ 3,067	(196)
Minnesota [Member] | Renewable Resource Adjustment [Member] | The 2017 Rate Reset [Member]
A - Approval Date		Oct. 30, 2017
Effective Date Requested or Approved		Nov. 1, 2017
Annual Revenue		$ 1,279
Rate rider rate (Kilowatt-Hour) | kWh		0.00049
Minnesota [Member] | Renewable Resource Adjustment [Member] | The 2018 Annual Update [Member]
A - Approval Date		Aug. 29, 2018
Effective Date Requested or Approved		Nov. 1, 2018
Annual Revenue		$ 5,886
Rate rider rate (Kilowatt-Hour) | kWh		0.00244
North Dakota [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue		$ 7,016	8,729	7,694
North Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
A - Approval Date		Dec. 14, 2016
Effective Date Requested or Approved		Jan. 1, 2017
Annual Revenue		$ 6,916
Rate		Various
North Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2015 Annual Update [Member]
A - Approval Date		Dec. 16, 2015
Effective Date Requested or Approved		Jan. 1, 2016
Annual Revenue		$ 9,985
Rate		Various
North Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2018 Rate Reset for Effect of TCJA [Member]
A - Approval Date		Feb. 27, 2018
Effective Date Requested or Approved		Mar. 1, 2018
Annual Revenue		$ 7,469
Rate		Various
North Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2018 Supplemental Update [Member]
A - Approval Date		Dec. 6, 2018
Effective Date Requested or Approved		Feb. 1, 2019
Annual Revenue		$ 4,801
Rate		Various
North Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2017 Annual Update [Member]
A - Approval Date		Nov. 29, 2017
Effective Date Requested or Approved		Jan. 1, 2018
Annual Revenue		$ 7,959
Rate		Various
North Dakota [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue		$ 7,318	9,782	11,089
North Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2017 Rate Reset [Member]
A - Approval Date		Dec. 20, 2017
Effective Date Requested or Approved		Jan. 1, 2018
Annual Revenue		$ 8,537
Rate of base		6.629%
North Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
A - Approval Date		Jun. 22, 2016
Effective Date Requested or Approved		Jul. 1, 2016
Annual Revenue		$ 10,359
Rate of base		7.904%
North Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2015 Annual Update [Member]
A - Approval Date		Jun. 17, 2015
Effective Date Requested or Approved		Jul. 1, 2015
Annual Revenue		$ 12,249
Rate of base		9.193%
North Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2018 Annual Update [Member]
A - Approval Date		Dec. 19, 2018
Effective Date Requested or Approved		Feb. 1, 2019
Annual Revenue		$ (378)
Rate of base		(0.31%)
North Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2018 Rate Reset for Effect of TCJA [Member]
A - Approval Date		Feb. 27, 2018
Effective Date Requested or Approved		Mar. 1, 2018
Annual Revenue		$ 7,718
Rate of base		5.593%
North Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2017 Annual Update [Member]
A - Approval Date		Jul. 12, 2017
Effective Date Requested or Approved		Aug. 1, 2017
Annual Revenue		$ 9,917
Rate of base		7.633%
North Dakota [Member] | Renewable Resource Adjustment [Member]
Annual Revenue		$ 8,529	7,620	7,800
North Dakota [Member] | Renewable Resource Adjustment [Member] | The 2017 Rate Reset [Member]
A - Approval Date		Dec. 20, 2017
Effective Date Requested or Approved		Jan. 1, 2018
Annual Revenue		$ 9,989
Rate of base		7.756%
North Dakota [Member] | Renewable Resource Adjustment [Member] | The 2016 Annual Update [Member]
A - Approval Date		Mar. 15, 2017
Effective Date Requested or Approved		Apr. 1, 2017
Annual Revenue		$ 9,156
Rate of base		7.005%
North Dakota [Member] | Renewable Resource Adjustment [Member] | The 2015 Annual Update [Member]
A - Approval Date		Jun. 22, 2016
Effective Date Requested or Approved		Jul. 1, 2016
Annual Revenue		$ 9,262
Rate of base		7.573%
North Dakota [Member] | Renewable Resource Adjustment [Member] | The 2014 Annual Update [Member]
A - Approval Date		Mar. 25, 2015
Effective Date Requested or Approved		Apr. 1, 2015
Annual Revenue		$ 5,441
Rate of base		4.069%
North Dakota [Member] | Renewable Resource Adjustment [Member] | The 2019 Annual Update [Member]
Effective Date Requested or Approved		Apr. 1, 2019
Annual Revenue		$ (236)
R - Request Date		Dec. 31, 2018
Rate of base		(0.224%)
North Dakota [Member] | Renewable Resource Adjustment [Member] | The 2018 Rate Reset for Effect of TCJA [Member]
A - Approval Date		Feb. 27, 2018
Effective Date Requested or Approved		Mar. 1, 2018
Annual Revenue		$ 9,650
Rate of base		7.493%
South Dakota [Member] | Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue		$ 628	598	468
South Dakota [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue		$ 1,664	1,843	1,820
South Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
A - Approval Date		Feb. 17, 2017
Effective Date Requested or Approved		Mar. 1, 2017
Annual Revenue		$ 2,053
Rate		Various
South Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2015 Annual Update [Member]
A - Approval Date		Feb. 12, 2016
Effective Date Requested or Approved		Mar. 1, 2016
Annual Revenue		$ 1,895
Rate		Various
South Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2014 Annual Update [Member]
A - Approval Date		Feb. 13, 2015
Effective Date Requested or Approved		Mar. 1, 2015
Annual Revenue		$ 1,538
Rate		Various
South Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2017 Annual Update [Member]
A - Approval Date		Feb. 28, 2018
Effective Date Requested or Approved		Mar. 1, 2018
Annual Revenue		$ 1,779
Rate		Various
South Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2018 Interim Rate Reset [Member]
A - Approval Date		Oct. 18, 2018
Effective Date Requested or Approved		Oct. 18, 2018
Annual Revenue		$ 1,171
Rate		Various
South Dakota [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue		$ 1,676	$ 2,345	$ 2,538
South Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
A - Approval Date		Oct. 26, 2016
Effective Date Requested or Approved		Nov. 1, 2016
Annual Revenue		$ 2,238
Rate rider rate (Kilowatt-Hour) | kWh		0.00536
South Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2015 Annual Update [Member]
A - Approval Date		Oct. 15, 2015
Effective Date Requested or Approved		Nov. 1, 2015
Annual Revenue		$ 2,728
Rate rider rate (Kilowatt-Hour) | kWh		0.00643
South Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2017 Annual Update [Member]
A - Approval Date		Oct. 13, 2017
Effective Date Requested or Approved		Nov. 1, 2017
Annual Revenue		$ 2,082
Rate rider rate (Kilowatt-Hour) | kWh		0.00483
South Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2018 Interim Rate Reset [Member]
A - Approval Date		Oct. 18, 2018
Effective Date Requested or Approved		Oct. 18, 2018
Annual Revenue		$ (189)
Rate rider rate (Kilowatt-Hour) | kWh		(0.00075)
[1]	Includes MNCIP costs recovered in base rates.

Note 3 - Rate and Regulatory _5

Note 3 - Rate and Regulatory Matters - Summary of Revenues Recorded Under Rate Riders (Details) - Otter Tail Power Company [Member] - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Minnesota [Member] | Conservation Improvement Program Costs and Incentives [Member]
Revenues recorded under rate riders	[1]	$ 12,028	$ 9,225	$ 12,920
Minnesota [Member] | Renewable Resource Adjustment [Member]
Revenues recorded under rate riders		3,067	(196)
Minnesota [Member] | Environmental Cost Recovery Rider [Member]
Revenues recorded under rate riders		(24)	8,148	12,443
Minnesota [Member] | Transmission Cost Recovery Rider [Member]
Revenues recorded under rate riders		(2,039)	2,973	5,795
North Dakota [Member] | Renewable Resource Adjustment [Member]
Revenues recorded under rate riders		8,529	7,620	7,800
North Dakota [Member] | Environmental Cost Recovery Rider [Member]
Revenues recorded under rate riders		7,318	9,782	11,089
North Dakota [Member] | Transmission Cost Recovery Rider [Member]
Revenues recorded under rate riders		7,016	8,729	7,694
South Dakota [Member] | Conservation Improvement Program Costs and Incentives [Member]
Revenues recorded under rate riders		628	598	468
South Dakota [Member] | Environmental Cost Recovery Rider [Member]
Revenues recorded under rate riders		1,676	2,345	2,538
South Dakota [Member] | Transmission Cost Recovery Rider [Member]
Revenues recorded under rate riders		$ 1,664	$ 1,843	$ 1,820
[1]	Includes MNCIP costs recovered in base rates.

Note 4 - Regulatory Assets an_3

Note 4 - Regulatory Assets and Liabilities (Details Textual)	12 Months Ended
	Dec. 31, 2018
Regulatory Noncurrent Asset, Remaining Recovery Period	13 years 270 days

Note 4 - Regulatory Assets an_4

Note 4 - Regulatory Assets and Liabilities - Amount of Regulatory Assets and Liabilities Recorded on Consolidated Balance Sheet (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017
Regulatory Assets - Current		$ 17,225		$ 22,551
Regulatory Assets - Long -Term		135,257		129,576
Regulatory Assets - Total		$ 152,482		$ 152,127
Regulatory Assets - Remaining Recovery/Refund Period (Month)
Regulatory Liabilities - Current		$ 738		$ 9,688
Regulatory Liabilities - Long -Term		226,469		232,893
Regulatory Liabilities, Total		$ 227,207		$ 242,581
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)
Net Regulatory Asset Position - Current		$ 16,487		$ 12,863
Net Regulatory Asset Position - Long-Term		(91,212)		(103,317)
Net Regulatory Asset/(Liability) Position		(74,725)		(90,454)
Deferred Income Taxes [Member]
Regulatory Liabilities - Current
Regulatory Liabilities - Long -Term		142,779		149,052
Regulatory Liabilities, Total		$ 142,779		$ 149,052
Regulatory Liabilities - Remaining Recovery/Refund Period		asset lives		asset lives
Accumulated Reserve for Estimated Removal Costs - Net of Salvage [Member]
Regulatory Liabilities - Current
Regulatory Liabilities - Long -Term		83,229		83,100
Regulatory Liabilities, Total		$ 83,229		$ 83,100
Regulatory Liabilities - Remaining Recovery/Refund Period		asset lives		asset lives
South Dakota Environmental Cost Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current		$ 207		$ 187
Regulatory Liabilities - Long -Term
Regulatory Liabilities, Total		$ 207		$ 187
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		1 year		1 year
MISO Schedule 26/26A Transmission Cost Recovery Rider True-up [Member]
Regulatory Liabilities - Current				$ 132
Regulatory Liabilities - Long -Term		187		48
Regulatory Liabilities, Total		$ 187		$ 180
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		2 years		2 years
South Dakota Transmission Cost Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current		$ 168		$ 151
Regulatory Liabilities - Long -Term
Regulatory Liabilities, Total		$ 168		$ 151
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		1 year		1 year
Revenue for Rate Case Expenses Subject to Refund - Minnesota [Member]
Regulatory Liabilities - Current				$ 208
Regulatory Liabilities - Long -Term		166
Regulatory Liabilities, Total		$ 166		$ 208
Regulatory Liabilities - Remaining Recovery/Refund Period		see below
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)				120 days
Refundable Fuel Clause Adjustment Revenues [Member]
Regulatory Liabilities - Current		$ 121		$ 5,778
Regulatory Liabilities - Long -Term
Regulatory Liabilities, Total		$ 121		$ 5,778
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		1 year		1 year
North Dakota Transmission Cost Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current		$ 60		$ 349
Regulatory Liabilities - Long -Term
Regulatory Liabilities, Total		$ 60		$ 349
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		1 year		1 year
Minnesota Environmental Cost Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current				$ 1,667
Regulatory Liabilities - Long -Term
Regulatory Liabilities, Total				$ 1,667
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)				330 days
North Dakota Renewable Resource Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current		$ 177
Regulatory Liabilities - Long -Term
Regulatory Liabilities, Total		$ 177
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		1 year
Minnesota Transmission Cost Recovery Rider Accrued Refund [member]
Regulatory Liabilities - Current				$ 802
Regulatory Liabilities - Long -Term				609
Regulatory Liabilities, Total				$ 1,411
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)				1 year 300 days
Minnesota Renewable Resource Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current				$ 409
Regulatory Liabilities - Long -Term
Regulatory Liabilities, Total				$ 409
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)				1 year
Other [Member]
Regulatory Liabilities - Current		$ 5		$ 5
Regulatory Liabilities - Long -Term		108		84
Regulatory Liabilities, Total		$ 113		$ 89
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		15 years		16 years
Prior Service Costs and Actuarial Losses on Pensions and Other Postretirement Benefits [Member]
Regulatory Assets - Current	[1]	$ 6,346		$ 9,090
Regulatory Assets - Long -Term	[1]	118,433		112,487
Regulatory Assets - Total	[1]	$ 124,779		$ 121,577
Regulatory Assets - Remaining Recovery/Refund Period	[1]	see below		see below
Conservation Improvement Program Costs and Incentives [Member]
Regulatory Assets - Current	[2]	$ 5,995		$ 7,385
Regulatory Assets - Long -Term	[2]	3,285		2,774
Regulatory Assets - Total	[2]	$ 9,280		$ 10,159
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]	1 year 270 days		1 year 270 days
Accumulated ARO Accretion/Depreciation Adjustment [Member]
Regulatory Assets - Current	[1]
Regulatory Assets - Long -Term	[1]	7,169		6,651
Regulatory Assets - Total	[1]	$ 7,169		$ 6,651
Regulatory Assets - Remaining Recovery/Refund Period	[1]	asset lives		asset lives
Deferred Income Taxes [Member]
Regulatory Assets - Current	[1]
Regulatory Assets - Long -Term	[1]	2,423
Regulatory Assets - Total	[1]	$ 2,423
Regulatory Assets - Remaining Recovery/Refund Period	[1]	asset lives
Deferred Marked-to-Market Losses [Member]
Regulatory Assets - Current	[1]	$ 1,661		$ 4,063
Regulatory Assets - Long -Term	[1]	743		2,405
Regulatory Assets - Total	[1]	$ 2,404		$ 6,468
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	2 years		3 years
Big Stone II Unrecovered Project Costs - Minnesota [Member]
Regulatory Assets - Current	[1]	$ 681		$ 650
Regulatory Assets - Long -Term	[1]	947		1,636
Regulatory Assets - Total	[1]	$ 1,628		$ 2,286
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	2 years 120 days		3 years 120 days
Debt Reacquisition Premiums [Member]
Regulatory Assets - Current	[1]	$ 207		$ 254
Regulatory Assets - Long -Term	[1]	753		960
Regulatory Assets - Total	[1]	$ 960		$ 1,214
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	13 years 270 days		14 years 270 days
Nonservice Costs Components of Postretirement Benefits Capitalized for Ratemaking Purposes and Subject to Deferred Recovery [Member]
Regulatory Assets - Current	[1]
Regulatory Assets - Long -Term	[1]	986
Regulatory Assets - Total	[1]	$ 986
Regulatory Assets - Remaining Recovery/Refund Period	[1]	asset lives
North Dakota Deferred Rate Case Expenses Subject to Recovery [Member]
Regulatory Assets - Current	[1]	$ 455		$ 309
Regulatory Assets - Long -Term	[1]
Regulatory Assets - Total		$ 455	[1]	$ 309
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	1 year		1 year
Big Stone II Unrecovered Project Costs - South Dakota [Member]
Regulatory Assets - Current	[1]	$ 100		$ 100
Regulatory Assets - Long -Term	[1]	342		442
Regulatory Assets - Total		$ 442	[1]	$ 542
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	4 years 150 days		5 years 150 days
Minnesota Energy Intensive Trade Exposed Rider Accrued Revenues [Member]
Regulatory Assets - Current	[1]	$ 328		$ 75
Regulatory Assets - Long -Term	[1]
Regulatory Assets - Total		$ 328	[1]	$ 75
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	120 days		1 year
Minnesota Renewable Resource Recovery Rider, Accrued Revenues [Member]
Regulatory Assets - Current	[2]	$ 452
Regulatory Assets - Long -Term	[2]
Regulatory Assets - Total	[2]	$ 452
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]	1 year
MISO Schedule 26/26A Transmission Cost Recovery Rider True-up [Member]
Regulatory Assets - Current	[1]	$ 240
Regulatory Assets - Long -Term	[1]			1,985
Regulatory Assets - Total		$ 240	[1]	$ 1,985
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	1 year		2 years
Minnesota Transmission Cost Recovery Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]	$ 444
Regulatory Assets - Long -Term	[2]
Regulatory Assets - Total	[2]	$ 444
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]	1 year
North Dakota Environmental Cost Recovery Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]	$ 17		$ 152
Regulatory Assets - Long -Term	[2]
Regulatory Assets - Total	[2]	$ 17		$ 152
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]	1 year		1 year
Minnesota Deferred Rate Case Expenses Subject to Recovery [Member]
Regulatory Assets - Current	[1]			$ 267
Regulatory Assets - Long -Term	[1]
Regulatory Assets - Total	[1]			$ 267
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]			120 days
North Dakota Renewable Resource Recovery Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]			$ 206
Regulatory Assets - Long -Term	[2]			236
Regulatory Assets - Total				$ 442
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]			1 year 90 days
South Dakota Deferred Rate Case Expenses Subject to Recovery [Member]
Regulatory Assets - Current	[1]	$ 178
Regulatory Assets - Long -Term	[1]
Regulatory Assets - Total	[1]	$ 178
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	1 year
Minnesota SPP Transmission Cost Recovery Tracker [Member]
Regulatory Assets - Current	[1]
Regulatory Assets - Long -Term	[1]	176
Regulatory Assets - Total	[1]	$ 176
Regulatory Assets - Remaining Recovery/Refund Period	[1]	see below
Minnesota Environmental Cost Recovery Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]	$ 121
Regulatory Assets - Long -Term	[2]
Regulatory Assets - Total	[2]	$ 121
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]	1 year
[1]	Costs subject to recovery without a rate of return.
[2]	Amount eligible for recovery under an alternative revenue program which includes an incentive or rate of return.

Note 5 - Common Shares and Ea_3

Note 5 - Common Shares and Earnings Per Share (Details Textual) - $ / shares	Jan. 01, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 03, 2018	Oct. 13, 2015	Apr. 16, 2012	Apr. 15, 2012
Maximum per Share Differences Between Basic and Diluted Earnings per Share in Total or from Continuing or Discontinued Operations		$ 0.016	$ 0.016	$ 0.016
The 2014 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		1,900,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant		1,121,330
The 1999 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		1,400,000					1,400,000	900,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant		366,867
Share-based Compensation Arrangements by Share-based Payment Award, Percentage of Market Price at Eligible Employees Granted Options to Purchase Shares	100.00%			85.00%
Open Market Share Purchases		7,757	4,202
Stock Issued During Period, Shares for Cash Invested under Employee Stock Purchase Plan			5,284	53,875
Dividend Reinvestment and Share Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant		1,383,178
Open Market Share Purchases		116,822
Shelf Registration, Shares					1,500,000
Prior Shelf Registration for Dividend Reinvestment and Share Purchase Plan [Member]
Open Market Share Purchases		53,853	87,634
Stock Issued During Period, Shares for Cash Invested under Employee Stock Purchase Plan			97,698	278,811
Shelf Registration, Shares						1,500,000

Note 5 - Common Shares and Ea_4

Note 5 - Common Shares and Earnings Per Share - Reconciliation of Company's Common Shares (Details)	12 Months Ended
	Dec. 31, 2018shares
Common Shares Outstanding, beginning balance (in shares)	39,557,491
Issuances:
Vesting of Restricted Stock Units (in shares)	26,575
Restricted Stock Issued to Directors (in shares)	18,200
Directors Deferred Compensation (in shares)	578
Retirements:
Shares Withheld for Individual Income Tax Requirements (in shares)	(71,208)
Common Shares Outstanding, ending balance (in shares)	39,664,884
Performance Awards 2015 [Member]
Issuances:
Executive Stock Performance Awards (2015 awards) (in shares)	114,648
Performance Awards 2016 and 2017 [Member]
Issuances:
Executive Stock Performance Awards (2015 awards) (in shares)	18,600

Note 5 - Common Shares and Ea_5

Note 5 - Common Shares and Earnings Per Share - Reconciliation of Weighted Average Common Shares Outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted Average Common Shares Outstanding – Basic (in shares)	39,599,944	39,457,261	38,546,459
Plus Outstanding Share Awards net of Share Reductions for Unrecognized Stock-Based Compensation Expense and Excess Tax Benefits:
Shares Expected to be Awarded for Stock Performance Awards Granted to Executive Officers based on Measurement Period-to-Date Performance (in shares)	212,043	210,784	118,644
Underlying Shares Related to Nonvested Restricted Stock Units Granted to Employees (in shares)	59,980	56,952	45,712
Nonvested Restricted Shares (in shares)	17,751	20,380	16,778
Shares Expected to be Issued Under the Deferred Compensation Program for Directors (in shares)	2,478	2,970	3,417
Total Dilutive Shares (in shares)	292,252	291,086	184,551
Weighted Average Common Shares Outstanding – Diluted (in shares)	39,892,196	39,748,347	38,731,010

Note 6 - Share-based Payments_2

Note 6 - Share-based Payments (Details Textual) - USD ($)	Apr. 09, 2018	Feb. 05, 2018	Jan. 01, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 31, 2014	Dec. 31, 2018	Dec. 31, 2017	Apr. 16, 2012	Apr. 15, 2012
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total				$ 4,300,000				$ 4,300,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition				1 year 328 days
Restricted Stock [Member] | Director [Member]
Allocated Share-based Compensation Expense, Total				$ 661,000	$ 658,000	$ 491,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	18,200			18,200	17,600	23,200
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 43.40			$ 43.40	$ 37.75	$ 28.66
Restricted Stock [Member] | Director [Member] | Share-based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	33.30%
Restricted Stock [Member] | Employees [Member]
Allocated Share-based Compensation Expense, Total				$ 16,000	$ 70,000	$ 96,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value
Restricted Stock Units (RSUs) [Member] | Employees [Member]
Allocated Share-based Compensation Expense, Total				$ 351,000	$ 331,000	$ 307,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period				14,780	10,995	17,220
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value				$ 38.99	$ 33.28	$ 24.54
Stock Performance Awards [Member] | Executive Officer [Member]
Allocated Share-based Compensation Expense, Total				$ 2,645,000	$ 2,007,000	$ 1,665,000
Period Specified for Average Adjusted Return				3 years
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized
Stock Performance Awards [Member] | Executive Officer [Member] | Granted in 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Forfeitures							8,900
Stock Performance Awards [Member] | Executive Officer [Member] | Performance Period 2016 To 2018 [Member]
Adjusted Return, Targeted Percentage of Equity								10.00%
Payout as a Percent of Target								145.17%
Stock Performance Awards [Member] | Executive Officer [Member] | Performance Period 2015 To 2017 [Member]
Adjusted Return, Targeted Percentage of Equity				10.00%
Payout as a Percent of Target				136.00%
Stock Performance Awards [Member] | Chief Executive Officer [Member] | Performance Period 2014 to 2016 [Member]
Payout as a Percent of Target						114.29%
Issue Price Per Share of Earned Shares Issued						$ 26.35
Value of Earned Shares Issued						$ 848,000
The 1999 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date						15.00%
Allocated Share-based Compensation Expense, Total						$ 173,000
Share-based Compensation Arrangements by Share-based Payment Award, Percentage of Market Price at Eligible Employees Granted Options to Purchase Shares			100.00%			85.00%
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized				1,400,000				1,400,000		1,400,000	900,000
The 1999 Stock Incentive Plan [Member] | Restricted Stock [Member] | Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period									0
The 2014 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized				1,900,000				1,900,000
The 2014 Stock Incentive Plan [Member] | Restricted Stock [Member] | Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period		15,200
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value		$ 41.325
The 2014 Stock Incentive Plan [Member] | Restricted Stock [Member] | Executive Officer [Member] | Share-based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage		25.00%
The 2014 Stock Incentive Plan [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period				4 years
The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		54,000
Targeted Aggregate Common Shares Award Return on Equity Component		27,000
Number of Trading Days		20 days
The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member] | Minimum [Member]
Percentage of Target Amount as Actual Payment		0.00%
The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period		81,000
Percentage of Target Amount as Actual Payment		150.00%
The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member] | Share-based Compensation Award, Tranche One [Member]
Targeted Aggregate Common Shares Award Total Shareholder Return Component		27,000

Note 6 - Share-based Payments -

Note 6 - Share-based Payments - Summary of Status of Directors' Restricted Stock Awards (Details) - Director [Member] - Restricted Stock [Member] - USD ($)	Apr. 09, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Nonvested, Beginning of Year (in shares)		46,800	46,334	38,217
Nonvested, Beginning of Year, weighted average grant-date fair value (in dollars per share)		$ 32.65	$ 29.71	$ 29.78
Units granted (in shares)	18,200	18,200	17,600	23,200
Grant-date fair value per award (in dollars per share)	$ 43.40	$ 43.40	$ 37.75	$ 28.66
Vested (in shares)		21,775	17,134	15,083
Vested, weighted average grant-date fair value (in dollars per share)		$ 31.94	$ 29.93	$ 28.28
Forfeited (in shares)
Forfeited, weighted average grant-date fair value (in dollars per share)
Nonvested, End of Year (in shares)		43,225	46,800	46,334
Nonvested, End of Year, weighted average grant-date fair value (in dollars per share)		$ 37.53	$ 32.65	$ 29.71
Compensation Expense Recognized		$ 661,000	$ 658,000	$ 491,000
Fair Value of Shares Vested in Year		$ 696,000	$ 513,000	$ 427,000

Note 6 - Share-based Payments_3

Note 6 - Share-based Payments - Summary of Status of Employees' Restricted Stock Awards (Details) - Employees [Member] - Restricted Stock [Member] - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Nonvested, Beginning of Year (in shares)	2,895	7,180	13,581
Nonvested, Beginning of Year, weighted average grant-date fair value (in dollars per share)	$ 29.41	$ 29.72	$ 28.56
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value
Vested (in shares)	2,895	4,285	6,401
Vested, weighted average grant-date fair value (in dollars per share)	$ 29.41	$ 29.94	$ 27.25
Forfeited (in shares)
Forfeited, weighted average grant-date fair value (in dollars per share)
Nonvested, End of Year (in shares)		2,895	7,180
Nonvested, End of Year, weighted average grant-date fair value (in dollars per share)		$ 29.41	$ 29.72
Allocated Share-based Compensation Expense, Total	$ 16,000	$ 70,000	$ 96,000
Fair Value of Awards Vested	$ 85,000	$ 128,000	$ 174,000

Note 6 - Share-based Payments_4

Note 6 - Share-based Payments - Summary of Status of Executive Restricted Stock Awards (Details) - Executive Officers [Member] - Restricted Stock [Member] - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Nonvested, Beginning of Year (in shares)	47,750	41,825	24,300
Nonvested, Beginning of Year, weighted average grant-date fair value (in dollars per share)	$ 32.71	$ 30.23	$ 31.682
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	15,200	15,900	22,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 41.325	$ 37.65	$ 28.915
Vested (in shares)	17,650	9,975	4,475
Vested, weighted average grant-date fair value (in dollars per share)	$ 32.462	$ 30.16	$ 31.69
Forfeited (in shares)
Forfeited, weighted average grant-date fair value (in dollars per share)
Nonvested, End of Year (in shares)	45,300	47,750	41,825
Nonvested, End of Year, weighted average grant-date fair value (in dollars per share)	$ 35.70	$ 32.71	$ 30.23
Allocated Share-based Compensation Expense, Total	$ 769,000	$ 576,000	$ 446,000
Fair Value of Awards Vested	$ 573,000	$ 301,000	$ 142,000

Note 6 - Share-based Payments_5

Note 6 - Share-based Payments - Summary of Awards Granted to Key Non-executive Employees (Details) - The 2014 Stock Incentive Plan [Member] - Restricted Stock Units (RSUs) [Member] - Key Employees [Member]	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Granted on April 9, 2018 [Member]
Vesting percentage	100.00%
Units granted (in shares) | shares	12,945
Grant-date fair value per award (in dollars per share) | $ / shares	$ 38.45
Granted on June 20, 2018 [Member]
Vesting percentage	100.00%
Units granted (in shares) | shares	1,000
Grant-date fair value per award (in dollars per share) | $ / shares	$ 42.46
Granted on September 25, 2018 [Member]
Vesting percentage	100.00%
Units granted (in shares) | shares	835
Grant-date fair value per award (in dollars per share) | $ / shares	$ 43.25

Note 6 - Share-based Payments_6

Note 6 - Share-based Payments - Summary of Status of Employees' Restricted Stock Unit Awards (Details) - Restricted Stock Units (RSUs) [Member] - Employees [Member] - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Nonvested, Beginning of Year (in shares)	46,440	47,370	46,600
Nonvested, Beginning of Year, weighted average grant-date fair value (in dollars per share)	$ 27.07	$ 25.19	$ 23.75
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	14,780	10,995	17,220
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 38.99	$ 33.28	$ 24.54
Vested (in shares)	8,925	11,550	12,250
Vested, weighted average grant-date fair value (in dollars per share)	$ 25.23	$ 25.30	$ 19.03
Forfeited (in shares)	2,825	375	4,200
Forfeited, weighted average grant-date fair value (in dollars per share)	$ 25.86	$ 26.92	$ 24.51
Nonvested, End of Year (in shares)	49,470	46,440	47,370
Nonvested, End of Year, weighted average grant-date fair value (in dollars per share)	$ 31.03	$ 27.07	$ 25.19
Compensation Expense Recognized	$ 351,000	$ 331,000	$ 307,000
Fair Value of Shares Vested in Year	$ 225,000	$ 292,000	$ 233,000

Note 6 - Share-based Payments_7

Note 6 - Share-based Payments - Summary of Stock Performance Awards Granted and Amounts Expensed (Details) - Executive Officer [Member] - Stock Performance Awards [Member] - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Maximum Shares Subject to Award (in shares)
Target Shares (in shares)
Compensation Expense Recognized	$ 2,645,000	$ 2,007,000	$ 1,665,000
Earned Shares (in shares)	356,937
Performance Period 2018 To 2020 [Member]
Maximum Shares Subject to Award (in shares)	81,000
Target Shares (in shares)	54,000
Compensation Expense Recognized	$ 1,121,000
Earned Shares (in shares)
Performance Period 2017 To 2019 [Member]
Maximum Shares Subject to Award (in shares)	89,250
Target Shares (in shares)	59,500
Compensation Expense Recognized	$ 729,000	854,000
Earned Shares (in shares)	7,500
Performance Period 2016 To 2018 [Member]
Maximum Shares Subject to Award (in shares)	122,250
Target Shares (in shares)	81,500
Compensation Expense Recognized	$ 772,000	580,000	798,000
Earned Shares (in shares)	113,298
Performance Period 2015 To 2017 [Member]
Maximum Shares Subject to Award (in shares)	126,450
Target Shares (in shares)	84,300
Compensation Expense Recognized	$ 23,000	573,000	535,000
Earned Shares (in shares)	114,648
Performance Period 2014 to 2016 [Member]
Maximum Shares Subject to Award (in shares)	159,450
Target Shares (in shares)	106,300
Compensation Expense Recognized			$ 332,000
Earned Shares (in shares)	121,491

Note 7 - Retained Earnings an_2

Note 7 - Retained Earnings and Dividend Restriction (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Capitalization, Long-term Debt and Equity, Total	$ 1,318,865	$ 1,187,272
Otter Tail Power Company [Member]
Equity to Total Capitalization Ratio	53.20%
Net Assets Restricted from Distribution	$ 477,000
Capitalization, Long-term Debt and Equity, Total	$ 1,200,000
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Minimum [Member]
Public Utilities, Requested Equity Capital Structure, Percentage	47.90%
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Maximum [Member]
Public Utilities, Requested Equity Capital Structure, Percentage	58.50%

Note 8 - Leases (Details Textua

Note 8 - Leases (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019
Capital Leases, Balance Sheet, Assets by Major Class, Net, Total	$ 0
Operating Leases, Rent Expense, Net, Total	$ 6,273,000	$ 6,237,000	$ 6,711,000
Subsequent Event [Member] | Accounting Standards Update 2016-02 [Member]
Operating Lease, Right-of-Use Asset				$ 20,000,000
Operating Lease, Liability, Total				$ 20,000,000

Note 8 - Leases - Operating Lea

Note 8 - Leases - Operating Lease Obligations (Details) $ in Thousands	Dec. 31, 2018USD ($)
2,019	$ 6,185
2,020	5,877
2,021	5,018
2,022	3,954
2,023	3,581
Beyond 2,023	6,743
Total	31,358
OTP [Member]
2,019	1,099
2,020	1,077
2,021	1,047
2,022	214
2,023	196
Beyond 2,023	448
Total	4,081
Nonelectric Companies [Member]
2,019	5,086
2,020	4,800
2,021	3,971
2,022	3,740
2,023	3,385
Beyond 2,023	6,295
Total	$ 27,277

Note 9 - Commitments and Cont_3

Note 9 - Commitments and Contingencies (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Land Easement Non-lease Payments Expense	$ 605,000	$ 593,000	$ 582,000
Loss Contingency, Estimate of Possible Loss	1,000,000
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member]
Estimated Liability of Refund Obligation	$ 1,600,000
Otter Tail Power Company [Member] | Construction Programs [Member]
Contract Expiration Year	2,021
Long-term Purchase Commitment, Amount	$ 64,500,000
Other Commitment, Total	$ 252,000	$ 280,000	$ 272,000
Otter Tail Power Company [Member] | Coal Purchase Commitments 1 [Member]
Contract Expiration Year	2,041
Otter Tail Power Company [Member] | Coal Purchase Commitments 2 [Member]
Contract Expiration Year	2,040
Otter Tail Power Company [Member] | Coal Purchase Commitments 3 [Member]
Contract Expiration Year	2,020
T. O. Plastics, Inc. [Member] | Contract Expiring on December 31, 2021 [Member]
Long-term Purchase Commitment, Amount	$ 5,000,000

Note 9 - Commitments and Cont_4

Note 9 - Commitments and Contingencies - Amounts of Commitments (Details) $ in Thousands	Dec. 31, 2018USD ($)
Otter Tail Power Company [Member]
2019, Land Easement Payments	$ 617
2020, Land Easement Payments	630
2021, Land Easement Payments	642
2022, Land Easement Payments	655
2023, Land Easement Payments	668
Beyond 2023, Land Easement Payments	7,612
Total, Land Easement Payments	10,824
Construction Program and Other Commitments [Member] | Subsidiaries [Member]
2019, Commitments Other than Operating Leases	43,887
2020, Commitments Other than Operating Leases	23,939
2021, Commitments Other than Operating Leases	1,681
2022, Commitments Other than Operating Leases
2023, Commitments Other than Operating Leases
Beyond 2023, Commitments Other than Operating Leases
Total, Commitments Other than Operating Leases	69,507
Capacity and Energy Requirements [Member] | Otter Tail Power Company [Member]
2019, Commitments Other than Operating Leases	24,925
2020, Commitments Other than Operating Leases	24,844
2021, Commitments Other than Operating Leases	12,988
2022, Commitments Other than Operating Leases	11,827
2023, Commitments Other than Operating Leases	11,827
Beyond 2023, Commitments Other than Operating Leases	143,099
Total, Commitments Other than Operating Leases	229,510
Coal Purchase Commitments [Member] | Otter Tail Power Company [Member]
2019, Commitments Other than Operating Leases	23,397
2020, Commitments Other than Operating Leases	22,645
2021, Commitments Other than Operating Leases	22,935
2022, Commitments Other than Operating Leases	22,793
2023, Commitments Other than Operating Leases	23,955
Beyond 2023, Commitments Other than Operating Leases	503,492
Total, Commitments Other than Operating Leases	$ 619,217

Note 10 - Short-term and Long_3

Note 10 - Short-term and Long-term Borrowings (Details Textual) - USD ($)	Sep. 17, 2018	Aug. 21, 2017	Dec. 31, 2018	Dec. 31, 2017	Feb. 07, 2018	Jan. 16, 2018	Sep. 23, 2016	Aug. 14, 2013	Oct. 29, 2012	Dec. 01, 2011
Short-term Debt, Average Outstanding Amount			$ 27,100,000
Short-term Debt, Weighted Average Interest Rate, at Point in Time			3.90%
Line of Credit Facility, Maximum Borrowing Capacity			$ 300,000,000			$ 122,000,000
Otter Tail Corporation Credit Agreement [Member]
Short-term Debt, Maximum Amount Outstanding During Period	$ 17,700,000
Short-term Debt, Average Outstanding Amount			$ 5,500,000
Short-term Debt, Weighted Average Interest Rate, at Point in Time			3.80%	2.80%
Line of Credit Facility, Maximum Borrowing Capacity			$ 130,000,000
Debt Instrument, Description of Variable Rate Basis			LIBOR
Otter Tail Corporation Credit Agreement [Member] | Maximum [Member]
Debt to Total Capitalization Ratio			0.6
Otter Tail Corporation Credit Agreement [Member] | Minimum [Member]
Interest and Dividend Coverage Ratio			1.5
Otter Tail Corporation Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate			1.50%	1.50%
Otter Tail Corporation Credit Agreement [Member] | Unsecured Revolving Credit Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity				$ 130,000,000
Line of Credit Facility, Maximum Borrowing Capacity, Subject to Conditions									$ 250,000,000
Reduced Line of Credit Facility, Borrowing Capacity by Outstanding Letters of Credit			$ 40,000,000
OTP Credit Agreement [Member]
Short-term Debt, Average Outstanding Amount			$ 21,600,000
Short-term Debt, Weighted Average Interest Rate, at Point in Time			3.00%	2.40%
Line of Credit Facility, Maximum Borrowing Capacity			$ 170,000,000			$ 122,000,000
Line of Credit Facility, Maximum Amount Outstanding During Period, by Outstanding Letters of Credit			$ 50,000,000
Debt Instrument, Description of Variable Rate Basis			LIBOR
OTP Credit Agreement [Member] | Maximum [Member]
Debt to Total Capitalization Ratio			0.6
OTP Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate			1.25%	1.25%
OTP Credit Agreement [Member] | Revolving Credit Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity									170,000,000
Line of Credit Facility, Maximum Borrowing Capacity, Subject to Conditions									$ 250,000,000
Series 2018 A senior Unsecured Notes Due February 7, 2048 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount					$ 100,000,000
Debt Instrument, Interest Rate, Stated Percentage					4.07%
The 2018 Note Purchase Agreement [Member]
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount					10.00%
Debt Instrument, Prepayment, Percentage of the Principal Amount Prepaid					100.00%
Debt Instrument, Percentage of Principal Amount to be Offered for Prepayment in the Event of a Change of Control					100.00%
The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member]
Debt Instrument, Face Amount							$ 80,000,000
Debt Instrument, Interest Rate, Stated Percentage			3.55%	3.55%			3.55%
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount							10.00%
Debt Instrument, Prepayment, Percentage of the Principal Amount Prepaid							100.00%
Debt Instrument, Percentage of Principal Amount to be Offered for Prepayment in the Event of a Change of Control							100.00%
The 9.000% Notes, due December 15, 2016 [Member]
Debt Instrument, Interest Rate, Stated Percentage			9.00%
Repayments of Debt			$ 52,330,000
Series A Senior Unsecured Notes due on February 27, 2029 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount								$ 60,000,000
Debt Instrument, Interest Rate, Stated Percentage								4.68%
Series B Senior Unsecured Notes due on February 27, 2044 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount								$ 90,000,000
Debt Instrument, Interest Rate, Stated Percentage								5.47%
Note Purchase Agreement 2013 [Member]
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount								10.00%
Debt Instrument, Prepayment, Percentage of the Principal Amount Prepaid								100.00%
Debt Instrument, Percentage of Principal Amount to be Offered for Prepayment in the Event of a Change of Control								100.00%
Priority Debt to Total Capitalization			20.00%
Interest-bearing Debt, Maximum Percentage of Total Capitalization			60.00%
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member]
Debt Instrument, Interest Rate, Stated Percentage			4.63%	4.63%
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount										$ 140,000,000
Debt Instrument, Interest Rate, Stated Percentage										4.63%
The 2007 Notes [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount			$ 122,000,000
Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member]
Debt Instrument, Interest Rate, Stated Percentage			6.15%	6.15%
Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount			$ 30,000,000
Debt Instrument, Interest Rate, Stated Percentage			6.15%
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member]
Debt Instrument, Interest Rate, Stated Percentage			6.37%	6.37%
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount			$ 42,000,000
Debt Instrument, Interest Rate, Stated Percentage			6.37%
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member]
Debt Instrument, Interest Rate, Stated Percentage			6.47%	6.47%
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount			$ 50,000,000
Debt Instrument, Interest Rate, Stated Percentage			6.47%
Senior Unsecured Notes 5.95%, Series A, Due August 20, 2017 [Member]
Debt Instrument, Interest Rate, Stated Percentage		5.95%
Repayments of Debt		$ 33,000,000
The 2011 and 2007 Note Purchase Agreements [Member]
Priority Debt to Total Capitalization			20.00%
The 2011 and 2007 Note Purchase Agreements [Member] | Maximum [Member]
Debt to Total Capitalization Ratio			0.6
The 2011 and 2007 Note Purchase Agreements [Member] | Minimum [Member]
Interest and Dividend Coverage Ratio			1.5
The 2011 and 2007 Note Purchase Agreements [Member] | Otter Tail Power Company [Member]
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount			10.00%
Debt Instrument, Prepayment, Percentage of the Principal Amount Prepaid			100.00%
Debt Instrument, Percentage of Principal Amount to be Offered for Prepayment in the Event of a Change of Control			100.00%
The 2016 Note Purchase Agreement [Member]
Priority Debt to Total Capitalization			10.00%
Priority Indebtedness Outstanding			$ 0

Note 10 - Short-term and Long_4

Note 10 - Short-term and Long-term Borrowings - Status of Lines of Credit (Details) - USD ($)	Dec. 31, 2018	Jan. 16, 2018	Dec. 31, 2017
Line of Credit Facility, Maximum Borrowing Capacity	$ 300,000,000	$ 122,000,000
In Use	18,599,000
Restricted due to Outstanding Letters of Credit	300,000
Available	281,101,000		$ 187,329,000
Otter Tail Corporation Credit Agreement [Member]
Line of Credit Facility, Maximum Borrowing Capacity	130,000,000
In Use	9,215,000
Restricted due to Outstanding Letters of Credit
Available	120,785,000		130,000,000
OTP Credit Agreement [Member]
Line of Credit Facility, Maximum Borrowing Capacity	170,000,000	$ 122,000,000
In Use	9,384,000
Restricted due to Outstanding Letters of Credit	300,000
Available	$ 160,316,000		$ 57,329,000

Note 10 - Short-term and Long_5

Note 10 - Short-term and Long-term Borrowings - Breakdown of Assignment of Consolidated Short-term and Long-term Debt Outstanding (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Short-Term Debt	$ 18,599	$ 112,371
Long-Term Debt	592,523	492,711
Less: Current Maturities net of Unamortized Debt Issuance Costs	172	186
Unamortized Long-Term Debt Issuance Costs	2,349	2,145
Total Long-Term Debt net of Unamortized Debt Issuance Costs	590,002	490,380
Total Short-Term and Long-Term Debt (with current maturities)	608,773	602,937
The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member]
Long-Term Debt	80,000	80,000
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member]
Long-Term Debt	140,000	140,000
Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member]
Long-Term Debt	30,000	30,000
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member]
Long-Term Debt	42,000	42,000
Senior Unsecured Notes 4.68%, Series A, Due February 27, 2029 [Member]
Long-Term Debt	60,000	60,000
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member]
Long-Term Debt	50,000	50,000
Senior Unsecured Notes 5.47%, Series B, Due February 27, 2044 [Member]
Long-Term Debt	90,000	90,000
Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member]
Long-Term Debt	100,000
North Dakota Development Note, 3.95%, Due April 1, 2018 [member]
Long-Term Debt		27
Partnership in Assisting Community Expansion (PACE) Note, 2.54%, due March 18, 2021 [Member]
Long-Term Debt	523	684
Otter Tail Power Company [Member]
Short-Term Debt	9,384	112,371
Long-Term Debt	512,000	412,000
Less: Current Maturities net of Unamortized Debt Issuance Costs
Unamortized Long-Term Debt Issuance Costs	1,942	1,684
Total Long-Term Debt net of Unamortized Debt Issuance Costs	510,058	410,316
Total Short-Term and Long-Term Debt (with current maturities)	519,442	522,687
Otter Tail Power Company [Member] | The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member]
Long-Term Debt
Otter Tail Power Company [Member] | Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member]
Long-Term Debt	140,000	140,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member]
Long-Term Debt	30,000	30,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member]
Long-Term Debt	42,000	42,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 4.68%, Series A, Due February 27, 2029 [Member]
Long-Term Debt	60,000	60,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member]
Long-Term Debt	50,000	50,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 5.47%, Series B, Due February 27, 2044 [Member]
Long-Term Debt	90,000	90,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member]
Long-Term Debt	100,000
Otter Tail Power Company [Member] | Partnership in Assisting Community Expansion (PACE) Note, 2.54%, due March 18, 2021 [Member]
Long-Term Debt
Parent Company [Member]
Short-Term Debt	9,215
Long-Term Debt	80,523	80,711
Less: Current Maturities net of Unamortized Debt Issuance Costs	172	186
Unamortized Long-Term Debt Issuance Costs	407	461
Total Long-Term Debt net of Unamortized Debt Issuance Costs	79,944	80,064
Total Short-Term and Long-Term Debt (with current maturities)	89,331	80,250
Parent Company [Member] | The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member]
Long-Term Debt	80,000	80,000
Parent Company [Member] | Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member]
Long-Term Debt
Parent Company [Member] | North Dakota Development Note, 3.95%, Due April 1, 2018 [member]
Long-Term Debt		27
Parent Company [Member] | Partnership in Assisting Community Expansion (PACE) Note, 2.54%, due March 18, 2021 [Member]
Long-Term Debt	$ 523	$ 684

Note 10 - Short-term and Long_6

Note 10 - Short-term and Long-term Borrowings - Breakdown of Assignment of Consolidated Short-term and Long-term Debt Outstanding (Details) (Parentheticals)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 23, 2016
The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member]
Debt Instrument, Interest Rate, Stated Percentage	3.55%	3.55%	3.55%
Long-Term Debt, Due Date	Dec. 15, 2026	Dec. 15, 2026
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member]
Debt Instrument, Interest Rate, Stated Percentage	4.63%	4.63%
Long-Term Debt, Due Date	Dec. 1, 2021	Dec. 1, 2021
Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member]
Debt Instrument, Interest Rate, Stated Percentage	6.15%	6.15%
Long-Term Debt, Due Date	Aug. 20, 2022	Aug. 20, 2022
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member]
Debt Instrument, Interest Rate, Stated Percentage	6.37%	6.37%
Long-Term Debt, Due Date	Aug. 20, 2027	Aug. 20, 2027
Senior Unsecured Notes 4.68%, Series A, Due February 27, 2029 [Member]
Debt Instrument, Interest Rate, Stated Percentage	4.68%	4.68%
Long-Term Debt, Due Date	Feb. 27, 2029	Feb. 27, 2029
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member]
Debt Instrument, Interest Rate, Stated Percentage	6.47%	6.47%
Long-Term Debt, Due Date	Aug. 20, 2037	Aug. 20, 2037
Senior Unsecured Notes 5.47%, Series B, Due February 27, 2044 [Member]
Debt Instrument, Interest Rate, Stated Percentage	5.47%	5.47%
Long-Term Debt, Due Date	Feb. 27, 2044	Feb. 27, 2044
Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member]
Debt Instrument, Interest Rate, Stated Percentage	4.07%
Long-Term Debt, Due Date	Feb. 7, 2048
North Dakota Development Note, 3.95%, Due April 1, 2018 [member]
Debt Instrument, Interest Rate, Stated Percentage		3.95%
Long-Term Debt, Due Date		Apr. 1, 2018
Partnership in Assisting Community Expansion (PACE) Note, 2.54%, due March 18, 2021 [Member]
Debt Instrument, Interest Rate, Stated Percentage	2.54%	2.54%
Long-Term Debt, Due Date	Mar. 18, 2021	Mar. 18, 2021

Note 10 - Short-term and Long_7

Note 10 - Short-term and Long-term Borrowings - Aggregate Amounts of Maturities on Bonds Outstanding and Other Long-term Obligations (Details) $ in Thousands	Dec. 31, 2018USD ($)
Aggregate amounts of debt maturities in 2019	$ 172
Aggregate amounts of debt maturities in 2020	185
Aggregate amounts of debt maturities in 2021	140,166
Aggregate amounts of debt maturities in 2022	30,000
Aggregate amounts of debt maturities in 2023

Note 11 - Pension Plan and Ot_3

Note 11 - Pension Plan and Other Postretirement Benefits (Details Textual)	Feb. 22, 2019USD ($)	Jul. 30, 2015USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Defined Contribution Plan, Cost			$ 4,532,000	$ 4,211,000	$ 3,877,000
Employee Stock Ownership Plan (ESOP), Cash Contributions to ESOP			$ 398,000	612,000	$ 647,000
Pension Plan [Member]
Defined Benefit Plan, Percentage Vesting After the Specified Employment Period			100.00%
Defined Benefit Plan, Vesting Period			5 years
Defined Benefit Plan, Age for Compensation			65
Defined Benefit Plan, Age Below Which Compensation is Reduced			62
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets for Next Fiscal Year			7.25%
Defined Benefit Plan, Plan Assets at Fair Value, Valuation Inputs, Period Over Which Investment Gains or Losses are Recognized			5 years
Pension Plan [Member] | SEI Energy Debt Fund [Member]
Amount Invested in Pension Fund Assets		$ 10,000,000
Pension Plan [Member] | Subsequent Event [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 10,000,000
Executive Survivor and Supplemental Retirement Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer			$ 1,505,000	1,175,000
Period of Benefit Payments to Beneficiaries on Deaths			15 years
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer			$ 3,183,000	$ 3,290,000
Health Insurance Benefits, Requisite Age			55 years
Health Insurance Benefits, Requisite Service Period			10 years
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year			$ 4,200,000
Defined Benefit Plan, Expected Future Prescription Drug Subsidy Receipt, Next Twelve Months			$ 400,000

Note 11 - Pension Plan and Ot_4

Note 11 - Pension Plan and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Service Cost		$ 6,459	$ 5,629	$ 5,518
Interest Cost		13,452	14,139	14,195
Expected Return on Assets		(21,199)	(19,229)	(19,454)
Amortization of Prior Service Cost:
From Regulatory Asset		16	120	189
From Other Comprehensive Income	[1]		3	5
Amortization of Net Actuarial Loss:
From Regulatory Asset		7,135	5,090	5,153
From Other Comprehensive Income	[1]	183	125	127
Net Periodic Pension Cost	[2]	6,046	5,877	5,733
Service Cost		6,459	5,629	5,518
Interest Cost		13,452	14,139	14,195
From Other Comprehensive Income1	[1]	183	125	127
Pension Plan [Member] | Costs Included in OTP Capital Expenditures [Member]
Amortization of Net Actuarial Loss:
Net Periodic Pension Cost		1,542	1,094	1,009
Pension Plan [Member] | Costs Included in Electric Operation and Maintenance Expenses [Member]
Amortization of Net Actuarial Loss:
Net Periodic Pension Cost		4,756	4,400	4,377
Pension Plan [Member] | Costs Included in Other Nonelectric Expenses [Member]
Amortization of Net Actuarial Loss:
Net Periodic Pension Cost		161	135	132
Pension Plan [Member] | Nonservice Costs Included in Nonservice Cost Components of Postretirement Benefits [Member]
Amortization of Net Actuarial Loss:
Net Periodic Pension Cost		(314)	200	176
Pension Plan [Member] | Nonservice Costs Capitalized as Regulatory Assets [Member]
Amortization of Net Actuarial Loss:
Net Periodic Pension Cost		(99)	48	39
Executive Survivor and Supplemental Retirement Plan [Member]
Service Cost		408	290	252
Interest Cost		1,589	1,686	1,667
Amortization of Prior Service Cost:
From Regulatory Asset		20	16	16
From Other Comprehensive Income	[3]	34	38	38
Amortization of Net Actuarial Loss:
From Regulatory Asset		206	285	293
From Other Comprehensive Income	[3]	722	440	446
Net Periodic Pension Cost	[4]	2,979	2,755	2,712
Service Cost		408	290	252
Interest Cost		1,589	1,686	1,667
From Other Comprehensive Income1	[3]	722	440	446
Executive Survivor and Supplemental Retirement Plan [Member] | Costs Included in Electric Operation and Maintenance Expenses [Member]
Amortization of Net Actuarial Loss:
Net Periodic Pension Cost		99	94	87
Executive Survivor and Supplemental Retirement Plan [Member] | Costs Included in Other Nonelectric Expenses [Member]
Amortization of Net Actuarial Loss:
Net Periodic Pension Cost		309	196	165
Executive Survivor and Supplemental Retirement Plan [Member] | Nonservice Costs Included in Nonservice Cost Components of Postretirement Benefits [Member]
Amortization of Net Actuarial Loss:
Net Periodic Pension Cost		2,571	2,465	2,460
Other Postretirement Benefits Plan [Member]
Service Cost		1,526	1,425	1,301
Interest Cost		2,583	2,712	2,503
Amortization of Prior Service Cost:
From Regulatory Asset			(4)	134
From Other Comprehensive Income	[1]		4	3
Amortization of Net Actuarial Loss:
From Regulatory Asset		1,648	936	379
From Other Comprehensive Income	[1]	42	19	9
Net Periodic Pension Cost	[5]	5,799	5,092	4,329
Service Cost		1,526	1,425	1,301
Interest Cost		2,583	2,712	2,503
From Other Comprehensive Income1	[1]	42	19	9
Effect of Medicare Part D Subsidy		(470)	(561)	(923)
Other Postretirement Benefits Plan [Member] | Costs Included in OTP Capital Expenditures [Member]
Amortization of Net Actuarial Loss:
Net Periodic Pension Cost		364	277	238
Other Postretirement Benefits Plan [Member] | Costs Included in Electric Operation and Maintenance Expenses [Member]
Amortization of Net Actuarial Loss:
Net Periodic Pension Cost		1,124	1,114	1,032
Other Postretirement Benefits Plan [Member] | Costs Included in Other Nonelectric Expenses [Member]
Amortization of Net Actuarial Loss:
Net Periodic Pension Cost		38	34	31
Other Postretirement Benefits Plan [Member] | Nonservice Costs Included in Nonservice Cost Components of Postretirement Benefits [Member]
Amortization of Net Actuarial Loss:
Net Periodic Pension Cost		3,253	2,955	2,474
Other Postretirement Benefits Plan [Member] | Nonservice Costs Capitalized as Regulatory Assets [Member]
Amortization of Net Actuarial Loss:
Net Periodic Pension Cost		$ 1,020	$ 712	$ 554
[1]	Corporate cost included in nonservice cost components of postretirement benefits.
[2]	Allocation of Costs: 2018 2017 2016 Service Costs included in OTP Capital Expenditures $ 1,542 $ 1,094 $ 1,009 Service costs included in electric operation and maintenance expenses 4,756 4,400 4,377 Service costs included in other nonelectric expenses 161 135 132 Nonservice costs capitalized (99) 48 39 Nonservice costs included in nonservice cost components of postretirement benefits (314) 200 176
[3]	Amortization of prior service costs and net actuarial losses from other comprehensive income are included in nonservice cost components of postretirement benefits on the face of the Company's consolidated statements of income.
[4]	Allocation of Costs: 2018 2017 2016 Service costs included in electric operation and maintenance expenses $ 99 $ 94 $ 87 Service costs included in other nonelectric expenses 309 196 165 Nonservice costs included in nonservice cost components of postretirement benefits 2,571 2,465 2,460
[5]	Allocation of Cost: 2018 2017 2016 Service Costs included in OTP capital expenditures $ 364 $ 277 $ 238 Service costs included in electric operation and maintenance expenses 1,124 1,114 1,032 Service costs included in other nonelectric expenses 38 34 31 Nonservice costs capitalized 1,020 712 554 Nonservice costs included in nonservice cost components of postretirement benefits 3,253 2,955 2,474

Note 11 - Pension Plan and Ot_5

Note 11 - Pension Plan and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) (Parentheticals) - Pension Plan [Member] - USD ($)		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Periodic Pension Cost	[1]	$ 6,046,000	$ 5,877,000	$ 5,733,000
Costs Included in OTP Capital Expenditures [Member]
Net Periodic Pension Cost		1,542,000	1,094,000	1,009,000
Costs Included in Electric Operation and Maintenance Expenses [Member]
Net Periodic Pension Cost		4,756,000	4,400,000	4,377,000
Costs Included in Other Nonelectric Expenses [Member]
Net Periodic Pension Cost		161,000	135,000	132,000
Nonservice Costs Capitalized as Regulatory Assets [Member]
Net Periodic Pension Cost		(99)	48	39
Nonservice Costs Included in Nonservice Cost Components of Postretirement Benefits [Member]
Net Periodic Pension Cost		$ (314)	$ 200	$ 176
[1]	Allocation of Costs: 2018 2017 2016 Service Costs included in OTP Capital Expenditures $ 1,542 $ 1,094 $ 1,009 Service costs included in electric operation and maintenance expenses 4,756 4,400 4,377 Service costs included in other nonelectric expenses 161 135 132 Nonservice costs capitalized (99) 48 39 Nonservice costs included in nonservice cost components of postretirement benefits (314) 200 176

Note 11 - Pension Plan and Ot_6

Note 11 - Pension Plan and Other Postretirement Benefits - Weighted-average Assumptions Used to Determine Net Periodic Benefit Cost (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Discount Rate	3.90%	4.60%	4.76%
Long-Term Rate of Return on Plan Assets	7.50%	7.50%	7.75%
Rate of Increase in Future Compensation Level		3.00%	3.13%
Pension Plan [Member] | Participants to Age 39 [Member]
Rate of Increase in Future Compensation Level	4.50%
Pension Plan [Member] | Participants Age 40 to 49 [Member]
Rate of Increase in Future Compensation Level	3.50%
Pension Plan [Member] | Participants Age 50 and Older [Member]
Rate of Increase in Future Compensation Level	2.75%
Executive Survivor and Supplemental Retirement Plan [Member]
Discount Rate	3.85%	4.60%	4.76%
Rate of Increase in Future Compensation Level	2.92%	3.00%	3.13%
Other Postretirement Benefits Plan [Member]
Discount Rate	3.81%	4.46%	4.57%

Note 11 - Pension Plan and Ot_7

Note 11 - Pension Plan and Other Postretirement Benefits - Amounts Recognized in Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets:
Total Regulatory Assets	$ 18,094	$ 18,927
Accumulated Other Comprehensive Loss:
Regulatory Asset	18,094	18,927
Other Postretirement Benefits Plan [Member]
Regulatory Assets:
Unrecognized Actuarial Loss	17,987	18,816
Projected Benefit Obligation Liability – Net Amount Recognized	(71,561)	(69,774)	$ (62,571)
Accumulated Other Comprehensive Loss:
Total Accumulated Other Comprehensive Loss	(107)	(111)
Projected Benefit Obligation Liability – Net Amount Recognized	(71,561)	(69,774)	(62,571)
Accumulated Other Comprehensive Income:	(107)	(111)
Unrecognized Actuarial Loss	17,987	18,816
Pension Plan [Member]
Regulatory Assets:
Unrecognized Prior Service Cost	5	21
Unrecognized Actuarial Loss	104,891	99,360
Total Regulatory Assets	104,896	99,381
Projected Benefit Obligation Liability – Net Amount Recognized	(328,442)	(352,718)	(314,637)
Accumulated Other Comprehensive Loss:
Unrecognized Prior Service Cost	9	9
Unrecognized Actuarial Loss	137	439
Total Accumulated Other Comprehensive Loss	146	448
Noncurrent Liability	58,659	67,399
Unrecognized Prior Service Cost	9	9
Projected Benefit Obligation Liability – Net Amount Recognized	(328,442)	(352,718)	(314,637)
Regulatory Asset	104,896	99,381
Accumulated Other Comprehensive Income:	146	448
Unrecognized Actuarial Loss	104,891	99,360
Executive Survivor and Supplemental Retirement Plan [Member]
Regulatory Assets:
Unrecognized Prior Service Cost	20	40
Unrecognized Actuarial Loss	1,768	3,229
Total Regulatory Assets	1,788	3,269
Projected Benefit Obligation Liability – Net Amount Recognized	(39,699)	(42,308)	(37,335)
Accumulated Other Comprehensive Loss:
Unrecognized Prior Service Cost	64	98
Unrecognized Actuarial Loss	6,455	9,024
Total Accumulated Other Comprehensive Loss	6,519	9,122
Unrecognized Prior Service Cost	64	98
Projected Benefit Obligation Liability – Net Amount Recognized	(39,699)	(42,308)	$ (37,335)
Regulatory Asset	1,788	3,269
Accumulated Other Comprehensive Income:	6,519	9,122
Unrecognized Actuarial Loss	$ 1,768	$ 3,229

Note 11 - Pension Plan and Ot_8

Note 11 - Pension Plan and Other Postretirement Benefits - Funded Status (Details) - Pension Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Benefit Obligation	$ (297,972)	$ (316,095)
Projected Benefit Obligation Liability – Net Amount Recognized	(328,442)	(352,718)	$ (314,637)
Fair Value of Plan Assets	269,783	285,319	$ 254,346
Funded Status	$ (58,659)	$ (67,399)

Note 11 - Pension Plan and Ot_9

Note 11 - Pension Plan and Other Postretirement Benefits - Reconciliation of Changes in Fair Value of Plan Assets and Plan's Benefit Obligations (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Reconciliation of Fair Value of Plan Assets:
Fair Value of Plan Assets, beginning balance		$ 285,319	$ 254,346
Actual Return on Plan Assets		(21,334)	44,181
Discretionary Company Contributions		20,000
Benefit Payments		(14,202)	(13,208)
Fair Value of Plan Assets, ending balance		$ 269,783	$ 285,319	$ 254,346
Estimated Asset Return		(7.30%)	17.80%
Reconciliation of Projected Benefit Obligation:
Projected Benefit Obligation, beginning balance		$ 352,718	$ 314,637
Service Cost		6,459	5,629	5,518
Interest Cost		13,452	14,139	14,195
Benefit Payments		(14,202)	(13,208)
Actuarial (Gain) Loss		(29,985)	31,521
Projected Benefit Obligation, ending balance		328,442	352,718	314,637
Reconciliation of Accrued Postretirement Cost:
Expense	[1]	(6,046)	(5,877)	(5,733)
Executive Survivor and Supplemental Retirement Plan [Member]
Reconciliation of Fair Value of Plan Assets:
Fair Value of Plan Assets, beginning balance
Actual Return on Plan Assets
Benefit Payments		(1,505)	(1,175)
Fair Value of Plan Assets, ending balance
Employer Contributions		1,505	1,175
Reconciliation of Projected Benefit Obligation:
Projected Benefit Obligation, beginning balance		42,308	37,335
Service Cost		408	290	252
Interest Cost		1,589	1,686	1,667
Benefit Payments		(1,505)	(1,175)
Actuarial (Gain) Loss		(3,101)	4,172
Projected Benefit Obligation, ending balance		39,699	42,308	37,335
Plan Amendments
Reconciliation of Accrued Postretirement Cost:
Expense	[2]	(2,979)	(2,755)	(2,712)
Employer Contributions		1,505	1,175
Other Postretirement Benefits Plan [Member]
Reconciliation of Fair Value of Plan Assets:
Fair Value of Plan Assets, beginning balance
Actual Return on Plan Assets
Benefit Payments		(6,684)	(6,534)
Fair Value of Plan Assets, ending balance
Employer Contributions		3,183	3,290
Participant Premium Payments		3,501	3,244
Reconciliation of Projected Benefit Obligation:
Projected Benefit Obligation, beginning balance		69,774	62,571
Service Cost		1,526	1,425	1,301
Interest Cost		2,583	2,712	2,503
Benefit Payments		(6,684)	(6,534)
Actuarial (Gain) Loss		861	6,356
Projected Benefit Obligation, ending balance		71,561	69,774	62,571
Participant Premium Payments		3,501	3,244
Reconciliation of Accrued Postretirement Cost:
Accrued Postretirement Cost at January 1		(50,958)	(49,156)
Expense	[3]	(5,799)	(5,092)	(4,329)
Employer Contributions		3,183	3,290
Accrued Postretirement Cost at December 31		$ (53,574)	$ (50,958)	$ (49,156)
[1]	Allocation of Costs: 2018 2017 2016 Service Costs included in OTP Capital Expenditures $ 1,542 $ 1,094 $ 1,009 Service costs included in electric operation and maintenance expenses 4,756 4,400 4,377 Service costs included in other nonelectric expenses 161 135 132 Nonservice costs capitalized (99) 48 39 Nonservice costs included in nonservice cost components of postretirement benefits (314) 200 176
[2]	Allocation of Costs: 2018 2017 2016 Service costs included in electric operation and maintenance expenses $ 99 $ 94 $ 87 Service costs included in other nonelectric expenses 309 196 165 Nonservice costs included in nonservice cost components of postretirement benefits 2,571 2,465 2,460
[3]	Allocation of Cost: 2018 2017 2016 Service Costs included in OTP capital expenditures $ 364 $ 277 $ 238 Service costs included in electric operation and maintenance expenses 1,124 1,114 1,032 Service costs included in other nonelectric expenses 38 34 31 Nonservice costs capitalized 1,020 712 554 Nonservice costs included in nonservice cost components of postretirement benefits 3,253 2,955 2,474

Note 11 - Pension Plan and O_10

Note 11 - Pension Plan and Other Postretirement Benefits - Weighted-average Assumptions Used to Determine Benefit Obligations (Details)	Dec. 31, 2018	Dec. 31, 2017
Pension Plan [Member]
Discount Rate	4.50%	3.90%
Pension Plan [Member] | Participants to Age 39 [Member]
Rate of Increase in Future Compensation Level	4.50%	4.50%
Pension Plan [Member] | Participants Age 40 to 49 [Member]
Rate of Increase in Future Compensation Level	3.50%	3.50%
Pension Plan [Member] | Participants Age 50 and Older [Member]
Rate of Increase in Future Compensation Level	2.75%	2.75%
Executive Survivor and Supplemental Retirement Plan [Member]
Discount Rate	4.46%	3.85%
Rate of Increase in Future Compensation Level	3.40%	2.92%
Other Postretirement Benefits Plan [Member]
Discount Rate	4.44%	3.81%

Note 11 - Pension Plan and O_11

Note 11 - Pension Plan and Other Postretirement Benefits - Measurement Dates (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Pension Plan [Member]
Net Periodic Pension Cost	Jan. 1, 2018	Jan. 1, 2017
End of Year Benefit Obligations	January 1, 2018 projected to December 31, 2018	January 1, 2017 projected to December 31, 2017
Market Value of Assets	Dec. 31, 2018	Dec. 31, 2017
Other Postretirement Benefits Plan [Member]
Net Periodic Postretirement Benefit Cost	Jan. 1, 2018	Jan. 1, 2017
End of Year Benefit Obligations	January 1, 2018 projected to December 31, 2018	January 1, 2017 projected to December 31, 2017

Note 11 - Pension Plan and O_12

Note 11 - Pension Plan and Other Postretirement Benefits - Estimated Amounts of Unrecognized Net Actuarial Losses and Prior Service Costs to be Amortized (Details) $ in Thousands	Dec. 31, 2018USD ($)
Pension Plan [Member]
Decrease in Regulatory Assets:
Amortization of Unrecognized Prior Service Cost	$ 5
Amortization of Unrecognized Actuarial Loss	4,642
Decrease in Accumulated Other Comprehensive Loss:
Amortization of Unrecognized Prior Service Cost	9
Amortization of Unrecognized Actuarial Loss	114
Total Estimated Amortization	4,770
Executive Survivor and Supplemental Retirement Plan [Member]
Decrease in Regulatory Assets:
Amortization of Unrecognized Prior Service Cost	5
Amortization of Unrecognized Actuarial Loss	124
Decrease in Accumulated Other Comprehensive Loss:
Amortization of Unrecognized Prior Service Cost	17
Amortization of Unrecognized Actuarial Loss	348
Total Estimated Amortization	494
Other Postretirement Benefits Plan [Member]
Decrease in Regulatory Assets:
Amortization of Unrecognized Actuarial Loss	1,570
Decrease in Accumulated Other Comprehensive Loss:
Amortization of Unrecognized Actuarial Loss	39
Total Estimated Amortization	$ 1,609

Note 11 - Pension Plan and O_13

Note 11 - Pension Plan and Other Postretirement Benefits - Benefit Payments, Which Reflect Expected Future Service, As Appropriate, Expected to Be Paid Out From Plan Assets (Details) $ in Thousands	Dec. 31, 2018USD ($)
2,019	$ 15,086
2,020	15,689
2,021	16,356
2,022	17,017
2,023	17,709
2024-2028	96,186
Executive Survivor and Supplemental Retirement Plan [Member]
2,019	1,468
2,020	1,527
2,021	1,618
2,022	2,214
2,023	2,635
2024-2028	14,215
Other Postretirement Benefits Plan [Member]
2,019	4,247
2,020	4,322
2,021	4,483
2,022	4,698
2,023	4,781
2024-2028	$ 24,183

Note 11 - Pension Plan and O_14

Note 11 - Pension Plan and Other Postretirement Benefits - Asset Allocation Ranges By Asset Class and PBO Funded Status (Details) - Pension Plan [Member]	Dec. 31, 2018
Equity Securities [Member] | Permitted Range, Less Than 85 Percent PBO [Member] | Minimum [Member]
Asset allocations	39.00%
Equity Securities [Member] | Permitted Range, Less Than 85 Percent PBO [Member] | Maximum [Member]
Asset allocations	59.00%
Equity Securities [Member] | Permitted Range, Equal to or Greater than 85 Percent PBO [Member] | Minimum [Member]
Asset allocations	34.00%
Equity Securities [Member] | Permitted Range, Equal to or Greater than 85 Percent PBO [Member] | Maximum [Member]
Asset allocations	54.00%
Equity Securities [Member] | Permitted Range, Equal to or Greater than 90 Percent PBO [Member] | Minimum [Member]
Asset allocations	24.00%
Equity Securities [Member] | Permitted Range, Equal to or Greater than 90 Percent PBO [Member] | Maximum [Member]
Asset allocations	44.00%
Equity Securities [Member] | Permitted Range, Equal to or Greater than 95% PBO [Member] | Minimum [Member]
Asset allocations	14.00%
Equity Securities [Member] | Permitted Range, Equal to or Greater than 95% PBO [Member] | Maximum [Member]
Asset allocations	34.00%
Equity Securities [Member] | Permitted Range, Equal to or Greater than 100 Percent PBO [Member] | Minimum [Member]
Asset allocations	0.00%
Equity Securities [Member] | Permitted Range, Equal to or Greater than 100 Percent PBO [Member] | Maximum [Member]
Asset allocations	20.00%
Investment Grade Fixed Income Securities [Member] | Permitted Range, Less Than 85 Percent PBO [Member] | Minimum [Member]
Asset allocations	22.00%
Investment Grade Fixed Income Securities [Member] | Permitted Range, Less Than 85 Percent PBO [Member] | Maximum [Member]
Asset allocations	42.00%
Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 85 Percent PBO [Member] | Minimum [Member]
Asset allocations	30.00%
Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 85 Percent PBO [Member] | Maximum [Member]
Asset allocations	50.00%
Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 90 Percent PBO [Member] | Minimum [Member]
Asset allocations	40.00%
Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 90 Percent PBO [Member] | Maximum [Member]
Asset allocations	60.00%
Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 95% PBO [Member] | Minimum [Member]
Asset allocations	53.00%
Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 95% PBO [Member] | Maximum [Member]
Asset allocations	73.00%
Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 100 Percent PBO [Member] | Minimum [Member]
Asset allocations	70.00%
Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 100 Percent PBO [Member] | Maximum [Member]
Asset allocations	100.00%
Below Investment Grade Fixed Income Securities [Member] | Permitted Range, Less Than 85 Percent PBO [Member] | Minimum [Member]
Asset allocations	0.00%	[1]
Below Investment Grade Fixed Income Securities [Member] | Permitted Range, Less Than 85 Percent PBO [Member] | Maximum [Member]
Asset allocations	15.00%	[1]
Below Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 85 Percent PBO [Member] | Minimum [Member]
Asset allocations	0.00%	[1]
Below Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 85 Percent PBO [Member] | Maximum [Member]
Asset allocations	15.00%	[1]
Below Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 90 Percent PBO [Member] | Minimum [Member]
Asset allocations	0.00%	[1]
Below Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 90 Percent PBO [Member] | Maximum [Member]
Asset allocations	15.00%	[1]
Below Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 95% PBO [Member] | Minimum [Member]
Asset allocations	0.00%	[1]
Below Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 95% PBO [Member] | Maximum [Member]
Asset allocations	10.00%	[1]
Below Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 100 Percent PBO [Member] | Minimum [Member]
Asset allocations	0.00%	[1]
Below Investment Grade Fixed Income Securities [Member] | Permitted Range, Equal to or Greater than 100 Percent PBO [Member] | Maximum [Member]
Asset allocations	10.00%	[1]
Other Asset Categories [Member] | Permitted Range, Less Than 85 Percent PBO [Member] | Minimum [Member]
Asset allocations	5.00%	[2]
Other Asset Categories [Member] | Permitted Range, Less Than 85 Percent PBO [Member] | Maximum [Member]
Asset allocations	20.00%	[2]
Other Asset Categories [Member] | Permitted Range, Equal to or Greater than 85 Percent PBO [Member] | Minimum [Member]
Asset allocations	5.00%	[2]
Other Asset Categories [Member] | Permitted Range, Equal to or Greater than 85 Percent PBO [Member] | Maximum [Member]
Asset allocations	20.00%	[2]
Other Asset Categories [Member] | Permitted Range, Equal to or Greater than 90 Percent PBO [Member] | Minimum [Member]
Asset allocations	5.00%	[2]
Other Asset Categories [Member] | Permitted Range, Equal to or Greater than 90 Percent PBO [Member] | Maximum [Member]
Asset allocations	20.00%	[2]
Other Asset Categories [Member] | Permitted Range, Equal to or Greater than 95% PBO [Member] | Minimum [Member]
Asset allocations	0.00%	[2]
Other Asset Categories [Member] | Permitted Range, Equal to or Greater than 95% PBO [Member] | Maximum [Member]
Asset allocations	15.00%	[2]
Other Asset Categories [Member] | Permitted Range, Equal to or Greater than 100 Percent PBO [Member] | Minimum [Member]
Asset allocations	0.00%	[2]
Other Asset Categories [Member] | Permitted Range, Equal to or Greater than 100 Percent PBO [Member] | Maximum [Member]
Asset allocations	15.00%	[2]
[1]	Includes (but not limited to) High Yield Bond Fund and Emerging Markets Debt funds.
[2]	Other category may include cash, alternatives, and/or other investment strategies that may be classified other than equity or fixed income, such as the Dynamic Asset Allocation fund.

Note 11 - Pension Plan and O_15

Note 11 - Pension Plan and Other Postretirement Benefits - Pension Plan Asset Allocations by Asset Category (Details) - Pension Plan [Member]	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan, Actual Plan Asset Allocations	100.00%	100.00%
Large Capitalization Equity Securities [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	17.50%	23.50%
International Equity Securities [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	17.00%	18.10%
Small and Mid Capitalization Equity Securities [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	6.70%	8.70%
SEI Dynamic Asset Allocation Fund [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	4.00%	5.00%
Emerging Markets Equity Funds [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	3.40%	5.50%
Equity Securities [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	48.60%	60.80%
Fixed-income Securities and Cash [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	47.10%	35.20%
Other - SEI Energy Debt Collective Fund [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	4.30%	4.00%

Note 11 - Pension Plan and O_16

Note 11 - Pension Plan and Other Postretirement Benefits - Pension Fund Assets Measured at Fair Value and NAV (Details) - Pension Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value of Plan Assets	$ 269,783	$ 285,319	$ 254,346
NAV Practical Expedient to Fair Valuation [Member] | Other - SEI Energy Debt Collective Fund [Member] | SEI Energy Debt Collective Fund [Member]
Fair Value of Plan Assets	11,476	11,320
Fair Value, Inputs, Level 1 [Member]
Fair Value of Plan Assets	$ 258,307	$ 273,999

Note 11 - Pension Plan and O_17

Note 11 - Pension Plan and Other Postretirement Benefits - Pension Fund Assets Measured at Fair Value (Details) - Pension Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value of Plan Assets	$ 269,783	$ 285,319	$ 254,346
Fair Value, Inputs, Level 1 [Member]
Fair Value of Plan Assets	258,307	273,999
Fair Value, Inputs, Level 1 [Member] | Large Capitalization Equity Securities [Member]
Fair Value of Plan Assets	47,198	66,946
Fair Value, Inputs, Level 1 [Member] | International Equity Securities [Member]
Fair Value of Plan Assets	45,912	51,636
Fair Value, Inputs, Level 1 [Member] | Small and Mid Capitalization Equity Securities [Member]
Fair Value of Plan Assets	17,971	24,848
Fair Value, Inputs, Level 1 [Member] | SEI Dynamic Asset Allocation Fund [Member]
Fair Value of Plan Assets	10,929	14,371
Fair Value, Inputs, Level 1 [Member] | Emerging Markets Equity Funds [Member]
Fair Value of Plan Assets	9,197	15,824
Fair Value, Inputs, Level 1 [Member] | Fixed Income Securities [Member]
Fair Value of Plan Assets	127,098	100,373
Fair Value, Inputs, Level 1 [Member] | Cash Management Money Market Fund [Member]
Fair Value of Plan Assets	$ 2	$ 1

Note 11 - Pension Plan and O_18

Note 11 - Pension Plan and Other Postretirement Benefits - Healthcare Cost-trend Rates (Details) - Other Postretirement Benefits Plan [Member]	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Healthcare Cost-Trend Rate Assumed for Next Year Pre-65	7.00%	5.85%
Healthcare Cost-Trend Rate Assumed for Next Year Post-65	7.00%	6.03%
Rate to Which the Cost-Trend Rate is Assumed to Decline	4.50%	4.50%
Year the Rate Reaches the Ultimate Trend Rate	2,038	2,038

Note 11 - Pension Plan and O_19

Note 11 - Pension Plan and Other Postretirement Benefits - Effects of One Percentage Change In Assumed Healthcare Cost-trend Rates (Details) - Other Postretirement Benefits Plan [Member] $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Effect of 1 Point Increase on the Postretirement Benefit Obligation	$ 9,095
Effect of 1 Point Decrease on the Postretirement Benefit Obligation	(7,586)
Effect of 1 Point Increase on Total of Service and Interest Cost	758
Effect of 1 Point Decrease on Total of Service and Interest Cost	(600)
Effect of 1 Point Increase on Expense	1,953
Effect of 1 Point Decrease on Expense	$ (1,589)

Note 12 - Fair Value of Finan_3

Note 12 - Fair Value of Financial Instruments (Details Textual) - London Interbank Offered Rate (LIBOR) [Member]	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Otter Tail Corporation Credit Agreement [Member]
Debt Instrument, Basis Spread on Variable Rate	1.50%	1.50%
OTP Credit Agreement [Member]
Debt Instrument, Basis Spread on Variable Rate	1.25%	1.25%

Note 12 - Fair Value of Finan_4

Note 12 - Fair Value of Financial Instruments - Summary of Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Reported Value Measurement [Member]
Cash and Cash Equivalents	$ 861	$ 16,216
Short-Term Debt	(18,599)	(112,371)
Long-Term Debt including Current Maturities	(590,174)	(490,566)
Estimate of Fair Value Measurement [Member]
Cash and Cash Equivalents	861	16,216
Short-Term Debt	(18,599)	(112,371)
Long-Term Debt including Current Maturities	$ (601,513)	$ (543,691)

Note 13 - Property, Plant and_3

Note 13 - Property, Plant and Equipment (Details Textual)	12 Months Ended
	Dec. 31, 2018
Electric Plant [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	5 years
Electric Plant [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	82 years
Nonelectric Plant [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	2 years
Nonelectric Plant [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	40 years

Note 13 - Property, Plant and_4

Note 13 - Property, Plant and Equipment - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Total Gross Plant	$ 2,429,467	$ 2,339,022
Less Accumulated Depreciation and Amortization	848,369	799,419
Net Plant	1,581,098	1,539,603
Electric Plant [Member]
Total Gross Plant	2,189,811	2,113,574
Less Accumulated Depreciation and Amortization	699,642	662,431
Net Plant	1,490,169	1,451,143
Nonelectric Operations Plant [Member]
Total Gross Plant	239,656	225,448
Less Accumulated Depreciation and Amortization	148,727	136,988
Net Plant	90,929	88,460
Electric Generation Equipment [Member] | Electric Plant [Member]
Total Gross Plant	905,224	897,732
Transmission Plant [Member] | Electric Plant [Member]
Total Gross Plant	512,832	500,352
Distribution Plant [Member] | Electric Plant [Member]
Total Gross Plant	502,261	482,867
General Plant [Member] | Electric Plant [Member]
Total Gross Plant	99,404	100,067
Electric Plant in Service [Member] | Electric Plant [Member]
Total Gross Plant	2,019,721	1,981,018
Construction in Progress [Member] | Electric Plant [Member]
Total Gross Plant	170,090	132,556
Construction in Progress [Member] | Nonelectric Plant [Member]
Total Gross Plant	11,536	8,511
Equipment [Member] | Nonelectric Plant [Member]
Total Gross Plant	170,634	160,263
Buildings and Leasehold Improvements [Member] | Nonelectric Plant [Member]
Total Gross Plant	53,011	52,280
Land [Member] | Nonelectric Plant [Member]
Total Gross Plant	4,475	4,394
Nonelectric Operations Plant [Member] | Nonelectric Plant [Member]
Total Gross Plant	$ 228,120	$ 216,937

Note 13 - Property, Plant and_5

Note 13 - Property, Plant and Equipment - Estimated Service Lives for Properties (Details)	12 Months Ended
	Dec. 31, 2018
Minimum [Member] | Electric Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	5 years
Minimum [Member] | Electric Plant [Member] | Electric Generation Equipment [Member]
Property, Plant and Equipment, Useful Life (Year)	9 years
Minimum [Member] | Electric Plant [Member] | Transmission Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	42 years
Minimum [Member] | Electric Plant [Member] | Distribution Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	5 years
Minimum [Member] | Electric Plant [Member] | General Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	5 years
Minimum [Member] | Nonelectric Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	2 years
Minimum [Member] | Nonelectric Plant [Member] | Equipment [Member]
Property, Plant and Equipment, Useful Life (Year)	2 years
Minimum [Member] | Nonelectric Plant [Member] | Buildings and Leasehold Improvements [Member]
Property, Plant and Equipment, Useful Life (Year)	5 years
Maximum [Member] | Electric Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	82 years
Maximum [Member] | Electric Plant [Member] | Electric Generation Equipment [Member]
Property, Plant and Equipment, Useful Life (Year)	82 years
Maximum [Member] | Electric Plant [Member] | Transmission Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	70 years
Maximum [Member] | Electric Plant [Member] | Distribution Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	68 years
Maximum [Member] | Electric Plant [Member] | General Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	50 years
Maximum [Member] | Nonelectric Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	40 years
Maximum [Member] | Nonelectric Plant [Member] | Equipment [Member]
Property, Plant and Equipment, Useful Life (Year)	12 years
Maximum [Member] | Nonelectric Plant [Member] | Buildings and Leasehold Improvements [Member]
Property, Plant and Equipment, Useful Life (Year)	40 years

Note 14 - Income Taxes (Details

Note 14 - Income Taxes (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	35.00%	35.00%
Increase (Decrease) in Percentage of Production Tax Credits	(53.00%)	4.40%
Wind Tax Credits, Amortization Period	25 years
Period for Unrecognized Tax Benefits Not Expected Change	1 year
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	$ 0
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability		$ 1,756

Note 14 - Income Taxes - Effect

Note 14 - Income Taxes - Effective Income Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Tax Computed at Federal Statutory Rate	$ 20,356	$ 34,893	$ 28,889
Increases (Decreases) in Tax from:
State Income Taxes Net of Federal Income Tax Expense	5,210	4,368	2,869
Differences Reversing in Excess of Federal Rates	(3,432)	551	77
Federal Production Tax Credits (PTCs)	(3,111)	(7,527)	(7,175)
Permanent Differences, R&D Tax Credits, Unitary Tax and Other Adjustments	(1,864)	(1,873)	(1,262)
North Dakota Wind Tax Credit Amortization – Net of Federal Taxes	(1,033)	(850)	(850)
Excess Tax deduction - Equity Method Stock Awards	(708)	(751)
Allowance for Funds Used During Construction – Equity	(431)	(322)	(280)
Employee Stock Ownership Plan Dividend Deduction	(298)	(509)	(537)
Investment Tax Credit Amortization	(98)	(164)	(350)
Corporate-owned Life Insurance	(3)	(845)	(680)
Section 199 Domestic Production Activities Deduction		(1,471)	(482)
Effect of TCJA Tax Rate Reduction on Value of Net Deferred Tax Assets		1,756
Income Tax Expense	$ 14,588	$ 27,256	$ 20,219
Overall Effective Federal, State and Foreign Income Tax Rate	15.00%	27.30%	24.50%
Current Federal Income Taxes	$ 4,960	$ 4,434	$ 989
Current State Income Taxes	1,395	1,128	1,208
Deferred Federal Income Taxes	8,065	25,648	23,774
Deferred State Income Taxes	4,410	4,587	2,623
Federal PTCs	(3,111)	(7,527)	(7,175)
North Dakota Wind Tax Credit Amortization – Net of Federal Taxes	(1,033)	(850)	(850)
Investment Tax Credit Amortization	(98)	(164)	(350)
Total Income Before Income Taxes	$ 96,933	$ 99,695	$ 82,540

Note 14 - Income Taxes - Deferr

Note 14 - Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets
Benefit Liabilities	$ 33,967	$ 32,328
Regulatory Tax Liability	33,228	39,465
Retirement Benefits Liabilities	32,664	31,894
North Dakota Wind Tax Credits	32,570	32,962
Federal PTCs	32,101	40,614
Cost of Removal	21,787	21,800
Differences Related to Property	6,842	6,499
Net Operating Loss Carryforward	2,489	3,203
Vacation Accrual	1,919	1,844
Investment Tax Credits	449	515
Other	3,218	668
Valuation Allowance	(600)
Total Deferred Tax Assets	200,634	211,792
Deferred Tax Liabilities
Differences Related to Property	(261,396)	(257,906)
Retirement Benefits Regulatory Asset	(32,664)	(31,894)
Excess Tax over Book Pension	(15,145)	(14,077)
North Dakota Wind Tax Credits	(4,386)	(4,112)
Impact of State Net Operating Losses on Federal Taxes	(523)	(673)
Other	(7,496)	(3,631)
Total Deferred Tax Liabilities	(321,610)	(312,293)
Deferred Income Taxes	$ (120,976)	$ (100,501)	$ (226,591)

Note 14 - Income Taxes - Expira

Note 14 - Income Taxes - Expiration of Tax Net Operating Losses and Tax Credits Available (Details) $ in Thousands	Dec. 31, 2018USD ($)
Domestic Tax Authority [Member]
Tax Credits	$ 34,586
Domestic Tax Authority [Member] | Tax Years 2022 to 2032 [Member]
Tax Credits
Domestic Tax Authority [Member] | Tax Years 2033 to 2038 [Member]
Tax Credits	34,586
Domestic Tax Authority [Member] | Tax Years 2039 to 2043 [Member]
Tax Credits
State and Local Jurisdiction [Member]
Tax Credits	33,106
State Net Operating Losses	2,489
State and Local Jurisdiction [Member] | Tax Years 2022 to 2032 [Member]
Tax Credits	273
State Net Operating Losses	2,396
State and Local Jurisdiction [Member] | Tax Years 2033 to 2038 [Member]
Tax Credits	3,326
State Net Operating Losses	93
State and Local Jurisdiction [Member] | Tax Years 2039 to 2043 [Member]
Tax Credits	29,507
State Net Operating Losses

Note 14 - Income Taxes - Unreco

Note 14 - Income Taxes - Unrecognized Tax Benefit Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance, beginning	$ 684	$ 891	$ 468
Increases Related to Tax Positions for Prior Years	6	28	406
Decreases Related to Tax Positions for Prior Years		(172)
Increases Related to Tax Positions for Current Year	778	143	114
Uncertain Positions Resolved During Year	(186)	(206)	(97)
Balance, ending	$ 1,282	$ 684	$ 891

Note 14 - Income Taxes - Impact

Note 14 - Income Taxes - Impacts of TCJA Adjustments to Deferred Taxes and Regulatory Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance, Deferred tax liability	$ 100,501	$ 226,591
Balance, Deferred tax regulatory liability	242,581
Change due to 2017Accruals and Amortizations, Deferred tax liability		20,012
TCJA Deferred Tax Valuation Adjustment, Deferred tax liability		(109,072)
Tax Effect on TCJA Deferred Tax Valuation Adjustment, Deferred tax liability		(38,786)
Effect of TCJA Tax Rate Reduction on Value of Net Deferred Tax Assets		1,756
Balance, Deferred tax liability	120,976	100,501	226,591
Balance, Deferred tax regulatory liability	227,207	242,581
Deferred Income Tax Charge [Member]
Balance, Deferred tax regulatory liability	$ 149,052	818
Change due to 2017Accruals and Amortizations, Deferred tax regulatory liability		376
TCJA Deferred Tax Valuation Adjustment, Deferred tax regulatory liability		109,072
Tax Effect on TCJA Deferred Tax Valuation Adjustment, Deferred tax regulatory liability		38,786
Balance, Deferred tax regulatory liability		$ 149,052	$ 818

Note 15 - Asset Retirement Ob_3

Note 15 - Asset Retirement Obligations (AROs) (Details Textual) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Asset Retirement Obligation, Liabilities Incurred
Otter Tail Power Company [Member]
Asset Retirement Obligation, Liabilities Incurred	$ 0
North Dakota [Member]
Number of Wind Turbines	92

Note 15 - Asset Retirement Ob_4

Note 15 - Asset Retirement Obligations (AROs) - Legal AROs, Capitalized Asset Retirement Costs and Accumulated Depreciation and a Settlement Activity Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Asset Retirement Obligations
Beginning Balance	$ 8,719	$ 8,341
Asset Retirement Obligation, Liabilities Incurred
Adjustments Due to Revisions in Cash Flow Estimates
Accrued Accretion	398	378
Settlements
Ending Balance	9,117	8,719
Asset Retirement Costs Capitalized
Beginning Balance	2,983	2,983
New Obligations Recognized
Adjustments Due to Revisions in Cash Flow Estimates
Settlements
Ending Balance	2,983	2,983
Accumulated Depreciation – Asset Retirement Costs Capitalized
Beginning Balance	915	795
New Obligations Recognized
Adjustments Due to Revisions in Cash Flow Estimates
Depreciation Expense	119	120
Settlements
Ending Balance	1,034	915
Original Capitalized Asset Retirement Cost – Retired
Accumulated Depreciation
Asset Retirement Obligation
Settlement Cost
Gain on Settlement – Deferred Under Regulatory Accounting

Note 16 - Subsequent Events (De

Note 16 - Subsequent Events (Details Textual) - shares	Feb. 13, 2019	Feb. 05, 2018	Dec. 31, 2018
Stock Performance Awards [Member] | Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized
Period Specified for Average Adjusted Return			3 years
The 2014 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized			1,900,000
The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		54,000
Number of Trading Days		20 days
Subsequent Event [Member] | The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member]
Period Specified for Average Adjusted Return	3 years
Number of Trading Days	20 days
Subsequent Event [Member] | The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	55,600
Actual Payment, Percentage of Target Amount	150.00%
Number of Shares Authorized for Actual Payment	83,400
Subsequent Event [Member] | The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member] | Minimum [Member]
Actual Payment, Percentage of Target Amount	0.00%
Subsequent Event [Member] | The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member] | Share-based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	27,800
Subsequent Event [Member] | The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member] | Share-based Compensation Award, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	27,800

Note 16 - Subsequent Events - S

Note 16 - Subsequent Events - Summary of Stock Incentive Awards to Executive Officers Under the 2014 Stock Incentive Plan (Details) - Subsequent Event [Member] - Executive Officer [Member]	Feb. 13, 2019$ / sharesshares
The 2014 Stock Incentive Plan [Member] | Restricted Stock Units (RSUs) [Member]
Units granted (in shares) | shares	15,600
Grant-date fair value per award (in dollars per share) | $ / shares	$ 49.6225
The 2014 Stock Incentive Plan Under Executive Agreement [Member] | Performance Shares [Member]
Units granted (in shares) | shares	47,800
Grant-date fair value per award (in dollars per share) | $ / shares	$ 42.875
Vesting date	Dec. 31, 2021
The 2014 Stock Incentive Plan Under Legacy Agreement [Member] | Performance Shares [Member]
Units granted (in shares) | shares	7,800
Grant-date fair value per award (in dollars per share) | $ / shares	$ 45.885
Vesting date	Dec. 31, 2021

Note 16 - Subsequent Events -_2

Note 16 - Subsequent Events - Summary of Stock Incentive Awards to Executive Officers Under the 2014 Stock Incentive Plan (Details) (Parentheticals) - The 2014 Stock Incentive Plan [Member] - Restricted Stock Units (RSUs) [Member]	Feb. 13, 2019	Dec. 31, 2018
Vesting period (Year)		4 years
Subsequent Event [Member] | Executive Officer [Member]
Vesting period (Year)	4 years
Subsequent Event [Member] | Executive Officer [Member] | Share-based Compensation Award, Tranche One [Member]
Vesting percentage	25.00%

Schedule 1 - Condensed Financ_3

Schedule 1 - Condensed Financial Information of Registrant - Condensed Balance Sheets, Parent Company (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Cash and Cash Equivalents	$ 861	$ 16,216
Accounts Receivable	75,144	68,466
Accounts Receivable from Subsidiaries	1,931	2,098
Interest Receivable from Subsidiaries	117	117
Notes Receivable from Subsidiaries	1,167	1,752
Other	6,114	12,491
Total Current Assets	241,420	239,127
Investments in Subsidiaries	8,961	8,629
Notes Receivable from Subsidiaries	79,422	79,611
Other Assets	35,759	36,006
Total Assets	2,052,517	2,004,278	1,912,385
Liabilities and Equity
Short-Term Debt	18,599	112,371
Less: Current Maturities--net of Unamortized Debt Issuance Costs	172	186
Accounts Payable to Subsidiaries	7	6
Notes Payable to Subsidiaries	60,626	61,908
Other	12,149	11,389
Total Current Liabilities	170,093	256,653
Other Noncurrent Liabilities	24,326	22,769
Commitments and Contingencies (note 9)
Capitalization (page 68)
Total Long-Term Debt net of Unamortized Debt Issuance Costs	590,002	490,380
Common Shareholder Equity	198,324	197,787
Capitalization, Long-term Debt and Equity, Total	1,318,865	1,187,272
Total Liabilities and Equity	2,052,517	2,004,278
Parent Company [Member]
Assets
Cash and Cash Equivalents		16,371	$ 6,218
Accounts Receivable
Accounts Receivable from Subsidiaries	1,931	2,098
Interest Receivable from Subsidiaries	117	117
Notes Receivable from Subsidiaries	1,167	1,752
Other	3,482	1,130
Total Current Assets	6,697	21,468
Investments in Subsidiaries	787,869	724,613
Notes Receivable from Subsidiaries	79,422	79,611
Deferred Income Taxes	21,100	27,923
Other Assets	31,547	31,559
Total Assets	926,635	885,174
Liabilities and Equity
Short-Term Debt	9,215
Less: Current Maturities--net of Unamortized Debt Issuance Costs	172	186
Accounts Payable to Subsidiaries	7	6
Notes Payable to Subsidiaries	60,626	61,908
Other	9,994	7,799
Total Current Liabilities	80,014	69,899
Other Noncurrent Liabilities	37,814	38,319
Capitalization (page 68)
Total Long-Term Debt net of Unamortized Debt Issuance Costs	79,944	80,064
Common Shareholder Equity	728,863	696,892
Capitalization, Long-term Debt and Equity, Total	808,807	776,956
Total Liabilities and Equity	$ 926,635	$ 885,174

Schedule 1 - Condensed Financ_4

Schedule 1 - Condensed Financial Information of Registrant - Condensed Statements of Income, Parent Company (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Loss
Operating Expenses	$ 787,058	$ 717,063	$ 686,908
Operating Loss	129,389	132,287	116,631
Other Income (Expense)
Interest Charges	(30,408)	(29,604)	(31,886)
Nonservice Cost Components of Postretirement Benefits	5,509	5,620	5,110
Other Income	3,461	2,632	2,905
Income Tax Expense	14,588	27,256	20,219
Net Income	82,345	72,439	62,321
Parent Company [Member]
Operating Loss
Revenue from Contracts with Customers
Operating Expenses	9,916	7,138	8,530
Operating Loss	(9,916)	(7,138)	(8,530)
Other Income (Expense)
Equity Income in Earnings of Subsidiaries	91,446	82,715	67,047
Interest Charges	(4,043)	(4,270)	(6,817)
Interest Charges to Subsidiaries	(387)	(244)	(173)
Interest Income from Subsidiaries	2,839	2,848	4,897
Nonservice Cost Components of Postretirement Benefits	1,422	1,215	1,159
Other Income	550	1,054	1,621
Total Other Income	88,983	80,888	65,416
Income Before Income Taxes	79,067	73,750	56,886
Income Tax Expense	(3,278)	1,311	(5,435)
Net Income	$ 82,345	$ 72,439	$ 62,321

Schedule 1 - Condensed Financ_5

Schedule 1 - Condensed Financial Information of Registrant - Condensed Statements of Cash Flows, Parent Company (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Flows from Operating Activities
Net Cash Provided by Operating Activities	$ 143,448	$ 173,577	$ 163,386
Cash Flows from Investing Activities
Return of Capital (Investment in Subsidiaries)	(4,372)	(4,168)	(4,402)
Net Cash (Used in) Provided by Investing Activities	(107,419)	(132,590)	(159,324)
Cash Flows from Financing Activities
Change in Checks Written in Excess of Cash	(345)	2,434	(3,363)
Net Short-Term Borrowings (Repayments)	(93,772)	69,488	(37,789)
Proceeds from Issuance of Common Stock		4,349	44,435
Common Stock Issuance Expenses	(108)		(562)
Payments for Retirement of Capital Stock	(3,011)	(1,799)	(104)
Proceeds from the Issuance of Long-Term Debt	100,000		130,000
Short-Term and Long-Term Debt Issuance Expenses	(761)	(380)	(888)
Payments for Retirement of Long-Term Debt	(189)	(48,231)	(87,547)
Dividends Paid and Other Distributions	(53,198)	(50,632)	(48,244)
Net Cash Used in Financing Activities	(51,384)	(24,771)	(4,062)
Net Change in Cash and Cash Equivalents	(15,355)	16,216
Cash and Cash Equivalents at Beginning of Period	16,216
Cash and Cash Equivalents at End of Period	861	16,216
Parent Company [Member]
Cash Flows from Operating Activities
Net Cash Provided by Operating Activities	56,947	50,205	83,296
Cash Flows from Investing Activities
Return of Capital (Investment in Subsidiaries)	(24,764)		9,912
Debt Repaid by (Issued to) Subsidiaries	774	151	(3,309)
Cash (Used in) Provided by Investing Activities	(623)	(121)	106
Net Cash (Used in) Provided by Investing Activities	(24,613)	30	6,709
Cash Flows from Financing Activities
Change in Checks Written in Excess of Cash	31		(428)
Net Short-Term Borrowings (Repayments)	9,215		(59,666)
(Repayments to) Borrowings from Subsidiaries	(1,281)	23,389	(60,948)
Proceeds from Issuance of Common Stock		4,349	44,435
Common Stock Issuance Expenses	(108)		(562)
Payments for Retirement of Capital Stock	(3,011)	(1,799)	(104)
Proceeds from the Issuance of Long-Term Debt			130,000
Short-Term and Long-Term Debt Issuance Expenses	(164)	(158)	(723)
Payments for Retirement of Long-Term Debt	(189)	(15,231)	(87,547)
Dividends Paid and Other Distributions	(53,198)	(50,632)	(48,244)
Net Cash Used in Financing Activities	(48,705)	(40,082)	(83,787)
Net Change in Cash and Cash Equivalents	(16,371)	10,153	6,218
Cash and Cash Equivalents at Beginning of Period	16,371	6,218
Cash and Cash Equivalents at End of Period		$ 16,371	$ 6,218

Schedule 1 - Condensed Financ_6

Schedule 1 - Condensed Financial Information of Registrant - Related Party Transactions (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable	$ 1,931	$ 2,098
Interest Receivable	117	117
Current Notes Receivable	1,167	1,752
Long-Term Notes Receivable	79,422	79,611
Accounts Payable	7	6
Current Notes Payable	60,626	61,908
Otter Tail Corporation [Member] | Otter Tail Power Company [Member]
Accounts Receivable	1,877	2,067
Interest Receivable
Current Notes Receivable
Long-Term Notes Receivable
Accounts Payable	7	6
Current Notes Payable
Otter Tail Corporation [Member] | Vinyltech Corporation [Member]
Accounts Receivable	4	2
Interest Receivable	17	17
Current Notes Receivable
Long-Term Notes Receivable	11,500	11,500
Accounts Payable
Current Notes Payable	15,305	20,603
Otter Tail Corporation [Member] | Northern Pipe Products, Inc. [Member]
Accounts Receivable		4
Interest Receivable	8	8
Current Notes Receivable
Long-Term Notes Receivable	5,522	5,711
Accounts Payable
Current Notes Payable	5,623	8,186
Otter Tail Corporation [Member] | BTD Manufacturing, Inc. [Member]
Accounts Receivable
Interest Receivable	77	77
Current Notes Receivable	415
Long-Term Notes Receivable	52,000	52,000
Accounts Payable
Current Notes Payable		7,260
Otter Tail Corporation [Member] | T. O. Plastics, Inc. [Member]
Accounts Receivable
Interest Receivable	15	15
Current Notes Receivable
Long-Term Notes Receivable	10,400	10,400
Accounts Payable
Current Notes Payable	14,308	13,446
Otter Tail Corporation [Member] | Wind Tower Business [Member]
Accounts Receivable
Interest Receivable
Current Notes Receivable		1,461
Long-Term Notes Receivable
Accounts Payable
Current Notes Payable
Otter Tail Corporation [Member] | Varistar Corporation [Member]
Accounts Receivable
Interest Receivable
Current Notes Receivable	752
Long-Term Notes Receivable
Accounts Payable
Current Notes Payable	25,390	12,413
Otter Tail Corporation [Member] | Dock and Boatlift Business [Member]
Accounts Receivable
Interest Receivable
Current Notes Receivable		291
Long-Term Notes Receivable
Accounts Payable
Current Notes Payable
Otter Tail Corporation [Member] | Otter Tail Assurance Limited [Member]
Accounts Receivable	50	25
Interest Receivable
Current Notes Receivable
Long-Term Notes Receivable
Accounts Payable
Current Notes Payable

Schedule 1 - Condensed Financ_7

Schedule 1 - Condensed Financial Information of Registrant - Dividends (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Dividends Paid to Parent by Subsidiaries	$ 53,134	$ 50,571	$ 77,779