Otter Tail (OTTR) 10-K 20 Feb 20 2019 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 06, 2020	Jun. 28, 2019
Document Information [Line Items]
Entity Central Index Key	0001466593
Entity Registrant Name	Otter Tail Corp
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2019
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
Document Transition Report	false
Entity File Number	0-53713
Entity Incorporation, State or Country Code	MN
Entity Tax Identification Number	27-0383995
Entity Address, Address Line One	215 South Cascade Street, Box 496
Entity Address, City or Town	Fergus Falls
Entity Address, State or Province	MN
Entity Address, Postal Zip Code	56538-0496
City Area Code	866
Local Phone Number	410-8780
Title of 12(b) Security	Common Shares, par value $5.00 per share
Trading Symbol	OTTR
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 2,040,017,347
Entity Common Stock, Shares Outstanding		40,214,375

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current Assets
Cash and Cash Equivalents	$ 21,199	$ 861
Accounts Receivable:
Trade (less allowance for doubtful accounts of $1,339 for 2019 and $1,407 for 2018)	77,947	75,144
Other	8,773	9,741
Inventories	97,851	106,270
Unbilled Receivables	20,911	23,626
Income Taxes Receivable	1,487	2,439
Regulatory Assets	21,650	17,225
Other	5,042	6,114
Total Current Assets	254,860	241,420
Investments	9,894	8,961
Other Assets	40,196	35,759
Goodwill	37,572	37,572
Other Intangibles–Net	11,290	12,450
Regulatory Assets	144,138	135,257
Right of Use Asset – Operating Leases	21,851	0
Plant
Electric Plant in Service	2,212,884	2,019,721
Nonelectric Operations	247,356	228,120
Construction Work in Progress	185,238	181,626
Total Gross Plant	2,645,478	2,429,467
Less Accumulated Depreciation and Amortization	891,684	848,369
Net Plant	1,753,794	1,581,098
Total Assets	2,273,595	2,052,517
Current Liabilities
Short-Term Debt	6,000	18,599
Current Maturities of Long-Term Debt	183	172
Accounts Payable	120,775	96,291
Accrued Salaries and Wages	22,730	24,857
Accrued Taxes	17,525	17,287
Regulatory Liabilities	7,480	738
Current Operating Lease Liabilities	4,136	0
Other Accrued Liabilities	10,912	12,149
Total Current Liabilities	189,741	170,093
Pensions Benefit Liability	98,970	98,358
Other Postretirement Benefits Liability	71,437	71,561
Long-Term Operating Lease Liabilities	18,193	0
Other Noncurrent Liabilities	30,833	24,326
Commitments and Contingencies (note 9)
Deferred Credits
Deferred Income Taxes	131,941	120,976
Deferred Tax Credits	18,626	19,974
Regulatory Liabilities	239,906	226,469
Other	2,885	1,895
Total Deferred Credits	393,358	369,314
Capitalization
Total Otter Tail Corporation Long-Term Debt net of Unamortized Debt Issuance Costs	689,581	590,002
Common Shares, Par Value $5 Per Share–Authorized, 50,000,000 Shares; Outstanding, 2019—40,157,591 Shares; 2018—39,664,884 Shares	200,788	198,324
Premium on Common Shares	364,790	344,250
Retained Earnings	222,341	190,433
Accumulated Other Comprehensive Loss	(6,437)	(4,144)
Total Common Equity	781,482	728,863
Total Capitalization	1,471,063	1,318,865
Total Liabilities and Equity	2,273,595	2,052,517
Cumulative Preferred Shares [Member]
Capitalization
Cumulative Shares	0	0
Cumulative Preference Shares [Member]
Capitalization
Cumulative Shares	$ 0	$ 0

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Allowance for doubtful accounts	$ 1,339	$ 1,407
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)	40,157,591	39,664,884
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, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Revenues:
Total	$ 919,503	$ 916,447	$ 849,350
Operating Expenses
Production Fuel – Electric	59,256	66,815	59,690
Electric Operation and Maintenance Expenses	153,529	155,534	146,914
Cost of Products Sold (depreciation included below)	355,119	354,559	316,562
Other Nonelectric Expenses	50,782	51,544	41,492
Depreciation and Amortization	78,086	74,666	72,545
Property Taxes – Electric	15,785	15,585	15,053
Total Operating Expenses	784,623	787,058	717,063
Operating Income	134,880	129,389	132,287
Interest Charges	31,411	30,408	29,604
Nonservice Cost Components of Postretirement Benefits	4,293	5,509	5,620
Other Income	5,112	3,461	2,632
Income Before Income Taxes	104,288	96,933	99,695
Income Tax Expense	17,441	14,588	27,256
Net Income	$ 86,847	$ 82,345	$ 72,439
Average Number of Common Shares Outstanding–Basic (in shares)	39,720,847	39,599,944	39,457,261
Average Number of Common Shares Outstanding–Diluted (in shares)	39,953,826	39,892,196	39,748,347
Basic Earnings Per Common Share (in dollars per share)	$ 2.19	$ 2.08	$ 1.84
Diluted Earnings Per Common Share (in dollars per share)	$ 2.17	$ 2.06	$ 1.82
Electricity [Member]
Operating Revenues:
Revenues from Contracts with Customers	$ 458,016	$ 450,637	$ 436,477
Changes in Accrued Revenues under Alternative Revenue Programs	1,032	(439)	(1,971)
Total Electric	459,048	450,198	434,506
Product [Member]
Operating Revenues:
Revenues from Contracts with Customers	460,455	466,249	414,844
Electricity, Purchased [Member]
Operating Expenses
Purchased Power – Electric System Use	$ 72,066	$ 68,355	$ 64,807

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Income	$ 86,847	$ 82,345	$ 72,439
Unrealized Gain (Loss) on Available-for-Sale Securities:
Reversal of Previously Recognized Losses (Gains) Realized on Sale of Investments and Included in Other Income During Period	16	(105)	(15)
Unrealized Gains (Losses) Arising During Period	147	(61)	115
Income Tax (Expense) Benefit	(34)	35	(35)
Change in Unrealized Gain (Loss) on Available-for-Sale Securities – net-of-tax	129	(131)	65
Pension and Postretirement Benefit Plans:
Actuarial (Losses) Gains net of Regulatory Allocation Adjustment	(2,779)	1,919	(3,791)
Amortization of Unrecognized Postretirement Benefit Losses and Costs (note 11)	565	985	629
Income Tax Benefit (Expense)	576	(755)	1,266
Adjustment to Income Tax Expense Related to 2017 Tax Cuts and Jobs Act	0	(531)	0
Pension and Postretirement Benefit Plans – net-of-tax	(1,638)	1,618	(1,896)
Total Other Comprehensive (Loss) Income	(1,509)	1,487	(1,831)
Total Comprehensive Income	$ 85,338	$ 83,832	$ 70,608

Consolidated Statements of Comm

Consolidated Statements of Common Shareholders' Equity - USD ($) $ in Thousands	AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]	Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member]	AOCI Attributable to Parent [Member]		Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Total
Balance (in shares) at Dec. 31, 2016					39,348,136
Balance at Dec. 31, 2016			$ (3,800)	[1]	$ 196,741	$ 337,684	$ 139,479	$ 670,104
At-the-Market Offering (in shares)					257,059
Common Stock Issuances, Net of Expenses					$ 1,285	3,684		4,969
Common Stock Retirements and Forfeitures (in shares)					(47,704)
Common Stock Retirements and Forfeitures					$ (239)	(1,560)		(1,799)
Net Income							72,439	72,439
Balance			(1,831)					(1,831)
Employee Stock Incentive Plan Expense						3,642		3,642
Common Dividends							(50,632)	(50,632)
Balance (in shares) at Dec. 31, 2017					39,557,491
Balance at Dec. 31, 2017	$ 46	$ (5,677)	(5,631)	[1]	$ 197,787	343,450	161,286	696,892
Before Tax	71	(9,462)	(9,391)
Tax Effect	(15)	2,991	2,976
Stranded Tax Effect	(10)	794	784
At-the-Market Offering (in shares)					178,601
Common Stock Issuances, Net of Expenses					$ 893			(93)
Common Stock Issuances, Net of Expenses						(986)
Common Stock Retirements and Forfeitures (in shares)					(71,208)
Common Stock Retirements and Forfeitures					$ (356)	(2,655)		(3,011)
Net Income							82,345	82,345
Balance			1,487					1,487
Employee Stock Incentive Plan Expense						4,441		4,441
Common Dividends							(53,198)	(53,198)
Balance (in shares) at Dec. 31, 2018					39,664,884
Balance at Dec. 31, 2018	(85)	(4,059)	(4,144)	[1]	$ 198,324	344,250	190,433	728,863
Before Tax	(95)	(6,558)	(6,653)
Tax Effect	20	1,705	1,725
Stranded Tax Effect	(10)	794	784
At-the-Market Offering (in shares)					547,931
Common Stock Issuances, Net of Expenses					$ 2,740	17,036		19,776
Common Stock Retirements and Forfeitures (in shares)					(55,224)
Common Stock Retirements and Forfeitures					$ (276)	(2,454)		(2,730)
Net Income							86,847	86,847
Balance			(1,509)					(1,509)
Employee Stock Incentive Plan Expense						5,958		5,958
Common Dividends							(55,723)	(55,723)
ASU 2018-02 2017 TCJA Stranded Tax Transfer			(784)				784
Balance (in shares) at Dec. 31, 2019					40,157,591
Balance at Dec. 31, 2019	54	(6,491)	(6,437)	[1]	$ 200,788	$ 364,790	$ 222,341	$ 781,482
Before Tax	68	(8,772)	(8,704)
Tax Effect	(14)	2,281	2,267
Stranded Tax Effect	$ 0	$ 0	$ 0
[1]	Accumulated Other Comprehensive Loss on December 31 is comprised of the following: (in thousands) 2019 2018 2017 Unrealized Gain (Loss) on Marketable Equity Securities: Before Tax $ 68 $ (95 ) $ 71 Tax Effect (14 ) 20 (15 ) Stranded Tax Effect - (10 ) (10 ) Unrealized Gain (Loss) on Marketable Equity Securities net-of-tax 54 (85 ) 46 Unamortized Actuarial Losses and Prior Service Costs Related to Pension and Postretirement Benefits: Before Tax (8,772 ) (6,558 ) (9,462 ) Tax Effect 2,281 1,705 2,991 Stranded Tax Effect - 794 794 Unamortized Actuarial Losses and Prior Service Costs Related to Pension and Postretirement Benefits net-of-tax (6,491 ) (4,059 ) (5,677 ) Accumulated Other Comprehensive Loss: Before Tax (8,704 ) (6,653 ) (9,391 ) Tax Effect 2,267 1,725 2,976 Stranded Tax Effect - 784 784 Net Accumulated Other Comprehensive Loss $ (6,437 ) $ (4,144 ) $ (5,631 )

Consolidated Statements of Co_2

Consolidated Statements of Common Shareholders' Equity (Parentheticals) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retained Earnings [Member]
Dividends declared per common share (in dollars per share)	$ 1.40	$ 1.34	$ 1.28

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Flows from Operating Activities
Net Income	$ 86,847	$ 82,345	$ 72,439
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Depreciation and Amortization	78,086	74,666	72,545
Deferred Tax Credits	(1,348)	(1,405)	(1,470)
Deferred Income Taxes	11,507	19,224	24,001
Change in Deferred Debits and Other Assets	(15,502)	941	(2,173)
Discretionary Contribution to Pension Plan	(22,500)	(20,000)	0
Change in Noncurrent Liabilities and Deferred Credits	33,534	(2,414)	19,257
Allowance for Equity/Other Funds Used During Construction	(2,553)	(2,194)	(986)
Stock Compensation Expense – Equity Awards	5,958	4,441	3,642
Other—Net	76	0	10
Cash (Used for) Provided by Current Assets and Current Liabilities:
Change in Receivables	(1,860)	(8,559)	(2,135)
Change in Inventories	8,419	(18,236)	(4,294)
Change in Other Current Assets	2,919	(754)	(3,060)
Change in Payables and Other Current Liabilities	(171)	14,997	(3,013)
Change in Interest Payable and Income Taxes Receivable	1,625	396	(1,186)
Net Cash Provided by Operating Activities	185,037	143,448	173,577
Cash Flows from Investing Activities
Capital Expenditures	(207,365)	(105,425)	(132,913)
Proceeds from Disposal of Noncurrent Assets	8,519	2,378	4,491
Cash Used for Investments and Other Assets	(10,626)	(4,372)	(4,168)
Net Cash Used in Investing Activities	(209,472)	(107,419)	(132,590)
Cash Flows from Financing Activities
Change in Checks Written in Excess of Cash	(2,814)	(345)	2,434
Net Short-Term (Repayments) Borrowings	(12,599)	(93,772)	69,488
Proceeds from Issuance of Common Stock	20,338	0	4,349
Common Stock Issuance Expenses	(577)	(108)	0
Payments for Retirement of Capital Stock	(2,730)	(3,011)	(1,799)
Proceeds from Issuance of Long-Term Debt	100,000	100,000	0
Short-Term and Long-Term Debt Issuance Expenses	(950)	(761)	(380)
Payments for Retirement of Long-Term Debt	(172)	(189)	(48,231)
Dividends Paid	(55,723)	(53,198)	(50,632)
Net Cash Provided by (Used in) Financing Activities	44,773	(51,384)	(24,771)
Net Change in Cash and Cash Equivalents	20,338	(15,355)	16,216
Cash and Cash Equivalents at Beginning of Period	861	16,216	0
Cash and Cash Equivalents at End of Period	$ 21,199	$ 861	$ 16,216

Consolidated Statements of Capi

Consolidated Statements of Capitalization - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018
Short-term Debt		$ 6,000	$ 18,599
Long-term Debt		692,351	592,523
Less: Current Maturities--net of Unamortized Debt Issuance Costs		183	172
Unamortized Long-Term Debt Issuance Costs		2,587	2,349
Total Otter Tail Corporation Long-Term Debt net of Unamortized Debt Issuance Costs		689,581	590,002
Total Common Shareholders’ Equity		781,482	728,863
Total Capitalization		1,471,063	1,318,865
Parent Company [Member]
Short-term Debt		6,000	9,215
Long-term Debt		80,351	80,523
Less: Current Maturities--net of Unamortized Debt Issuance Costs		183	172
Unamortized Long-Term Debt Issuance Costs		356	407
Total Otter Tail Corporation Long-Term Debt net of Unamortized Debt Issuance Costs		79,812	79,944
Total Common Shareholders’ Equity		781,482	728,863
Total Capitalization		861,294	808,807
Otter Tail Power Company [Member]
Short-term Debt			9,384
Long-term Debt		612,000	512,000
Unamortized Long-Term Debt Issuance Costs		2,231	1,942
Total Otter Tail Corporation Long-Term Debt net of Unamortized Debt Issuance Costs		609,769	510,058
Otter Tail Corporation Credit Agreement [Member] | Parent Company [Member]
Short-term Debt		6,000	9,215
Otter Tail Power Company Credit Agreement [Member] | Parent Company [Member]
Short-term Debt		0	9,384
The 3.55% Guaranteed Senior Notes, due December 15, 2026 [Member] | Parent Company [Member]
Long-term Debt		80,000	80,000
Partnership in Assisting Community Expansion Note, 2.54%, due March 18, 2021 [Member] | Parent Company [Member]
Long-term Debt		351	523
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 3.07%, Series 2019A, Due October 10, 2029 [Member] | Otter Tail Power Company [Member]
Long-term Debt	[1]	10,000	0
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 3.52%, Series 2019B, Due October 10, 2039 [Member]
Long-term Debt		26,000
Senior Unsecured Notes 3.52%, Series 2019B, Due October 10, 2039 [Member] | Otter Tail Power Company [Member]
Long-term Debt		26,000	0
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	100,000
Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member] | Otter Tail Power Company [Member]
Long-term Debt		100,000	100,000
Senior Unsecured Notes 3.82%, Sericies C, Due October 10, 2049 [Member]
Long-term Debt		64,000
Senior Unsecured Notes 3.82%, Sericies C, Due October 10, 2049 [Member] | Otter Tail Power Company [Member]
Long-term Debt		$ 64,000	$ 0
[1]	Holder is COBANK, a cooperative lender. Interest payments are subject to cash credits which may result in a lower effective interest rate.

Consolidated Statements of Ca_2

Consolidated Statements of Capitalization (Parentheticals)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
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
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 3.07%, Series 2019A, Due October 10, 2029 [Member] | Otter Tail Power Company [Member]
Long-term Debt, Interest Rate	3.07%
Long-term Debt, Due Date	Oct. 10, 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 3.52%, Series 2019B, Due October 10, 2039 [Member]
Long-term Debt, Interest Rate	3.52%
Long-term Debt, Due Date	Oct. 10, 2039
Senior Unsecured Notes 3.52%, Series 2019B, Due October 10, 2039 [Member] | Otter Tail Power Company [Member]
Long-term Debt, Interest Rate	3.52%
Long-term Debt, Due Date	Oct. 10, 2039
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%	4.07%
Long-term Debt, Due Date	Feb. 7, 2048	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%	4.07%
Long-term Debt, Due Date	Feb. 7, 2048	Feb. 7, 2048
Senior Unsecured Notes 3.82%, Sericies C, Due October 10, 2049 [Member]
Long-term Debt, Interest Rate	3.82%
Long-term Debt, Due Date	Oct. 10, 2049
Senior Unsecured Notes 3.82%, Sericies C, Due October 10, 2049 [Member] | Otter Tail Power Company [Member]
Long-term Debt, Interest Rate	3.82%
Long-term Debt, Due Date	Oct. 10, 2049

Note 1 - Summary of Significant

Note 1 - Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Notes to Financial Statements
Significant Accounting Policies and New Accounting Pronouncements [Text Block]	1. 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 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 $1,728,000 in 2019, 2018 2017. 2019, 2018 2017. 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 to 40 years). The cost of additions includes contracted work, direct labor and materials, allocable overheads and capitalized interest. No interest was capitalized on nonelectric plant in 2019, 2018 2017. 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 five December 31, 2019 2018 Jointly Owned Facilities (dollars in thousands) OTP Ownership Percentage Electric Plant in Service Construction Work in Progress Accumulated Depreciation Net Plant December 31, 2019 Big Stone Plant 53.9 % $ 337,197 $ 384 $ (98,654 ) $ 238,927 Coyote Station 35.0 % 184,493 83 (108,248 ) 76,328 Big Stone South–Ellendale 345 kV line 1 50.0 % 106,343 - (819 ) 105,524 Fargo–Monticello 345 kV line 14.2 % 78,184 - (7,011 ) 71,173 Big Stone South–Brookings 345 kV line 50.0 % 53,036 - (2,016 ) 51,020 Brookings–Southeast Twin Cities 345 kV line 4.8 % 26,286 - (2,086 ) 24,200 Bemidji–Grand Rapids 230 kV line 14.8 % 16,331 - (233 ) 16,098 December 31, 2018 Big Stone Plant 53.9 % $ 336,051 $ 361 $ (92,007 ) $ 244,405 Coyote Station 35.0 % 177,713 2,588 (100,997 ) 79,304 Big Stone South–Ellendale 345 kV line 1 50.0 % - 106,490 - 106,490 Fargo–Monticello 345 kV line 14.2 % 78,184 - (5,891 ) 72,293 Big Stone South–Brookings 345 kV line 50.0 % 53,235 (150 ) (1,264 ) 51,821 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 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 In 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, 2019. 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 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. Provisions for sales returns, early payment terms discounts, volume-based variable pricing incentives and warranty costs are recorded as reductions to revenue at the time revenue is recognized based on customer history, historical information and current trends. 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. A 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), Energy Intensive Trade Exposed and Conservation Improvement Program riders. ● In North Dakota: TCR, ECR, RRA and Generation Cost Recovery (GCR) riders. ● In South Dakota: TCR, ECR, Phase-in Rate Plan 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 changes in accrued revenues under ARPs on a separate line in the revenue section of the Company’s consolidated statement of income. See table in note 3 December 31, 2019, 2018 2017. Manufacturing Segment Revenues no Plastics Segment Revenues no one See operating revenue table in note 2 December 31, 2019, 2018 2017. 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. Use 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) 2019 2018 Cost Method: Economic Development Loan Pools $ 24 $ 34 Other 73 123 Equity Method Partnerships 27 26 Marketable Debt Securities Classified as Available-for-Sale 8,184 7,484 Marketable Equity Securities Classified as Available-for-Sale 1,586 1,294 Total Investments $ 9,894 $ 8,961 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, 2019. Inventories Inventories, valued at the lower of cost or net realizable value, consist of the following: December 31, December 31, (in thousands) 2019 2018 Finished Goods $ 31,863 $ 37,130 Work in Process 16,508 20,393 Raw Material, Fuel and Supplies 49,480 48,747 Total Inventories $ 97,851 $ 106,270 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, 2019 December 31, 2018: December 31, 2019 (in thousands) Level 1 Level 2 Level 3 Assets: Investments: Equity Funds – Held by Captive Insurance Company $ 1,586 Corporate Debt Securities – Held by Captive Insurance Company $ 2,124 Government-Backed and Government-Sponsored Enterprises’ Debt Securities – Held by Captive Insurance Company 6,060 Other Assets: Money Market and Mutual Funds – Retirement Plans 2,363 Total Assets $ 3,949 $ 8,184 December 31, 2018 (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 The level 2 third may 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 2019 2018: (in thousands) Gross Balance December 31, 2018 Accumulated Impairments Balance (net of impairments) December 31, 2018 Adjustments to Goodwill in 2019 Balance (net of impairments) December 31, 2019 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, 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 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, 2019 December 31, 2018: December 31, 2019 (in thousands) Gross Carrying Amount Accumulated Amortization Net Carrying Amount Remaining Amortization Periods (months) Amortizable Intangible Assets: Customer Relationships $ 22,491 $ 11,259 $ 11,232 88 - 188 Other 179 121 58 8 - 45 Total $ 22,670 $ 11,380 $ 11,290 December 31, 2018 (in thousands) 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 The amortization expense for these intangible assets was: (in thousands) 2019 2018 2017 Amortization Expense – Intangible Assets $ 1,186 $ 1,315 $ 1,347 The estimated annual amortization expense for these intangible assets for the next five (in thousands) 2020 2021 2022 2023 2024 Estimated Amortization Expense – Intangible Assets $ 1,140 $ 1,105 $ 1,105 $ 1,104 $ 1,099 Supplemental Disclosures of Cash Flow Information As of December 31, (in thousands) 2019 2018 Noncash Investing Activities: Transactions Related to Capital Additions not Settled in Cash $ 37,429 $ 13,757 (in thousands) 2019 2018 2017 Cash Paid During the Year for: Interest (net of amount capitalized) $ 30,132 $ 28,109 $ 29,791 Income Taxes $ 4,797 $ 6,109 $ 5,064 New Accounting Standards Adopted ASU 2016 02 February 2016 No. 2016 02, Leases (Topic 842 2016 02 2016 02 842, 840 2016 02 December 15, 2018, 842 2016 02 January 1, 2019. 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 first 2019. no no ASU 2018 02 February 2018 No. 2018 02, Income Statement—Reporting Comprehensive Income (Topic 220 2018 02 2018 02, 2017 2018 02 December 15, 2018, 2018 02 The Company adopted the updates in ASU 2018 02 January 1, 2019, not Support for the determination of the stranded tax effects resulting from the enactment of the TCJA in AOCI/(L) is provided in the table below. (in thousands) Unrealized Gains on Available-for- Sale Securities Unamortized Actuarial Losses and Prior Service Costs on Pension and Other Postretirement Benefits AOCI/(L) Balance on December 22, 2017 – Pre-tax $ 71 $ (5,672 ) $ (5,601 ) Effect of TCJA 14% Federal Tax Rate Reduction on Gross Deferred Tax Amounts $ 10 $ (794 ) $ (784 ) New Accounting Standards Pending Adoption ASU 2016 13 June 2016 No. 2016 13, Financial Instruments—Credit Losses (Topic 326 326 , may 326 December 15, 2019. not 326 first 2020. not not ASU 2018 15 August 2018 No. 2018 15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350 40 350 40, Internal-Use Software 2018 15 2018 15 350 40 2018 15 2018 15 December 15, 2019 2018 15 first 2020 no

Note 2 - Business Segment Infor

Note 2 - Business Segment Information	12 Months Ended
	Dec. 31, 2019
Notes to Financial Statements
Segment Reporting Disclosure [Text Block]	2. The accounting policies of the segments are described under note 1 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 single customer accounted for over 10% 2019, 2018 2017. no 10% 2019, 2019 2019 2019 2019 2019 2019 one All the Company’s long-lived assets are within the United States and sales within the United States accounted for 98.8% of sales in 2019, 2018 2017. 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 2019, 2018 2017 (in thousands) 2019 2018 2017 Operating Revenue Electric Segment: Retail Sales Revenue from Contracts with Customers $ 405,446 $ 388,690 $ 376,902 Changes in Accrued ARP Revenues 1,032 (439 ) (1,971 ) Total Retail Sales Revenue 406,478 388,251 374,931 Transmission Services Revenue 40,542 46,947 46,664 Wholesale Revenues – Company Generation 5,007 7,735 5,173 Other Electric Revenues 7,070 7,322 7,769 Total Electric Segment Revenues 459,097 450,255 434,537 Manufacturing Segment: Metal Parts and Tooling 236,032 223,765 189,242 Plastic Products and Tooling 35,173 35,836 33,939 Other 5,999 8,808 6,557 Total Manufacturing Segment Revenues 277,204 268,409 229,738 Plastics Segment – Sale of PVC Pipe Products 183,257 197,840 185,132 Intersegment Eliminations (55 ) (57 ) (57 ) Total $ 919,503 $ 916,447 $ 849,350 Cost of Products Sold Manufacturing $ 215,179 $ 205,699 $ 176,473 Plastics 139,974 148,881 140,107 Intersegment Eliminations (34 ) (21 ) (18 ) Total $ 355,119 $ 354,559 $ 316,562 Other Nonelectric Expenses Manufacturing $ 29,895 $ 29,650 $ 23,785 Plastics 11,393 12,323 11,564 Corporate 9,515 9,607 6,182 Intersegment Eliminations (21 ) (36 ) (39 ) Total $ 50,782 $ 51,544 $ 41,492 (in thousands) 2019 2018 2017 Depreciation and Amortization Electric $ 60,044 $ 55,935 $ 53,276 Manufacturing 14,261 14,794 15,379 Plastics 3,451 3,719 3,817 Corporate 330 218 73 Total $ 78,086 $ 74,666 $ 72,545 Operating Income (Loss) Electric $ 98,417 $ 88,031 $ 94,797 Manufacturing 17,869 18,266 14,101 Plastics 28,439 32,917 29,644 Corporate (9,845 ) (9,825 ) (6,255 ) Total $ 134,880 $ 129,389 $ 132,287 Interest Charges Electric $ 26,548 $ 26,365 $ 25,334 Manufacturing 2,345 2,230 2,215 Plastics 718 609 633 Corporate and Intersegment Eliminations 1,800 1,204 1,422 Total $ 31,411 $ 30,408 $ 29,604 Income Tax Expense (Benefit) Electric $ 12,867 $ 5,685 $ 17,013 Manufacturing 2,784 3,393 989 Plastics 7,309 8,728 7,448 Corporate (5,519 ) (3,218 ) 1,806 Total $ 17,441 $ 14,588 $ 27,256 Net Income (Loss) Electric $ 59,046 $ 54,431 $ 49,446 Manufacturing 12,899 12,839 11,050 Plastics 20,572 23,819 21,696 Corporate (5,670 ) (8,744 ) (9,753 ) Total $ 86,847 $ 82,345 $ 72,439 Capital Expenditures Electric $ 187,362 $ 87,287 $ 118,444 Manufacturing 14,268 13,316 9,916 Plastics 5,452 4,199 4,432 Corporate 283 623 121 Total $ 207,365 $ 105,425 $ 132,913 Identifiable Assets Electric $ 1,931,525 $ 1,728,534 $ 1,690,224 Manufacturing 195,742 187,556 167,023 Plastics 92,049 91,630 87,230 Corporate 54,279 44,797 59,801 Total $ 2,273,595 $ 2,052,517 $ 2,004,278

Note 3 - Rate and Regulatory Ma

Note 3 - Rate and Regulatory Matters	12 Months Ended
	Dec. 31, 2019
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 general rate proceedings and descriptions of rate riders and a summary of rate 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 2019, 2018 2017. Major Capital Expenditure Projects Merricourt Wind Energy Center (Merricourt) November 16, 2016 150 November 16, 2016, 2020 October 26, 2017 January 10, 2018. November 3, 2017. March 6, 2019 April 2019. In connection with action by the FERC, OTP and EDF-US agreed, in the First Amendment to the Purchase Agreement and the TEPC Agreement dated June 11, 2019, July 16, 2019 August 2019. December 31, 2019, October 2020. Astoria Station 2021. November 3, 2017, August 3, 2018 September 26, 2018 March 6, 2019 December 2018 January 2019. May 2019. December 31, 2019, 2020 2021. Big Stone South–Ellendale Multi-Value Transmission Project (MVP) February 6, 2019, 162 December 2011. General Rates Minnesota 2016 March 2017 May 1, 2017. 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 million was applied as a credit to Minnesota customers’ electric bills beginning November 17, 2017. 1 2 9.41% 2016 April 16, 2016, November 2017 North Dakota November 2, 2017 $13.1 December 20, 2017 January 1, 2018. February 27, 2018 March 1, 2018. On March 23, 2018 In a September 26, 2018 March 2018 $7.1 4.8% 10.3%. not February 1, 2019, April 2019 January February 2018. South Dakota April 20, 2018 first two October 18, 2018. second The SDPUC approved a partial settlement on March 1, 2019 second January 1, 2018 October 17, 2018 first 2019. May 14, 2019. May 30, 2019 June 28, 2019 August 1, 2019. October 18, 2018 October 2019 On July 9, 2019 2018 May 30, 2019 first October 18, 2018, October 1, 2019. To ensure rates are appropriately set under the stipulation, the parties agreed to establish an earnings sharing mechanism to share with customers any weather-normalized earnings above the authorized ROE of 8.75%. OTP's annual weather-normalized earnings are reported each year by June 1 9.50% 30 Rate Riders OTP has several rate riders in place in each of its state jurisdictional service areas. These rate riders are designed to recover expenses, costs and returns on rate base investments not 2019, 2018 2017 2019, 2018 2017. Minnesota Minnesota Conservation Improvement Programs (MNCIP) not May 25, 2016 2017 2018 2019 2017 2018 2019 May 20, 2019 2017 2019 one 2020, 30% 10%   Based on results from MNCIP 2019, 2018 2017 2019, 2018 2017, 2017 2019. Transmission Cost Recovery Rider may In OTP’s 2016 May 1, 2017, two August 18, 2017 On June 11, 2018 July 11, 2018 second 2020. On November 30, 2018 two two April 1, 2019, not December 31, 2019, Environmental Cost Recovery Rider 2010 2016 November 2017. 2018 zero December 1, 2018. Renewable Resource Adjustment November 1, 2017, 2017 2018. June 21, 2019 December 19, 2019 Fuel and Purchased Power Costs Recovery December 2017 October 31, 2019 2020 January 1, 2020. North Dakota Renewable Resource Adjustment Effective in February 2019 2017 On December 31, 2019 November 30, 2019. Transmission Cost Recovery Rider 2017 26 26A 2017 On December 18, 2019 seven Environmental Cost Recovery Rider 2017 February 1, 2019, February 1, 2019 December 31, 2019. Generation Cost Recovery Rider May 15, 2019 July 1, 2019. South Dakota Transmission Cost Recovery Rider January 29, 2018 October 18, 2018, 2018 OTP made a supplemental filing for the South Dakota TCR rider on February 1, 2019. February 15, 2019 March 1, 2019. January 1, 2019 January 1, 2020. On September 17, 2019 2018 October 1, 2019. Environmental Cost Recovery Rider October 18, 2018 October 2019 Phase-In Rate Plan Rider May 31, 2019 On August 21, 2019 September 1, 2019. Rate Rider Updates The following table provides summary information on the status of updates since January 1, 2016 Rate Rider R - Request Date A - Approval Date Effective Date Requested or Approved Annual Revenue ($000s) Rate Minnesota Conservation Improvement Program 2018 Incentive and Cost Recovery A – December 27, 2019 January 1, 2020 $ 11,926 $0.00710 /kwh 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 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 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 Renewable Resource Adjustment 2019 Annual Update – Revised A – December 19, 2019 January 1, 2020 $ 12,506 $0.00467 /kwh 2018 Annual Update A – August 29, 2018 November 1, 2018 $ 5,886 $0.00219 /kwh 2017 Rate Reset A – October 30, 2017 November 1, 2017 $ 1,279 $0.00049 /kwh North Dakota Renewable Resource Adjustment 2020 Annual Update R – December 31, 2019 April 1, 2020 $ 3,828 3.744% of base 2019 Annual Update A – May 1, 2019 June 1, 2019 $ (235 ) -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 Transmission Cost Recovery 2019 Annual Update A – December 18, 2019 January 1, 2020 $ 5,739 Various 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 Environmental Cost Recovery 2019 Update A – October 22, 2019 November 1, 2019 $ - 0% of base 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 Generation Cost Recovery 2019 Initial Request A – May 15, 2019 July 1, 2019 $ 2,720 2.547% of base South Dakota Transmission Cost Recovery 2020 Annual Update R – October 31, 2019 March 1, 2020 $ 2,407 Various 2019 Rate Reset A – September 17, 2019 October 1, 2019 $ 2,046 Various 2019 Annual Update A – February 20, 2019 March 1, 2019 $ 1,638 Various 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 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 Phase-In Rate Plan Recovery 2019 Initial Request A – August 21, 2019 September 1, 2019 $ 864 3.345% of base 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) 2019 2018 2017 Minnesota Conservation Improvement Program Costs and Incentives $ 8,271 $ 8,127 $ 6,008 Renewable Resource Adjustment 5,513 3,067 (196 ) Transmission Cost Recovery 2,497 (2,039 ) 2,973 Environmental Cost Recovery (1 ) (24 ) 8,148 North Dakota Transmission Cost Recovery 5,292 7,016 8,729 Generation Cost Recovery 878 - - Environmental Cost Recovery 550 7,318 9,782 Renewable Resource Adjustment 230 8,529 7,620 South Dakota Transmission Cost Recovery 2,165 1,664 1,843 Conservation Improvement Program Costs and Incentives 851 628 598 Environmental Cost Recovery (29 ) 1,676 2,345 Phase-In Rate Plan (125 ) - - Total $ 26,092 $ 35,962 $ 47,850 TCJA The TCJA, passed in December 2017, 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 June 1, 2019. one January 2018 May 2019 August September 2019. As described above, OTP’s recent general rate cases in North Dakota and South Dakota reflected the impact of the TCJA in interim rates. OTP accrued refund liabilities for the time periods during which revenues were collected under rates set to recover higher levels of federal income taxes than OTP incurred under the lower federal tax rates in the TCJA. FERC Wholesale power sales and transmission rates are subject to the jurisdiction of the FERC under the Federal Power Act of 1935 MVPs ROE November 12, 2013 may 15 November 12, 2013 February 11, 2015. December 22, 2015 September 28, 2016 September 2016 On November 6, 2014 January 5, 2015 November 12, 2013 0.5% September 28, 2016. On February 12, 2015 may 12.38% second second 15 February 12, 2015 May 11, 2016. June 18, 2015 February 16, 2016. June 30, 2016 Based on the probable reduction by the FERC in the ROE component of the MISO Tariff, OTP had a $2.7 million liability on its balance sheet as of December 31, 2016, first 15 February June 2017 2016 December 31, 2016 September 30, 2019. On March 1, 2019 two On November 21, 2019 two 206 9.88%. two first 12.38% September 28, 2016, second not 9.88% first As a result of the FERC granting rehearing on the first December 31, 2019. $3.0 ● an additional $0.2 million refund related to the first 10.32%, September 28, 2016 9.88% ● a $1.3 million refund for the period from September 28, 2016 December 31, 2019 9.88% first January 5, 2015, ● a $1.5 million refund related to the second second In response to the FERC’s November 21, 2019 November 21, 2019

Note 4 - Regulatory Assets and

Note 4 - Regulatory Assets and Liabilities	12 Months Ended
	Dec. 31, 2019
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, 2019 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 $ 129,102 $ 138,192 see below Accumulated Asset Retirement Obligation (ARO) Accretion/Depreciation Adjustment 1 - 7,772 7,772 asset lives Conservation Improvement Program Costs and Incentives 2 4,024 2,844 6,868 21 Minnesota Transmission Cost Recovery Rider Accrued Revenues 2 4,208 - 4,208 12 MISO Schedule 26/26A Transmission Cost Recovery Rider True-ups 1 2,033 968 3,001 24 Nonservice Costs Components of Postretirement Benefits Capitalized for Ratemaking Purposes and Subject to Deferred Recovery 1 - 1,681 1,681 asset lives Big Stone II Unrecovered Project Costs – Minnesota 1 715 225 940 16 Debt Reacquisition Premiums 1 201 548 749 153 Deferred Marked-to-Market Losses 1 743 - 743 12 Big Stone II Unrecovered Project Costs – South Dakota 1 144 253 397 33 South Dakota Deferred Rate Case Expenses Subject to Recovery 1 138 245 383 34 North Dakota Deferred Rate Case Expenses Subject to Recovery 1 122 244 366 36 Minnesota SPP Transmission Cost Recovery Tracker 1 - 202 202 see below Minnesota Renewable Resource Rider Accrued Revenues 2 131 - 131 12 South Dakota Transmission Cost Recovery Rider Accrued Revenues 2 97 - 97 2 Deferred Lease Expenses 1 - 54 54 39 Minnesota Environmental Cost Recovery Rider Accrued Revenues 2 4 - 4 12 Total Regulatory Assets $ 21,650 $ 144,138 $ 165,788 Regulatory Liabilities: Deferred Income Taxes $ - $ 141,707 $ 141,707 asset lives Accumulated Reserve for Estimated Removal Costs – Net of Salvage - 97,726 97,726 asset lives Refundable Fuel Clause Adjustment Revenues 3,982 - 3,982 12 North Dakota Renewable Resource Recovery Rider Accrued Refund 1,515 - 1,515 12 North Dakota Transmission Cost Recovery Rider Accrued Refund 700 - 700 12 Prior Service Costs and Actuarial Gains on Postretirement Benefits 471 - 471 12 Revenue for Rate Case Expenses Subject to Refund – Minnesota - 401 401 see below South Dakota Phase-In Rate Plan Rider Accrued Refund 355 - 355 9 North Dakota Generation Cost Recovery Rider Accrued Refund 287 - 287 6 Minnesota Energy Intensive Trade Exposed Rider Accrued Refund 164 - 164 12 Other 6 72 78 168 Total Regulatory Liabilities $ 7,480 $ 239,906 $ 247,386 Net Regulatory Asset/(Liability) Position $ 14,170 $ (95,768 ) $ (81,598 ) 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, 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 Accumulated ARO Accretion/Depreciation Adjustment 1 - 7,169 7,169 asset lives Conservation Improvement Program Costs and Incentives 2 5,995 3,285 9,280 21 Minnesota Transmission Cost Recovery Rider Accrued Revenues 2 444 - 444 12 MISO Schedule 26/26A Transmission Cost Recovery Rider True-up 1 240 - 240 12 Nonservice Costs Components of Postretirement Benefits Capitalized for Ratemaking Purposes and Subject to Deferred Recovery 1 - 986 986 asset lives Big Stone II Unrecovered Project Costs – Minnesota 1 681 947 1,628 28 Debt Reacquisition Premiums 1 207 753 960 165 Deferred Marked-to-Market Losses 1 1,661 743 2,404 24 Big Stone II Unrecovered Project Costs – South Dakota 1 100 342 442 53 South Dakota Deferred Rate Case Expenses Subject to Recovery 1 178 - 178 12 North Dakota Deferred Rate Case Expenses Subject to Recovery 1 455 - 455 12 Minnesota SPP Transmission Cost Recovery Tracker 1 - 176 176 see below Minnesota Renewable Resource Recovery Rider Accrued Revenues 2 452 - 452 12 Minnesota Environmental Cost Recovery Rider Accrued Revenues 2 121 - 121 12 Deferred Income Taxes 1 - 2,423 2,423 asset lives Minnesota Energy Intensive Trade Exposed Rider Accrued Revenues 1 328 - 328 4 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 Refundable Fuel Clause Adjustment Revenues 121 - 121 12 North Dakota Renewable Resource Recovery Rider Accrued Refund 177 - 177 12 North Dakota Transmission Cost Recovery Rider Accrued Refund 60 - 60 12 Revenue for Rate Case Expenses Subject to Refund – Minnesota - 166 166 see below MISO Schedule 26/26A Transmission Cost Recovery Rider True-up - 187 187 24 South Dakota Transmission Cost Recovery Rider Accrued Refund 168 - 168 12 South Dakota Environmental Cost Recovery Rider Accrued Refund 207 - 207 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. The regulatory asset and liability related to prior service costs and actuarial losses on pensions and other postretirement benefits represents benefit costs and actuarial losses and gains subject to recovery or refund through rates as they are expensed. These unrecognized benefit costs and actuarial losses and gains are required to be recognized as components of Accumulated Other Comprehensive Income in equity under ASC Topic 715, Compensation—Retirement Benefits The Accumulated ARO Accretion/Depreciation Adjustment will accrete and be amortized over the lives of property with asset retirement obligations. Conservation Improvement Program Costs and Incentives represent mandated conservation expenditures and incentives recoverable through retail electric rates. 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 were recoverable from Minnesota customers as of the balance sheet date. MISO Schedule 26/26A 26/26A 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. 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. Debt Reacquisition Premiums are being recovered from OTP customers over the remaining original lives of the reacquired debt issues, the longest of which is 153 All Deferred Marked-to-Market Losses recorded as of the balance sheet date relate to forward purchases of energy scheduled for delivery through December 2020. 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. South Dakota Deferred Rate Case Expenses Subject to Recovery relate to costs incurred in conjunction with OTP’s most recent rate case in South Dakota and are currently being recovered beginning with the establishment of interim rates in October 2018. North Dakota Deferred Rate Case Expenses Subject to Recovery relate to costs incurred in conjunction with OTP’s most recent rate case in North Dakota currently being recovered beginning with the establishment of interim rates in January 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 had not The Minnesota Renewable Resource Recovery Rider Accrued Revenues relate to revenues earned on qualifying renewable resource costs incurred to serve Minnesota customers that were recoverable from Minnesota customers as of the balance sheet date. The South Dakota Transmission Cost Recovery Rider Accrued Revenues relate to revenues earned on qualifying transmission system facilities and operating costs incurred to serve South Dakota customers that were recoverable from South Dakota customers as of the balance sheet date. Deferred Lease Expenses: Under ASC 842 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 were recoverable from Minnesota customers as of the balance sheet date. 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 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 were subject to recovery from other Minnesota customers as of the balance sheet date. 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 were recoverable from North Dakota customers as of the balance sheet date. The Accumulated Reserve for Estimated Removal Costs – Net of Salvage is reduced as actual removal costs, net of salvage revenues, are incurred. The North Dakota Renewable Resource Recovery Rider Accrued Refund relates to amounts collected for qualifying renewable resource costs incurred to serve North Dakota customers that were refundable to North Dakota customers as of the balance sheet date. 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 were refundable to North Dakota customers as of the balance sheet date. 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. The South Dakota Phase-In Rate Plan Rider Accrued Refund relates to amounts collected for actual and forecasted costs for Astoria Station, Merricourt, and additional load growth that were refundable to South Dakota customers as of the balance sheet date. The North Dakota Generation Cost Recovery Rider Accrued Refund relates to revenues collected under the rider in excess of returns allowed on recoverable costs incurred for the North Dakota share of OTP’s investment in Astoria Station, a natural gas-fired combustion turbine generation facility under construction near Astoria, South Dakota. The balance represents amounts subject to refund to North Dakota customers that had been billed to North Dakota customers as of the balance sheet date. The Minnesota Energy Intensive Trade Exposed Rider Accrued Refund relates to over-collected amounts from Minnesota retail customers for fuel and purchased power costs reductions provided to customers in energy intensive trade exposed industries that were subject to refund to Minnesota customers as of the balance sheet date. 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 were refundable to South Dakota customers as of the balance sheet date. 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 were refundable to South Dakota customers as of the balance sheet date. 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, 2019
Notes to Financial Statements
Stockholders Equity and Earnings per Share [Text Block]	5. Shelf Registration and Common Share Distribution Agreement On May 3, 2018 may May 3, 2021. On November 8, 2019, may Under the Distribution Agreement, the Company will designate the minimum price and maximum number of common shares to be sold through KeyBanc on any given trading day or over a specified period of trading days, and KeyBanc will use commercially reasonable efforts to sell such shares on such days, subject to certain conditions. Sales of the shares, if any, will be made by means of ordinary brokers’ transactions on the Nasdaq Global Select Market at market prices or as otherwise agreed with KeyBanc. The Company may not not 2019 Following is a reconciliation of the Company’s common shares outstanding from December 31, 2018 December 31, 2019: Common Shares Outstanding, December 31, 2018 39,664,884 Issuances: At-the-Market Offering 347,000 Executive Stock Performance Awards (2016 awards earned) 102,198 Automatic Dividend Reinvestment and Share Purchase Plan: Dividends Reinvested 29,599 Cash Invested 23,740 Vesting of Restricted Stock Units 29,100 Restricted Stock Issued to Directors 15,700 Directors Deferred Compensation 594 Retirements: Shares Withheld for Individual Income Tax Requirements (55,224 ) Common Shares Outstanding, December 31, 2019 40,157,591 2014 The 2014 2014 April 2014, 2014 December 31, 2019. 2014 December 13, 2023. Employee Stock Purchase Plan The 1999 six January 1, 2017 June 30, 2019, six June 30, 2019, six December 31, 2019. January 2020 December 31, 2019, 2019, 2018, 2017. Dividend Reinvestment and Share Purchase Plan On May 3, 2018, May 3, 2021. October 2019, 2019, 2018, December 31, 2019. 2018 2017. 2017. Earnings Per Share The numerator used in the calculation of both basic and diluted earnings per common share is net income with no 2019, 2018 2017. not 2019 2018 2017 Weighted Average Common Shares Outstanding – Basic 39,720,847 39,599,944 39,457,261 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 147,001 212,043 210,784 Underlying Shares Related to Nonvested Restricted Stock Units Granted to Employees 65,421 59,980 56,952 Nonvested Restricted Shares 15,377 17,751 20,380 Shares Expected to be Issued Under the Employee Stock Purchase Plan 3,228 - - Shares Expected to be Issued Under the Deferred Compensation Program for Directors 1,952 2,478 2,970 Total Dilutive Shares 232,979 292,252 291,086 Weighted Average Common Shares Outstanding – Diluted 39,953,826 39,892,196 39,748,347 The effect of dilutive shares on earnings per share for the years ended December 31, 2019, 2018 2017, no $0.016

Note 6 - Share-based Payments

Note 6 - Share-based Payments	12 Months Ended
	Dec. 31, 2019
Notes to Financial Statements
Share-based Payment Arrangement [Text Block]	6. Purchase Plan The Purchase Plan allows eligible employees to purchase the Company’s common shares through payroll withholding at a discount of up to 15% off the market price at the end of each six January 1, 2017 June 30, 2019, six June 30, 2019, six 718, Compensation—Stock Compensation 718 , 15% 15% six December 31, 2019. Restricted Stock Granted to Directors Under the 1999 2014 1999 April 8, 2017. 718 April 8, 2019, April 8, 2019 2019 April 8 2020 2022 not Presented below is a summary of the status of directors’ restricted stock awards for the years ended December 31: Directors’ Restricted Stock Awards 2019 2018 2017 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 43,225 $ 37.53 46,800 $ 32.65 46,334 $ 29.71 Granted 15,700 49.73 18,200 43.40 17,600 37.75 Vested 18,320 36.02 21,775 31.94 17,134 29.93 Forfeited - - - Nonvested, End of Year 40,605 42.93 43,225 37.53 46,800 32.65 Compensation Expense Recognized $ 776,000 $ 661,000 $ 658,000 Fair Value of Shares Vested in Year $ 660,000 $ 696,000 $ 513,000 Restricted Stock Granted to Employees Under the 1999 2014 1999 April 8, 2017. 718 2014. Presented below is a summary of the status of employees’ restricted stock awards for the years ended December 31: Employees’ Restricted Stock Awards 2019 2018 2017 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 Granted - - - Vested - - 2,895 29.41 4,285 29.94 Forfeited - - - Nonvested, End of Year - - - - 2,895 29.41 Compensation Expense Recognized $ - $ 16,000 $ 70,000 Fair Value of Awards Vested $ - $ 85,000 $ 128,000 Restricted Stock Units Granted to Executive Officers and Key Employees On February 13, 2019, 2014 December 17, 2019, 2014 2019 February 6 2020 2023. Presented below is a summary of the status of dividend equivalent restricted stock unit awards granted to executive officers and key employees for the years ended December 31: Dividend Equivalent Restricted Stock Unit Awards 2019 2018 2017 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 45,300 $ 35.70 47,750 $ 32.71 41,825 $ 30.23 Granted 16,200 49.72 15,200 41.325 15,900 37.65 Vested 15,850 34.46 17,650 32.462 9,975 30.16 Forfeited - - - Nonvested, End of Year 45,650 41.10 45,300 35.70 47,750 32.71 Compensation Expense Recognized $ 1,055,000 $ 769,000 $ 576,000 Fair Value of Awards Vested $ 546,000 $ 573,000 $ 301,000 Restricted Stock Units Granted to Employees In 2019, 2014 four 2019 April 8, 2023. not 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 2019 2018 2017 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 49,470 $ 31.03 46,440 $ 27.07 47,370 $ 25.19 Granted 13,270 44.45 14,780 38.99 10,995 33.28 Vested 13,250 27.62 8,925 25.23 11,550 25.30 Forfeited 1,900 34.57 2,825 25.86 375 26.92 Nonvested, End of Year 47,590 35.58 49,470 31.03 46,440 27.07 Compensation Expense Recognized $ 427,000 $ 351,000 $ 331,000 Fair Value of Awards Vested $ 366,000 $ 225,000 $ 292,000 Stock Performance Awards Granted to Executive Officers Agreements for stock performance awards have been granted under the 2014 three January 1 3 three no On February 13, 2019 2014 2019 2021 2019 3 27,800 January 1, 2019 December 31, 2021, January 1, 2019 20 January 1, 2022. may zero There are 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 2019 2018 2017 2019-2021 83,400 55,600 $ 2,168,000 2018-2020 81,000 54,000 897,000 $ 1,121,000 2017-2019 89,250 59,500 524,000 729,000 $ 854,000 69,997 2016-2018 122,250 81,500 8,000 772,000 580,000 113,298 2015-2017 126,450 84,300 - 23,000 573,000 114,648 Total $ 3,597,000 $ 2,645,000 $ 2,007,000 297,943 Stock-based payment expense recognized in 2019, 2018 2017 2019 2021, 2018 2020 2017 2019 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 2017 2019 2020 19 39 3 3 The earned shares shown in the table above for the 2016 2018 2019 1 41 3 3 The earned shares shown in the table above for the 2015 2017 2018 2 42 3 3 As of December 31, 2019,

Note 7 - Retained Earnings and

Note 7 - Retained Earnings and Dividend Restriction	12 Months Ended
	Dec. 31, 2019
Notes to Financial Statements
Retained Earnings Restrictions [Text Block]	7. 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, 2019, 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 46.0% and 56.2% based on OTP’s 2019 July 19, 2019. December 31, 2019, 2019

Note 8 - Leases

Note 8 - Leases	12 Months Ended
	Dec. 31, 2019
Notes to Financial Statements
Lessee, Operating Leases [Text Block]	8. The Company adopted ASU 2016 02 842 January 1, 2019, not 842 12 January 1, 2019. not not The Company elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allows for the carry forward of lease classifications determined under the requirements of ASC Topic 840. not The Company enters into leases for coal rail cars, warehouse and office space, land and certain office, manufacturing and material handling equipment under varying terms and conditions. The lengths of the leases vary from less than one ten None January 1, 2019, 50 no The right-of-use asset operating leases in place at the time of adoption were capitalized on the basis of their remaining payment obligation balances, discounted to present value based on the Company’s incremental borrowing rates (IBRs) appropriate to the leased asset and lease terms. The remaining payments for operating lease right-of-use assets are being charged to expense on a straight-line basis over the life of the lease. For the Company’s current lease obligations, no no The breakdown of right-of-use assets and lease liabilities as of December 31, 2019 (in thousands) Electric Manufacturing Plastics Corporate Total Right of Use Assets – Operating Leases: Gross $ 4,137 $ 20,347 $ 666 $ 769 $ 25,919 Accumulated Amortization (1,166 ) (2,375 ) (395 ) (132 ) (4,068 ) Net of Accumulated Amortization $ 2,971 $ 17,972 $ 271 $ 637 $ 21,851 Obligations: Current Operating Lease Liabilities $ 1,116 $ 2,609 $ 256 $ 155 $ 4,136 Long-Term Operating Lease Liabilities 2,176 15,470 15 532 18,193 Total Lease Liabilities $ 3,292 $ 18,079 $ 271 $ 687 $ 22,329 The amounts of the Company’s right-of-use operating lease obligations as of December 31, 2019 five 2020 2024 2024 Right-of-Use Operating Leases (in thousands) OTP Nonelectric Total 2020 $ 1,243 $ 3,969 $ 5,212 2021 1,228 3,717 4,945 2022 326 3,588 3,914 2023 268 3,303 3,571 2024 217 2,870 3,087 Beyond 2024 283 5,304 5,587 Total Minimum Obligations $ 3,565 $ 22,751 $ 26,316 Interest Component of Obligations (273 ) (3,714 ) (3,987 ) Present Value of Minimum Obligations, December 31, 2019 $ 3,292 $ 19,037 $ 22,329 The weighted-average remaining lease term for the Company’s outstanding lease liabilities is 6.0 years and the weighted-average discount rate is 5.3%. A reconciliation of the Company’s operating lease obligations on adoption of ASC Topic 842 January 1, 2019 December 31, 2019 (in thousands) OTP Nonelectric Total Operating Lease Obligations, January 1, 2019 $ 3,609 $ 16,760 $ 20,369 Non-cash Acquisition of Right-of-Use Assets 758 5,560 6,318 Lease Adjustments/Modifications 71 (187 ) (116 ) Lease Obligation Payments (1,316 ) (4,055 ) (5,371 ) Interest Component of Lease Obligation Payment 170 959 1,129 Operating Lease Obligations, December 31, 2019 $ 3,292 $ 19,037 $ 22,329 The lease modifications in the above table include adjustments in future minimum lease obligations on several units of leased equipment along with reductions for equipment leases terminated prior to the end of the original lease term when the equipment being leased was replaced with new equipment under a new lease. 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 CWIP 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 products sold or other nonelectric expenses, as appropriate, on the Company’s consolidated statements of income. The allocation of right-of-use asset and variable lease costs, including non-cash costs related to straight-line amortization of escalating lease payments, for 2019 Operating Lease Cost Variable Lease Cost Total Lease Cost Plant in Service or CWIP $ 29 $ - $ 29 Inventory 952 - 952 Cost of Products Sold 3,848 1,068 4,916 Electric Operation and Maintenance Expenses 334 - 334 Other Nonelectric Expenses 208 - 208 Total $ 5,371 $ 1,068 $ 6,439 Prior to adopting ASU 842, 840 Leases 840 840, $6,237,000 2018 2017,

Note 9 - Commitments and Contin

Note 9 - Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Notes to Financial Statements
Commitments and Contingencies Disclosure [Text Block]	9. Construction and Other Purchase Commitments At December 31, 2019 2019, 2018 2017, On October 1, 2019 six January 1, 2020. no not six 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 2043. 2040. 2020. December 31, 2020. no December 31, 2023. no October 2019, two October, two OTP Land Easements OTP has commitments to make future payments for land easements not 2034 2019, 2018 2017, 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, 2019, (in thousands) Construction Program and Other Commitments Capacity and Energy Requirements Coal Purchase Commitments OTP Land Easement Payments 2020 $ 269,774 $ 24,844 $ 22,644 $ 630 2021 47,230 12,988 22,935 642 2022 22 11,827 22,793 655 2023 11 11,827 23,955 668 2024 6 11,801 24,369 682 Beyond 2024 - 131,913 479,123 6,931 Total $ 317,043 $ 205,200 $ 595,819 $ 10,208 Contingencies OTP had a $3.0 million refund liability on its balance sheet as of December 31, 2019. 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 that could impact the Company’s consolidated financial statements are those related to environmental remediation, risks associated with warranty claims relating to divested businesses that could exceed established reserve amounts, risks associated with adverse regulatory decisions that could impact the recovery of fixed asset costs in future rates and litigation matters. On August 30, 2019 seven State implementation of pollution control plans to improve visibility and air quality at national parks under the EPA’s Regional Haze Rule (RHR) could require OTP to incur significant new costs, which could, dependent on determinations by state regulatory commissions on approval to recover such costs from customers, negatively impact OTP’s and the Company’s net income, financial position and cash flows. OTP understands that the North Dakota Department of Environmental Quality (NDDEQ) intends to require sources subject to RHR Round 2 1. December 31, 2028 July 2021. first 2020 2020. may not 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, 2019, not

Note 10 - Short-term and Long-t

Note 10 - Short-term and Long-term Borrowings	12 Months Ended
	Dec. 31, 2019
Notes to Financial Statements
Debt Disclosure [Text Block]	10. Short-Term Debt The following table presents the status of the Company’s lines of credit as of December 31, 2019 December 31, 2018: (in thousands) Line Limit In Use on December 31, 2019 Restricted due to Outstanding Letters of Credit Available on December 31, 2019 Available on December 31, 2018 Otter Tail Corporation Credit Agreement $ 170,000 $ 6,000 $ - $ 164,000 $ 120,785 OTP Credit Agreement 170,000 - 15,476 154,524 160,316 Total $ 340,000 $ 6,000 $ 15,476 $ 318,524 $ 281,101 Under the Otter Tail Corporation Credit Agreement (as defined below), the maximum amount of debt outstanding in 2019 July 17, 2019 2019 2019 2018. 2019 October 2, 2019 2019 2019 2018. 2019 October 1, 2019 2019 December 31, 2019 On October 29, 2012 October 31, 2019 one October 31, 2023 October 31, 2024, 1.50%, not not On October 29, 2012 may October 31, 2019 one October 31, 2023 October 31, 2024. not not not not Both the Otter Tail Corporation Credit Agreement and the OTP Credit Agreement currently expire on October 31, 2024. January 1, 2022. not may zero Long-Term Debt Issuances and Retirements 2019 On September 12, 2019, 2019 2019A October 10, 2029 ( 2019A 2019B October 10, 2039 ( 2019B 2019C October 10, 2049 ( 2019C 2020A February 25, 2030 ( 2020A 2020B August 20, 2030 ( 2020B 2020C February 25, 2040 ( 2020C 2020D February 25, 2050 ( 2020D 2019A 2019B 2019C 2020A 2020B 2020C On October 10, 2019, 2019A 2019B 2019C 2019 2019 2020A 2020C 2020D February 25, 2020, 2020B August 20, 2020, OTP may 2019 not 2019 no 2019 2019A April 10, 2029, ( 2019B April 10, 2039 2019C April 10, 2049 2019 100% 2019 The 2019 2019 2019 not 2019 not 2019 2019 2019 2019 2019 no 2018 On November 14, 2017, 2018 2018A February 7, 2048 ( 2018 2018 February 7, 2018. 2018 OTP may 2018 not no 2018 2018 August 7, 2047 2018 2018 2018 The 2018 2018 2018 not 2018 not 2018 2018 2018 2018 2018 2018 no 2016 On September 23, 2016 2016 December 15, 2026 ( 2026 2026 December 13, 2016. 2016 2026 2016 2026 December 15, 2016, $50 February 2016 The Company may 2026 not 2026 no 2016 2026 September 15, 2026 2026 2016 2016 2016 2016 2026 2026 The 2016 2016 2016 2016 not 2013 On August 14, 2013 2013 February 27, 2029 ( February 27, 2044 ( 2013 2013 February 27, 2014. The 2013 may 2013 not 2013 no 2013 November 27, 2028 November 27, 2043, 100% 2013 2013 The 2013 2013 2013 not 2013 not 2013 2013 2013 2013 2013 2013 no 2007 2011 On December 1, 2011, December 1, 2021 July 29, 2011 ( 2011 three 2022; 2027; 2037 2007 2007 August 20, 2007 ( 2007 The 2011 2007 may not 2011 2011 2011 2007 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, 2019 December 31, 2018: December 31, 2019 (in thousands) OTP Otter Tail Corporation Otter Tail Corporation Consolidated Short-Term Debt $ - $ 6,000 $ 6,000 Long-Term Debt: 3.55% Guaranteed Senior Notes, due December 15, 2026 $ 80,000 $ 80,000 Senior Unsecured Notes 4.63%, Series 2011A, due December 1, 2021 $ 140,000 140,000 Senior Unsecured Notes 6.15%, Series 2007B, due August 20, 2022 30,000 30,000 Senior Unsecured Notes 6.37%, Series 2007C, due August 20, 2027 42,000 42,000 Senior Unsecured Notes 4.68%, Series 2013A, due February 27, 2029 60,000 60,000 Senior Unsecured Notes 3.07%, Series 2019A, due October 10, 2029 1 10,000 10,000 Senior Unsecured Notes 6.47%, Series 2007D, due August 20, 2037 50,000 50,000 Senior Unsecured Notes 3.52%, Series 2019B, due October 10, 2039 26,000 26,000 Senior Unsecured Notes 5.47%, Series 2013B, due February 27, 2044 90,000 90,000 Senior Unsecured Notes 4.07%, Series 2018A, due February 7, 2048 100,000 100,000 Senior Unsecured Notes 3.82%, Series 2019C, due October 10, 2049 64,000 64,000 PACE Note, 2.54%, due March 18, 2021 351 351 Total $ 612,000 $ 80,351 $ 692,351 Less: Current Maturities net of Unamortized Debt Issuance Costs - 183 183 Unamortized Long-Term Debt Issuance Costs 2,231 356 2,587 Total Long-Term Debt net of Unamortized Debt Issuance Costs $ 609,769 $ 79,812 $ 689,581 Total Short-Term and Long-Term Debt (with current maturities) $ 609,769 $ 85,995 $ 695,764 1 Holder is COBANK, a cooperative lender. Interest payments are subject to cash credits which may December 31, 2018 (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%, Series 2011A, due December 1, 2021 $ 140,000 140,000 Senior Unsecured Notes 6.15%, Series 2007B, due August 20, 2022 30,000 30,000 Senior Unsecured Notes 6.37%, Series 2007C, due August 20, 2027 42,000 42,000 Senior Unsecured Notes 4.68%, Series 2013A, due February 27, 2029 60,000 60,000 Senior Unsecured Notes 6.47%, Series 2007D, due August 20, 2037 50,000 50,000 Senior Unsecured Notes 5.47%, Series 2013B, 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 The aggregate amounts of maturities on bonds outstanding and other long-term obligations at December 31, 2019 five (in thousands) 2020 2021 2022 2023 2024 Aggregate Amounts of Debt Maturities $ 183 $ 140,168 $ 30,000 $ - $ - Financial Covenants The Company and OTP were in compliance with the financial covenants in these debt agreements as of December 31, 2019. No The Company’s and OTP’s borrowing agreements are subject to certain financial covenants. Specifically: ● Under the OTC Credit Agreement and the 2016 may not 1.00 1.00 ● Under the 2016 may not ● Under the OTP Credit Agreement, OTP may not 1.00. ● Under the 2007 2011 may not 1.00 1.00, may not ● Under the 2013 2018 2019 may not may not

Note 11 - Pension Plan and Othe

Note 11 - Pension Plan and Other Postretirement Benefits	12 Months Ended
	Dec. 31, 2019
Notes to Financial Statements
Pension and Other Postretirement Benefits Disclosure [Text Block]	11. 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 five 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) 2019 2018 2017 Service Cost–Benefit Earned During the Period $ 5,491 $ 6,459 $ 5,629 Interest Cost on Projected Benefit Obligation 14,412 13,452 14,139 Expected Return on Assets (21,297 ) (21,199 ) (19,229 ) Amortization of Prior Service Cost: From Regulatory Asset 5 16 120 From Other Comprehensive Income 1 9 - 3 Amortization of Net Actuarial Loss: From Regulatory Asset 4,642 7,135 5,090 From Other Comprehensive Income 1 114 183 125 Net Periodic Pension Cost 2 $ 3,376 $ 6,046 $ 5,877 1 Corporate cost included in nonservice cost components of postretirement benefits. 2 Allocation of costs: 2019 2018 2017 Service c osts included in OTP capital expenditures $ 1,365 $ 1,542 $ 1,094 Service costs included in electric operation and maintenance expenses 3,994 4,756 4,400 Service costs included in other nonelectric expenses 132 161 135 Nonservice costs capitalized (526 ) (99 ) 48 Nonservice costs included in n onservice cost components of postretirement benefits (1,589 ) (314 ) 200 Weighted average assumptions used to determine net periodic pension cost for the year ended December 31: 2019 2018 2017 Discount Rate 4.50 % 3.90 % 4.60 % Long-Term Rate of Return on Plan Assets 7.25 % 7.50 % 7.50 % Rate of Increase in Future Compensation Level See below See below 3.00 % 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 following table presents amounts recognized in the consolidated balance sheets as of December 31: (in thousands) 2019 2018 Regulatory Assets: Unrecognized Prior Service Cost $ - $ 5 Unrecognized Actuarial Loss 120,592 104,891 Total Regulatory Assets $ 120,592 $ 104,896 Accumulated Other Comprehensive Loss: Unrecognized Prior Service Cost $ - $ 9 Unrecognized Actuarial (Gain) Loss (82 ) 137 Total Accumulated Other Comprehensive Loss $ (82 ) $ 146 Noncurrent Liability $ 55,004 $ 58,659 Funded status as of December 31: (in thousands) 2019 2018 Accumulated Benefit Obligation $ (346,723 ) $ (297,972 ) Projected Benefit Obligation $ (384,785 ) $ (328,442 ) Fair Value of Plan Assets 329,781 269,783 Funded Status $ (55,004 ) $ (58,659 ) 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, 2019: (in thousands) 2019 2018 Reconciliation of Fair Value of Plan Assets: Fair Value of Plan Assets at January 1 $ 269,783 $ 285,319 Actual Return on Plan Assets 52,640 (21,334 ) Discretionary Company Contributions 22,500 20,000 Benefit Payments (15,142 ) (14,202 ) Fair Value of Plan Assets at December 31 $ 329,781 $ 269,783 Estimated Asset Return 19.3 % (7.3 %) Reconciliation of Projected Benefit Obligation: Projected Benefit Obligation at January 1 $ 328,442 $ 352,718 Service Cost 5,491 6,459 Interest Cost 14,412 13,452 Benefit Payments (15,142 ) (14,202 ) Actuarial Loss (Gain) 51,582 (29,985 ) Projected Benefit Obligation at December 31 $ 384,785 $ 328,442 Weighted average assumptions used to determine benefit obligations at December 31: 2019 2018 Discount Rate 3.47 % 4.50 % 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 2020 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: 2019 2018 Net Periodic Pension Cost January 1, 2019 January 1, 2018 End of Year Benefit Obligations January 1, 2019 projected to December 31, 2019 January 1, 2018 projected to December 31, 2018 Market Value of Assets December 31, 2019 December 31, 2018 Cash flows no December 31, 2019 January 2020. The following benefit payments, which reflect expected future service, as appropriate, are expected to be paid out from plan assets: (in thousands) 2020 2021 2022 2023 2024 Years 2025-2029 $ 15,908 $ 16,477 $ 17,116 $ 17,768 $ 18,374 $ 98,994 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 or the SEI Energy Debt Collective Fund . The Company’s pension plan asset allocations at December 31, 2019 2018, Asset Allocation 2019 2018 Global MGD Volatility Fund ( mixed equities fund 20.4 % - Large Capitalization Equity Securities 11.3 % 17.5 % International Equity Securities 9.3 % 17.0 % Emerging Markets Equity Fund 4.2 % 3.4 % Small and Mid-Capitalization Equity Securities 4.1 % 6.7 % SEI Dynamic Asset Allocation Fund 3.1 % 4.0 % Equity Securities 52.4 % 48.6 % Fixed-Income Securities and Cash 44.7 % 47.1 % Other – SEI Energy Debt Collective Fund 2.9 % 4.3 % 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) 2019 2018 Assets in Level 1 of the Fair Value Hierarchy $ 320,241 $ 258,307 SEI Energy Debt Collective Fund at NAV 9,540 11,476 Total Assets $ 329,781 $ 269,783 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) 2019 2018 Global MGD Volatility Fund ( mixed equities fund $ 67,184 $ - Large Capitalization Equity Securities Mutual Fund 37,357 47,198 International Equity Securities Mutual Funds 30,653 45,912 Small and Mid-Capitalization Equity Securities Mutual Fund 13,447 17,971 SEI Dynamic Asset Allocation Mutual Fund 10,168 10,929 Emerging Markets Equity Fund 13,792 9,197 Fixed Income Securities Mutual Funds 147,639 127,098 Cash Management – Money Market Fund 1 2 Total Assets $ 320,241 $ 258,307 The investments held by the SEI Energy Debt Collective Fund on December 31, 2019 2018 95 three 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 that provides for defined benefit payments to these employees on their retirement for life or to their beneficiaries on their death. In addition, the ESSRP provides for survivor benefit payments to beneficiaries of executive officers. On December 26, 2019, December 31, 2019. In connection with amending and restating the ESSRP, the Board of Directors also approved the making of special employer contributions to named participants in the Otter Tail Corporation Executive Restoration Plus Plan (the ERPP) who will be affected by the ESSRP freeze in order to offset the impact of the freeze for those participants. The following table lists components of net periodic pension benefit cost for the year ended December 31: (in thousands) 2019 2018 2017 Service Cost–Benefit Earned During the Period $ 418 $ 408 $ 290 Interest Cost on Projected Benefit Obligation 1,735 1,589 1,686 Amortization of Prior Service Cost: From Regulatory Asset 5 20 16 From Other Comprehensive Income 1 17 34 38 Amortization of Net Actuarial Loss: From Regulatory Asset 124 206 285 From Other Comprehensive Income 1 348 722 440 Net Periodic Pension Cost 2 $ 2,647 $ 2,979 $ 2,755 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 Allocation of costs: 2019 2018 2017 Service costs included in electric operation and maintenance expenses $ 104 $ 99 $ 94 Service costs included in other nonelectric expenses 314 309 196 Nonservice costs included in n onservice cost components of postretirement benefits 2,229 2,571 2,465 Weighted average assumptions used to determine net periodic pension cost for the year ended December 31: 2019 2018 2017 Discount Rate 4.46 % 3.85 % 4.60 % Rate of Increase in Future Compensation Level 3.40 % 2.92 % 3.00 % The following table presents amounts recognized in the consolidated balance sheets as of December 31: (in thousands) 2019 2018 Regulatory Assets: Unrecognized Prior Service Cost $ - $ 20 Unrecognized Actuarial Loss 2,170 1,768 Total Regulatory Assets $ 2,170 $ 1,788 Projected Benefit Obligation Liability – Net Amount Recognized $ (43,966 ) $ (39,699 ) Accumulated Other Comprehensive Loss: Unrecognized Prior Service Cost $ 1 $ 64 Unrecognized Actuarial Loss 9,170 6,455 Total Accumulated Other Comprehensive Loss $ 9,171 $ 6,519 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, 2019 December 31 (in thousands) 2019 2018 Reconciliation of Fair Value of Plan Assets: Fair Value of Plan Assets at January 1 $ - $ - Actual Return on Plan Assets - - Employer Contributions 1,475 1,505 Benefit Payments (1,475 ) (1,505 ) Fair Value of Plan Assets at December 31 $ - $ - Reconciliation of Projected Benefit Obligation: Projected Benefit Obligation at January 1 $ 39,699 $ 42,308 Service Cost 418 408 Interest Cost 1,735 1,589 Benefit Payments (1,475 ) (1,505 ) Curtailments (1,671 ) - Actuarial Loss (Gain) 5,260 (3,101 ) Projected Benefit Obligation at December 31 $ 43,966 $ 39,699 Weighted average assumptions used to determine benefit obligations at December 31: 2019 2018 Discount Rate 3.36 % 4.46 % Rate of Increase in Future Compensation Level: 3.50 % 3.40 % Cash flows no Years (in thousands) 2020 2021 2022 2023 2024 2025-2029 $ 1,571 $ 1,681 $ 2,279 $ 2,680 $ 2,627 $ 13,976 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 no In 2019, 65 no 65 2020 2019. 2020, The following table lists components of net periodic postretirement benefit cost for the year ended December 31: (in thousands) 2019 2018 2017 Service Cost–Benefit Earned During the Period $ 1,286 $ 1,526 $ 1,425 Interest Cost on Projected Benefit Obligation 3,083 2,583 2,712 Amortization of Prior Service Cost From Regulatory Asset - - (4 ) From Other Comprehensive Income 1 - - 4 Amortization of Net Actuarial Loss From Regulatory Asset 1,571 1,648 936 From Other Comprehensive Income 1 38 42 19 Net Periodic Postretirement Benefit Cost 2 $ 5,978 $ 5,799 $ 5,092 Effect of Medicare Part D Subsidy $ (179 ) $ (470 ) $ (561 ) 1 Corporate cost included in nonservice cost components of postretirement benefits. 2 Allocation of cost: 2019 2018 2017 Service costs included in OTP capital expenditures $ 320 $ 364 $ 277 Service costs included in electric operation and maintenance expenses 935 1,124 1,114 Service costs included in other nonelectric expenses 31 38 34 Nonservice costs capitalized 1,167 1,020 712 Nonservice costs included in n onservice cost components of postretirement benefits 3,525 3,253 2,955 Weighted average assumptions used to determine net periodic postretirement benefit cost for the year ended December 31: 2019 2018 2017 Discount Rate 4.44 % 3.81 % 4.46 % The following table presents amounts recognized in the consolidated balance sheets as of December 31: (in thousands) 2019 2018 Regulatory Asset: Unrecognized Prior Service Credit $ (20,363 ) - Unrecognized Net Actuarial Loss (Gain) 35,322 $ 18,094 Net Regulatory Asset $ 14,959 $ 18,094 Projected Benefit Obligation Liability – Net Amount Recognized $ (71,437 ) $ (71,561 ) Accumulated Other Comprehensive (Income) Loss: Unrecognized Prior Service Credit $ (501 ) - Unrecognized Net Actuarial Loss (Gain) 184 $ (107 ) Accumulated Other Comprehensive (Income) Loss: $ (317 ) $ (107 ) 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, 2019: (in thousands) 2019 2018 Reconciliation of Fair Value of Plan Assets: Fair Value of Plan Assets at January 1 $ - $ - Actual Return on Plan Assets - - Company Contributions 2,757 3,183 Benefit Payments (Net of Medicare Part D Subsidy) (7,164 ) (6,684 ) Participant Premium Payments 4,407 3,501 Fair Value of Plan Assets at December 31 $ - $ - Reconciliation of Projected Benefit Obligation: Projected Benefit Obligation at January 1 $ 71,561 $ 69,774 Service Cost (Net of Medicare Part D Subsidy) 1,286 1,526 Interest Cost (Net of Medicare Part D Subsidy) 3,083 2,583 Benefit Payments (Net of Medicare Part D Subsidy) (7,164 ) (6,684 ) Participant Premium Payments 4,407 3,501 Plan Amendments (20,864 ) - Actuarial Loss 19,128 861 Projected Benefit Obligation at December 31 $ 71,437 $ 71,561 Reconciliation of Accrued Postretirement Cost: Accrued Postretirement Cost at January 1 $ (53,574 ) $ (50,958 ) Expense (5,978 ) (5,799 ) Net Company Contribution 2,757 3,183 Accrued Postretirement Cost at December 31 $ (56,795 ) $ (53,574 ) Weighted average assumptions used to determine benefit obligations at December 31: 2019 2018 Discount Rate 3.43 % 4.44 % Assumed healthcare cost-trend rates as of December 31: 2019 2018 Healthcare Cost-Trend Rate Assumed for Next Year 6.72 % 7.00 % 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 Measurement Dates: 2019 2018 Net Periodic Postretirement Benefit Cost January 1, 2019 January 1, 2018 End of Year Benefit Obligations January 1, 2019 projected to December 31, 2019 January 1, 2018 projected to December 31, 2018 Cash flows 2020. 2020. Years (in thousands) 2020 2021 2022 2023 2024 2025-2029 $ 3,323 $ 3,470 $ 3,653 $ 3,720 $ 3,778 $ 19,852 401K The Company sponsors a 401K 2019, 2018 2017. 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 $374,000 for 2019, 2018 2017.

Note 12 - Fair Value of Financi

Note 12 - Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2019
Notes to Financial Statements
Fair Value Disclosures [Text Block]	12. 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, 2019 December 31, 2018 Long-Term Debt including Current Maturities 2 820. December 31, 2019 December 31, 2018 (in thousands) Carrying Amount Fair Value Carrying Amount Fair Value Cash and Cash Equivalents $ 21,199 $ 21,199 $ 861 $ 861 Short-Term Debt (6,000 ) (6,000 ) (18,599 ) (18,599 ) Long-Term Debt including Current Maturities (689,764 ) (742,279 ) (590,174 ) (601,513 )

Note 13 - Property, Plant and E

Note 13 - Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2019
Notes to Financial Statements
Property, Plant and Equipment Disclosure [Text Block]	13. (in thousands) December 31, 2019 December 31, 2018 Electric Plant in Service Production $ 915,996 $ 905,224 Transmission 647,474 512,832 Distribution 526,146 502,261 General 123,268 99,404 Electric Plant in Service 2,212,884 2,019,721 Construction Work in Progress 177,584 170,090 Total Gross Electric Plant 2,390,468 2,189,811 Less Accumulated Depreciation and Amortization 731,110 699,642 Net Electric Plant $ 1,659,358 $ 1,490,169 Nonelectric Operations Plant Equipment $ 187,904 $ 170,634 Buildings and Leasehold Improvements 53,412 53,011 Land 6,040 4,475 Nonelectric Operations Plant 247,356 228,120 Construction Work in Progress 7,654 11,536 Total Gross Nonelectric Plant 255,010 239,656 Less Accumulated Depreciation and Amortization 160,574 148,727 Net Nonelectric Operations Plant $ 94,436 $ 90,929 Net Plant $ 1,753,794 $ 1,581,098 The estimated service lives for rate-regulated properties is 5 to 82 years. For nonelectric property the estimated useful lives are from 2 to 40 years. Service Life Range (years) Low High Electric Fixed Assets: Production Plant 9 82 Transmission Plant 51 75 Distribution Plant 15 70 General Plant 5 57 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, 2019
Notes to Financial Statements
Income Tax Disclosure [Text Block]	14. The total income tax expense differs from the amount computed by applying the federal income tax rate (21% in 2019 2018, 2017 (in thousands) 2019 2018 2017 Tax Computed at Federal Statutory Rate $ 21,901 $ 20,356 $ 34,893 Increases (Decreases) in Tax from: State Income Taxes Net of Federal Income Tax Expense 3,561 5,210 4,368 Differences Reversing in Excess of Federal Rates (3,357 ) (3,432 ) 551 Permanent Differences, R&D Tax Credits, Unitary Tax and Other Adjustments (1,315 ) (1,864 ) (1,873 ) North Dakota Wind Tax Credit Amortization – Net of Federal Taxes (1,033 ) (1,033 ) (850 ) Corporate-owned Life Insurance (749 ) (3 ) (845 ) Excess Tax deduction – Equity Method Stock Awards (744 ) (708 ) (751 ) Allowance for Funds Used During Construction – Equity (501 ) (431 ) (322 ) Employee Stock Ownership Plan Dividend Deduction (281 ) (298 ) (509 ) Investment Tax Credit Amortization (41 ) (98 ) (164 ) Federal Production Tax Credits (PTCs) - (3,111 ) (7,527 ) Section 199 Domestic Production Activities Deduction - - (1,471 ) Effect of TCJA Tax Rate Reduction on Value of Net Deferred Tax Assets - - 1,756 Income Tax Expense $ 17,441 $ 14,588 $ 27,256 Overall Effective Federal and State Income Tax Rate 16.7 % 15.0 % 27.3 % Income Tax Expense Includes the Following: Current Federal Income Taxes $ 5,156 $ 4,960 $ 4,434 Current State Income Taxes 1,333 1,395 1,128 Deferred Federal Income Taxes 8,859 8,065 25,648 Deferred State Income Taxes 3,167 4,410 4,587 Federal PTCs - (3,111 ) (7,527 ) North Dakota Wind Tax Credit Amortization – Net of Federal Taxes (1,033 ) (1,033 ) (850 ) Investment Tax Credit Amortization (41 ) (98 ) (164 ) Total $ 17,441 $ 14,588 $ 27,256 Total Income Before Income Taxes $ 104,288 $ 96,933 $ 99,695 The Company's deferred tax assets and liabilities were composed of the following on December 31: (in thousands) 2019 2018 Deferred Tax Assets Benefit Liabilities $ 36,246 $ 33,967 Retirement Benefits Liabilities 36,206 32,664 Regulatory Tax Liability 35,700 33,228 North Dakota Wind Tax Credits 31,611 32,570 Cost of Removal 25,604 21,787 Federal PTCs 20,017 32,101 Differences Related to Property 6,979 6,842 Lease Liability 5,733 - Vacation Accrual 1,884 1,919 Net Operating Loss Carryforward 1,860 2,489 Investment Tax Credits 408 449 Other 344 3,218 Valuation Allowance (800 ) (600 ) Total Deferred Tax Assets $ 201,792 $ 200,634 Deferred Tax Liabilities Differences Related to Property $ (268,495 ) $ (261,396 ) Retirement Benefits Regulatory Asset (36,206 ) (32,664 ) Excess Tax over Book Pension (17,556 ) (15,145 ) Right of Use Asset (5,705 ) - North Dakota Wind Tax Credits (3,126 ) (4,386 ) Impact of State Net Operating Losses on Federal Taxes (385 ) (523 ) Other (2,260 ) (7,496 ) Total Deferred Tax Liabilities $ (333,733 ) $ (321,610 ) Deferred Income Taxes $ (131,941 ) $ (120,976 ) In November 2018, 2018 2017 one 2017 2018. Schedule of expiration of tax credits and tax net operating losses available as of December 31, 2019: (in thousands) Amount 2022-2032 2033-2038 2039-2043 United States Federal Tax Credits $ 23,002 $ - $ 22,220 $ 782 State Net Operating Losses 1,860 1,833 27 - State Tax Credits 32,177 - 2,643 29,534 The following table summarizes the activity related to the Company’s unrecognized tax benefits: (in thousands) 2019 2018 2017 Balance on January 1 $ 1,282 $ 684 $ 891 Increases Related to Tax Positions for Prior Years 37 6 28 Decreases Related to Tax Positions for Prior Years - - (172 ) Increases Related to Tax Positions for Current Year 339 778 143 Uncertain Positions Resolved During Year (170 ) (186 ) (206 ) Balance on December 31 $ 1,488 $ 1,282 $ 684 The balance of unrecognized tax benefits as of December 31, 2019 December 31, 2019 not December 31, 2019. The Company and its subsidiaries file a consolidated U.S. federal income tax return and various state income tax returns. As of December 31, 2019, no 2016 2015 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 2017. 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, 2019
Notes to Financial Statements
Asset Retirement Obligation Disclosure [Text Block]	15. The Company’s AROs are related to OTP’s coal-fired generation plants and its 92 wind turbines located in North Dakota. The AROs include items such as site restoration, closure of ash pits, and removal of certain structures, generators, asbestos and storage tanks. The Company has legal obligations associated with the retirement of a variety of other long-lived tangible assets used in electric operations where the estimated settlement costs are individually and collectively immaterial. The Company has no OTP recorded no new AROs in 2019. 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, 2019 2018 (in thousands) 2019 2018 Asset Retirement Obligations Beginning Balance $ 9,117 $ 8,719 New Obligations Recognized - - Adjustments Due to Revisions in Cash Flow Estimates 3,099 - Accrued Accretion 440 398 Settlements - - Ending Balance $ 12,656 $ 9,117 Asset Retirement Costs Capitalized Beginning Balance $ 2,983 $ 2,983 New Obligations Recognized - - Adjustments Due to Revisions in Cash Flow Estimates 3,099 - Settlements - - Ending Balance $ 6,082 $ 2,983 Accumulated Depreciation – Asset Retirement Costs Capitalized Beginning Balance $ 1,034 $ 915 New Obligations Recognized - - Adjustments Due to Revisions in Cash Flow Estimates - - Depreciation Expense 163 119 Settlements - - Ending Balance $ 1,197 $ 1,034 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, 2019
Notes to Financial Statements
Subsequent Events [Text Block]	16. Stock Incentive Awards On February 12, 2020 2014 Award Shares/Units Granted Vesting Restricted Stock Units Granted 15,300 25% per year through February 6, 2024 Stock Performance Awards Granted: Under Executive Agreement 47,600 December 31, 2022 Under Legacy Agreement 7,400 December 31, 2022 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. Restricted stock units granted to executive officers and certain key employees 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 55,000 shares. For target performance the participants would earn an aggregate of 27,500 common shares for achieving the target set for the Company’s 3-year average adjusted ROE. The participants would also earn an aggregate of 27,500 common shares based on the Company’s total shareholder return relative to the total shareholder return of the companies that comprise the EEI Index over the performance measurement period of January 1, 2020 December 31, 2022, January 1, 2020 20 January 1, 2023. may zero no 718, Under the 2020 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, 2019
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) 2019 2018 Assets Current Assets Cash and Cash Equivalents $ 4,959 $ - Accounts Receivable - - Accounts Receivable from Subsidiaries 2,144 1,931 Interest Receivable from Subsidiaries 117 117 Notes Receivable from Subsidiaries - 1,167 Income Taxes Receivable 1,487 - Other 1,050 3,482 Total Current Assets 9,757 6,697 Investments in Subsidiaries 860,646 787,869 Notes Receivable from Subsidiaries 79,251 79,422 Deferred Income Taxes 25,505 21,100 Right of Use Assets - Operating 637 - Other Assets 35,503 31,547 Total Assets $ 1,011,299 $ 926,635 Liabilities and Equity Current Liabilities Short-Term Debt $ 6,000 $ 9,215 Current Maturities of Long-Term Debt 183 172 Accounts Payable to Subsidiaries 7 7 Notes Payable to Subsidiaries 89,611 60,626 Current Operating Lease Liabilities 156 - Other 9,473 9,994 Total Current Liabilities 105,430 80,014 Long Term Operating Lease Liabilities 533 - Other Noncurrent Liabilities 44,042 37,814 Commitments and Contingencies Capitalization Long-Term Debt, Net of Current Maturities 79,812 79,944 Common Shareholder Equity 781,482 728,863 Total Capitalization 861,294 808,807 Total Liabilities and Equity $ 1,011,299 $ 926,635 See accompanying notes to condensed financial statements. Otter Tail Corporation (PARENT COMPANY) Condensed Statements of Income--For the Years Ended December 31 (in thousands) 2019 2018 2017 Operating Loss Revenue from Contracts with Customers $ - $ - $ - Operating Expenses 10,529 9,916 7,138 Operating Loss (10,529 ) (9,916 ) (7,138 ) Other Income (Expense) Equity Income in Earnings of Subsidiaries 93,731 91,446 82,715 Interest Charges (4,863 ) (4,043 ) (4,270 ) Interest Charges to Subsidiaries (306 ) (387 ) (244 ) Interest Income from Subsidiaries 3,063 2,839 2,848 Nonservice Cost Components of Postretirement Benefits (1,297 ) (1,422 ) (1,215 ) Other Income 1,566 550 1,054 Total Other Income 91,894 88,983 80,888 Income Before Income Taxes 81,365 79,067 73,750 Income Tax (Benefit) Expense (5,482 ) (3,278 ) 1,311 Net Income $ 86,847 $ 82,345 $ 72,439 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) 2019 2018 2017 Cash Flows from Operating Activities Net Cash Provided by Operating Activities $ 52,263 $ 56,947 $ 50,205 Cash Flows from Investing Activities Investment in Subsidiaries (34,990 ) (24,764 ) - Debt Repaid by Subsidiaries 1,338 774 151 Cash Used in Investing Activities (257 ) (623 ) (121 ) Net Cash (Used in) Provided by Investing Activities (33,909 ) (24,613 ) 30 Cash Flows from Financing Activities Change in Checks Written in Excess of Cash (31 ) 31 - Net Short-Term (Repayments) Borrowings (3,215 ) 9,215 - Borrowings from (Repayments to) Subsidiaries 28,985 (1,281 ) 23,389 Proceeds from Issuance of Common Stock 20,338 - 4,349 Common Stock Issuance Expenses (577 ) (108 ) - Payments for Retirement of Capital Stock (2,730 ) (3,011 ) (1,799 ) Short-Term and Long-Term Debt Issuance Expenses (270 ) (164 ) (158 ) Payments for Retirement of Long-Term Debt (172 ) (189 ) (15,231 ) Dividends Paid (55,723 ) (53,198 ) (50,632 ) Net Cash Used in Financing Activities (13,395 ) (48,705 ) (40,082 ) Net Change in Cash and Cash Equivalents 4,959 (16,371 ) 10,153 Cash and Cash Equivalents at Beginning of Period - 16,371 6,218 Cash and Cash Equivalents at End of Period $ 4,959 $ - $ 16,371 See accompanying notes to condensed financial statements. Otter Tail Corporation (Parent Company) Notes to Condensed Financial Statements For the years ended December 31, 2019, 2018 2017 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, 2019: (in thousands) Accounts Receivable Interest Receivable Current Notes Receivable Long-Term Notes Receivable Accounts Payable Current Notes Payable Otter Tail Power Company $ 2,056 $ - $ - $ - $ 7 $ - Northern Pipe Products, Inc. - 8 - 5,351 - 3,056 Vinyltech Corporation 4 17 - 11,500 - 15,099 BTD Manufacturing, Inc. - 77 - 52,000 - 18,474 T.O. Plastics, Inc. - 15 - 10,400 - 3,099 Varistar Corporation - - - - - 49,883 Otter Tail Assurance Limited 84 - - - - - $ 2,144 $ 117 $ - $ 79,251 $ 7 $ 89,611 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 $ - Northern Pipe Products, Inc. - 8 - 5,522 - 5,623 Vinyltech Corporation 4 17 - 11,500 - 15,305 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 Dividends Dividends paid to Otter Tail Corporation (the Parent) from its subsidiaries were as follows: (in thousands) 2019 2018 2017 Cash Dividends Paid to Parent by Subsidiaries $ 55,660 $ 53,134 $ 50,571 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, 2019
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
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 $1,728,000 in 2019, 2018 2017. 2019, 2018 2017. 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 to 40 years). The cost of additions includes contracted work, direct labor and materials, allocable overheads and capitalized interest. No interest was capitalized on nonelectric plant in 2019, 2018 2017.
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 five December 31, 2019 2018 Jointly Owned Facilities (dollars in thousands) OTP Ownership Percentage Electric Plant in Service Construction Work in Progress Accumulated Depreciation Net Plant December 31, 2019 Big Stone Plant 53.9 % $ 337,197 $ 384 $ (98,654 ) $ 238,927 Coyote Station 35.0 % 184,493 83 (108,248 ) 76,328 Big Stone South–Ellendale 345 kV line 1 50.0 % 106,343 - (819 ) 105,524 Fargo–Monticello 345 kV line 14.2 % 78,184 - (7,011 ) 71,173 Big Stone South–Brookings 345 kV line 50.0 % 53,036 - (2,016 ) 51,020 Brookings–Southeast Twin Cities 345 kV line 4.8 % 26,286 - (2,086 ) 24,200 Bemidji–Grand Rapids 230 kV line 14.8 % 16,331 - (233 ) 16,098 December 31, 2018 Big Stone Plant 53.9 % $ 336,051 $ 361 $ (92,007 ) $ 244,405 Coyote Station 35.0 % 177,713 2,588 (100,997 ) 79,304 Big Stone South–Ellendale 345 kV line 1 50.0 % - 106,490 - 106,490 Fargo–Monticello 345 kV line 14.2 % 78,184 - (5,891 ) 72,293 Big Stone South–Brookings 345 kV line 50.0 % 53,235 (150 ) (1,264 ) 51,821 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 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 In 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, 2019.
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
Revenue [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. Provisions for sales returns, early payment terms discounts, volume-based variable pricing incentives and warranty costs are recorded as reductions to revenue at the time revenue is recognized based on customer history, historical information and current trends. 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. A 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), Energy Intensive Trade Exposed and Conservation Improvement Program riders. ● In North Dakota: TCR, ECR, RRA and Generation Cost Recovery (GCR) riders. ● In South Dakota: TCR, ECR, Phase-in Rate Plan 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 changes in accrued revenues under ARPs on a separate line in the revenue section of the Company’s consolidated statement of income. See table in note 3 December 31, 2019, 2018 2017. Manufacturing Segment Revenues no Plastics Segment Revenues no one See operating revenue table in note 2 December 31, 2019, 2018 2017.
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]	Use 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) 2019 2018 Cost Method: Economic Development Loan Pools $ 24 $ 34 Other 73 123 Equity Method Partnerships 27 26 Marketable Debt Securities Classified as Available-for-Sale 8,184 7,484 Marketable Equity Securities Classified as Available-for-Sale 1,586 1,294 Total Investments $ 9,894 $ 8,961 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, 2019.
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) 2019 2018 Finished Goods $ 31,863 $ 37,130 Work in Process 16,508 20,393 Raw Material, Fuel and Supplies 49,480 48,747 Total Inventories $ 97,851 $ 106,270
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, 2019 December 31, 2018: December 31, 2019 (in thousands) Level 1 Level 2 Level 3 Assets: Investments: Equity Funds – Held by Captive Insurance Company $ 1,586 Corporate Debt Securities – Held by Captive Insurance Company $ 2,124 Government-Backed and Government-Sponsored Enterprises’ Debt Securities – Held by Captive Insurance Company 6,060 Other Assets: Money Market and Mutual Funds – Retirement Plans 2,363 Total Assets $ 3,949 $ 8,184 December 31, 2018 (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 The level 2 third may
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 2019 2018: (in thousands) Gross Balance December 31, 2018 Accumulated Impairments Balance (net of impairments) December 31, 2018 Adjustments to Goodwill in 2019 Balance (net of impairments) December 31, 2019 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, 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 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, 2019 December 31, 2018: December 31, 2019 (in thousands) Gross Carrying Amount Accumulated Amortization Net Carrying Amount Remaining Amortization Periods (months) Amortizable Intangible Assets: Customer Relationships $ 22,491 $ 11,259 $ 11,232 88 - 188 Other 179 121 58 8 - 45 Total $ 22,670 $ 11,380 $ 11,290 December 31, 2018 (in thousands) 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 The amortization expense for these intangible assets was: (in thousands) 2019 2018 2017 Amortization Expense – Intangible Assets $ 1,186 $ 1,315 $ 1,347 The estimated annual amortization expense for these intangible assets for the next five (in thousands) 2020 2021 2022 2023 2024 Estimated Amortization Expense – Intangible Assets $ 1,140 $ 1,105 $ 1,105 $ 1,104 $ 1,099
Cash Flow Supplemental [Policy Text Block]	Supplemental Disclosures of Cash Flow Information As of December 31, (in thousands) 2019 2018 Noncash Investing Activities: Transactions Related to Capital Additions not Settled in Cash $ 37,429 $ 13,757 (in thousands) 2019 2018 2017 Cash Paid During the Year for: Interest (net of amount capitalized) $ 30,132 $ 28,109 $ 29,791 Income Taxes $ 4,797 $ 6,109 $ 5,064
New Accounting Pronouncements, Policy [Policy Text Block]	New Accounting Standards Adopted ASU 2016 02 February 2016 No. 2016 02, Leases (Topic 842 2016 02 2016 02 842, 840 2016 02 December 15, 2018, 842 2016 02 January 1, 2019. 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 first 2019. no no ASU 2018 02 February 2018 No. 2018 02, Income Statement—Reporting Comprehensive Income (Topic 220 2018 02 2018 02, 2017 2018 02 December 15, 2018, 2018 02 The Company adopted the updates in ASU 2018 02 January 1, 2019, not Support for the determination of the stranded tax effects resulting from the enactment of the TCJA in AOCI/(L) is provided in the table below. (in thousands) Unrealized Gains on Available-for- Sale Securities Unamortized Actuarial Losses and Prior Service Costs on Pension and Other Postretirement Benefits AOCI/(L) Balance on December 22, 2017 – Pre-tax $ 71 $ (5,672 ) $ (5,601 ) Effect of TCJA 14% Federal Tax Rate Reduction on Gross Deferred Tax Amounts $ 10 $ (794 ) $ (784 ) New Accounting Standards Pending Adoption ASU 2016 13 June 2016 No. 2016 13, Financial Instruments—Credit Losses (Topic 326 326 , may 326 December 15, 2019. not 326 first 2020. not not ASU 2018 15 August 2018 No. 2018 15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350 40 350 40, Internal-Use Software 2018 15 2018 15 350 40 2018 15 2018 15 December 15, 2019 2018 15 first 2020 no

Note 1 - Summary of Significa_2

Note 1 - Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 Big Stone Plant 53.9 % $ 337,197 $ 384 $ (98,654 ) $ 238,927 Coyote Station 35.0 % 184,493 83 (108,248 ) 76,328 Big Stone South–Ellendale 345 kV line 1 50.0 % 106,343 - (819 ) 105,524 Fargo–Monticello 345 kV line 14.2 % 78,184 - (7,011 ) 71,173 Big Stone South–Brookings 345 kV line 50.0 % 53,036 - (2,016 ) 51,020 Brookings–Southeast Twin Cities 345 kV line 4.8 % 26,286 - (2,086 ) 24,200 Bemidji–Grand Rapids 230 kV line 14.8 % 16,331 - (233 ) 16,098 December 31, 2018 Big Stone Plant 53.9 % $ 336,051 $ 361 $ (92,007 ) $ 244,405 Coyote Station 35.0 % 177,713 2,588 (100,997 ) 79,304 Big Stone South–Ellendale 345 kV line 1 50.0 % - 106,490 - 106,490 Fargo–Monticello 345 kV line 14.2 % 78,184 - (5,891 ) 72,293 Big Stone South–Brookings 345 kV line 50.0 % 53,235 (150 ) (1,264 ) 51,821 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
Investment [Table Text Block]	(in thousands) 2019 2018 Cost Method: Economic Development Loan Pools $ 24 $ 34 Other 73 123 Equity Method Partnerships 27 26 Marketable Debt Securities Classified as Available-for-Sale 8,184 7,484 Marketable Equity Securities Classified as Available-for-Sale 1,586 1,294 Total Investments $ 9,894 $ 8,961
Schedule of Inventory, Current [Table Text Block]	December 31, December 31, (in thousands) 2019 2018 Finished Goods $ 31,863 $ 37,130 Work in Process 16,508 20,393 Raw Material, Fuel and Supplies 49,480 48,747 Total Inventories $ 97,851 $ 106,270
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]	December 31, 2019 (in thousands) Level 1 Level 2 Level 3 Assets: Investments: Equity Funds – Held by Captive Insurance Company $ 1,586 Corporate Debt Securities – Held by Captive Insurance Company $ 2,124 Government-Backed and Government-Sponsored Enterprises’ Debt Securities – Held by Captive Insurance Company 6,060 Other Assets: Money Market and Mutual Funds – Retirement Plans 2,363 Total Assets $ 3,949 $ 8,184 December 31, 2018 (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
Schedule of Goodwill [Table Text Block]	(in thousands) Gross Balance December 31, 2018 Accumulated Impairments Balance (net of impairments) December 31, 2018 Adjustments to Goodwill in 2019 Balance (net of impairments) December 31, 2019 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, 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
Schedule of Finite-Lived Intangible Assets [Table Text Block]	December 31, 2019 (in thousands) Gross Carrying Amount Accumulated Amortization Net Carrying Amount Remaining Amortization Periods (months) Amortizable Intangible Assets: Customer Relationships $ 22,491 $ 11,259 $ 11,232 88 - 188 Other 179 121 58 8 - 45 Total $ 22,670 $ 11,380 $ 11,290 December 31, 2018 (in thousands) 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
Finite-lived Intangible Assets Amortization Expense [Table Text Block]	(in thousands) 2019 2018 2017 Amortization Expense – Intangible Assets $ 1,186 $ 1,315 $ 1,347
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block]	(in thousands) 2020 2021 2022 2023 2024 Estimated Amortization Expense – Intangible Assets $ 1,140 $ 1,105 $ 1,105 $ 1,104 $ 1,099
Schedule of Cash Flow, Supplemental Disclosures [Table Text Block]	As of December 31, (in thousands) 2019 2018 Noncash Investing Activities: Transactions Related to Capital Additions not Settled in Cash $ 37,429 $ 13,757 (in thousands) 2019 2018 2017 Cash Paid During the Year for: Interest (net of amount capitalized) $ 30,132 $ 28,109 $ 29,791 Income Taxes $ 4,797 $ 6,109 $ 5,064
Schedule of New Accounting Pronouncements and Changes in Accounting Principles [Table Text Block]	(in thousands) Unrealized Gains on Available-for- Sale Securities Unamortized Actuarial Losses and Prior Service Costs on Pension and Other Postretirement Benefits AOCI/(L) Balance on December 22, 2017 – Pre-tax $ 71 $ (5,672 ) $ (5,601 ) Effect of TCJA 14% Federal Tax Rate Reduction on Gross Deferred Tax Amounts $ 10 $ (794 ) $ (784 )

Note 2 - Business Segment Inf_2

Note 2 - Business Segment Information (Tables)	12 Months Ended
	Dec. 31, 2019
Notes Tables
Schedule of Segment Reporting Information, by Segment [Table Text Block]	(in thousands) 2019 2018 2017 Operating Revenue Electric Segment: Retail Sales Revenue from Contracts with Customers $ 405,446 $ 388,690 $ 376,902 Changes in Accrued ARP Revenues 1,032 (439 ) (1,971 ) Total Retail Sales Revenue 406,478 388,251 374,931 Transmission Services Revenue 40,542 46,947 46,664 Wholesale Revenues – Company Generation 5,007 7,735 5,173 Other Electric Revenues 7,070 7,322 7,769 Total Electric Segment Revenues 459,097 450,255 434,537 Manufacturing Segment: Metal Parts and Tooling 236,032 223,765 189,242 Plastic Products and Tooling 35,173 35,836 33,939 Other 5,999 8,808 6,557 Total Manufacturing Segment Revenues 277,204 268,409 229,738 Plastics Segment – Sale of PVC Pipe Products 183,257 197,840 185,132 Intersegment Eliminations (55 ) (57 ) (57 ) Total $ 919,503 $ 916,447 $ 849,350 Cost of Products Sold Manufacturing $ 215,179 $ 205,699 $ 176,473 Plastics 139,974 148,881 140,107 Intersegment Eliminations (34 ) (21 ) (18 ) Total $ 355,119 $ 354,559 $ 316,562 Other Nonelectric Expenses Manufacturing $ 29,895 $ 29,650 $ 23,785 Plastics 11,393 12,323 11,564 Corporate 9,515 9,607 6,182 Intersegment Eliminations (21 ) (36 ) (39 ) Total $ 50,782 $ 51,544 $ 41,492 (in thousands) 2019 2018 2017 Depreciation and Amortization Electric $ 60,044 $ 55,935 $ 53,276 Manufacturing 14,261 14,794 15,379 Plastics 3,451 3,719 3,817 Corporate 330 218 73 Total $ 78,086 $ 74,666 $ 72,545 Operating Income (Loss) Electric $ 98,417 $ 88,031 $ 94,797 Manufacturing 17,869 18,266 14,101 Plastics 28,439 32,917 29,644 Corporate (9,845 ) (9,825 ) (6,255 ) Total $ 134,880 $ 129,389 $ 132,287 Interest Charges Electric $ 26,548 $ 26,365 $ 25,334 Manufacturing 2,345 2,230 2,215 Plastics 718 609 633 Corporate and Intersegment Eliminations 1,800 1,204 1,422 Total $ 31,411 $ 30,408 $ 29,604 Income Tax Expense (Benefit) Electric $ 12,867 $ 5,685 $ 17,013 Manufacturing 2,784 3,393 989 Plastics 7,309 8,728 7,448 Corporate (5,519 ) (3,218 ) 1,806 Total $ 17,441 $ 14,588 $ 27,256 Net Income (Loss) Electric $ 59,046 $ 54,431 $ 49,446 Manufacturing 12,899 12,839 11,050 Plastics 20,572 23,819 21,696 Corporate (5,670 ) (8,744 ) (9,753 ) Total $ 86,847 $ 82,345 $ 72,439 Capital Expenditures Electric $ 187,362 $ 87,287 $ 118,444 Manufacturing 14,268 13,316 9,916 Plastics 5,452 4,199 4,432 Corporate 283 623 121 Total $ 207,365 $ 105,425 $ 132,913 Identifiable Assets Electric $ 1,931,525 $ 1,728,534 $ 1,690,224 Manufacturing 195,742 187,556 167,023 Plastics 92,049 91,630 87,230 Corporate 54,279 44,797 59,801 Total $ 2,273,595 $ 2,052,517 $ 2,004,278

Note 3 - Rate and Regulatory _2

Note 3 - Rate and Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2019
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 2018 Incentive and Cost Recovery A – December 27, 2019 January 1, 2020 $ 11,926 $0.00710 /kwh 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 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 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 Renewable Resource Adjustment 2019 Annual Update – Revised A – December 19, 2019 January 1, 2020 $ 12,506 $0.00467 /kwh 2018 Annual Update A – August 29, 2018 November 1, 2018 $ 5,886 $0.00219 /kwh 2017 Rate Reset A – October 30, 2017 November 1, 2017 $ 1,279 $0.00049 /kwh North Dakota Renewable Resource Adjustment 2020 Annual Update R – December 31, 2019 April 1, 2020 $ 3,828 3.744% of base 2019 Annual Update A – May 1, 2019 June 1, 2019 $ (235 ) -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 Transmission Cost Recovery 2019 Annual Update A – December 18, 2019 January 1, 2020 $ 5,739 Various 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 Environmental Cost Recovery 2019 Update A – October 22, 2019 November 1, 2019 $ - 0% of base 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 Generation Cost Recovery 2019 Initial Request A – May 15, 2019 July 1, 2019 $ 2,720 2.547% of base South Dakota Transmission Cost Recovery 2020 Annual Update R – October 31, 2019 March 1, 2020 $ 2,407 Various 2019 Rate Reset A – September 17, 2019 October 1, 2019 $ 2,046 Various 2019 Annual Update A – February 20, 2019 March 1, 2019 $ 1,638 Various 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 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 Phase-In Rate Plan Recovery 2019 Initial Request A – August 21, 2019 September 1, 2019 $ 864 3.345% of base
Schedule of Revenues Recorded under Rate Riders [Table Text Block]	Rate Rider (in thousands) 2019 2018 2017 Minnesota Conservation Improvement Program Costs and Incentives $ 8,271 $ 8,127 $ 6,008 Renewable Resource Adjustment 5,513 3,067 (196 ) Transmission Cost Recovery 2,497 (2,039 ) 2,973 Environmental Cost Recovery (1 ) (24 ) 8,148 North Dakota Transmission Cost Recovery 5,292 7,016 8,729 Generation Cost Recovery 878 - - Environmental Cost Recovery 550 7,318 9,782 Renewable Resource Adjustment 230 8,529 7,620 South Dakota Transmission Cost Recovery 2,165 1,664 1,843 Conservation Improvement Program Costs and Incentives 851 628 598 Environmental Cost Recovery (29 ) 1,676 2,345 Phase-In Rate Plan (125 ) - - Total $ 26,092 $ 35,962 $ 47,850

Note 4 - Regulatory Assets an_2

Note 4 - Regulatory Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2019
Notes Tables
Schedule of Regulatory Assets and Liabilities [Table Text Block]	December 31, 2019 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 $ 129,102 $ 138,192 see below Accumulated Asset Retirement Obligation (ARO) Accretion/Depreciation Adjustment 1 - 7,772 7,772 asset lives Conservation Improvement Program Costs and Incentives 2 4,024 2,844 6,868 21 Minnesota Transmission Cost Recovery Rider Accrued Revenues 2 4,208 - 4,208 12 MISO Schedule 26/26A Transmission Cost Recovery Rider True-ups 1 2,033 968 3,001 24 Nonservice Costs Components of Postretirement Benefits Capitalized for Ratemaking Purposes and Subject to Deferred Recovery 1 - 1,681 1,681 asset lives Big Stone II Unrecovered Project Costs – Minnesota 1 715 225 940 16 Debt Reacquisition Premiums 1 201 548 749 153 Deferred Marked-to-Market Losses 1 743 - 743 12 Big Stone II Unrecovered Project Costs – South Dakota 1 144 253 397 33 South Dakota Deferred Rate Case Expenses Subject to Recovery 1 138 245 383 34 North Dakota Deferred Rate Case Expenses Subject to Recovery 1 122 244 366 36 Minnesota SPP Transmission Cost Recovery Tracker 1 - 202 202 see below Minnesota Renewable Resource Rider Accrued Revenues 2 131 - 131 12 South Dakota Transmission Cost Recovery Rider Accrued Revenues 2 97 - 97 2 Deferred Lease Expenses 1 - 54 54 39 Minnesota Environmental Cost Recovery Rider Accrued Revenues 2 4 - 4 12 Total Regulatory Assets $ 21,650 $ 144,138 $ 165,788 Regulatory Liabilities: Deferred Income Taxes $ - $ 141,707 $ 141,707 asset lives Accumulated Reserve for Estimated Removal Costs – Net of Salvage - 97,726 97,726 asset lives Refundable Fuel Clause Adjustment Revenues 3,982 - 3,982 12 North Dakota Renewable Resource Recovery Rider Accrued Refund 1,515 - 1,515 12 North Dakota Transmission Cost Recovery Rider Accrued Refund 700 - 700 12 Prior Service Costs and Actuarial Gains on Postretirement Benefits 471 - 471 12 Revenue for Rate Case Expenses Subject to Refund – Minnesota - 401 401 see below South Dakota Phase-In Rate Plan Rider Accrued Refund 355 - 355 9 North Dakota Generation Cost Recovery Rider Accrued Refund 287 - 287 6 Minnesota Energy Intensive Trade Exposed Rider Accrued Refund 164 - 164 12 Other 6 72 78 168 Total Regulatory Liabilities $ 7,480 $ 239,906 $ 247,386 Net Regulatory Asset/(Liability) Position $ 14,170 $ (95,768 ) $ (81,598 ) 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 Accumulated ARO Accretion/Depreciation Adjustment 1 - 7,169 7,169 asset lives Conservation Improvement Program Costs and Incentives 2 5,995 3,285 9,280 21 Minnesota Transmission Cost Recovery Rider Accrued Revenues 2 444 - 444 12 MISO Schedule 26/26A Transmission Cost Recovery Rider True-up 1 240 - 240 12 Nonservice Costs Components of Postretirement Benefits Capitalized for Ratemaking Purposes and Subject to Deferred Recovery 1 - 986 986 asset lives Big Stone II Unrecovered Project Costs – Minnesota 1 681 947 1,628 28 Debt Reacquisition Premiums 1 207 753 960 165 Deferred Marked-to-Market Losses 1 1,661 743 2,404 24 Big Stone II Unrecovered Project Costs – South Dakota 1 100 342 442 53 South Dakota Deferred Rate Case Expenses Subject to Recovery 1 178 - 178 12 North Dakota Deferred Rate Case Expenses Subject to Recovery 1 455 - 455 12 Minnesota SPP Transmission Cost Recovery Tracker 1 - 176 176 see below Minnesota Renewable Resource Recovery Rider Accrued Revenues 2 452 - 452 12 Minnesota Environmental Cost Recovery Rider Accrued Revenues 2 121 - 121 12 Deferred Income Taxes 1 - 2,423 2,423 asset lives Minnesota Energy Intensive Trade Exposed Rider Accrued Revenues 1 328 - 328 4 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 Refundable Fuel Clause Adjustment Revenues 121 - 121 12 North Dakota Renewable Resource Recovery Rider Accrued Refund 177 - 177 12 North Dakota Transmission Cost Recovery Rider Accrued Refund 60 - 60 12 Revenue for Rate Case Expenses Subject to Refund – Minnesota - 166 166 see below MISO Schedule 26/26A Transmission Cost Recovery Rider True-up - 187 187 24 South Dakota Transmission Cost Recovery Rider Accrued Refund 168 - 168 12 South Dakota Environmental Cost Recovery Rider Accrued Refund 207 - 207 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 )

Note 5 - Common Shares and Ea_2

Note 5 - Common Shares and Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2019
Notes Tables
Schedule of Common Stock Outstanding Roll Forward [Table Text Block]	Common Shares Outstanding, December 31, 2018 39,664,884 Issuances: At-the-Market Offering 347,000 Executive Stock Performance Awards (2016 awards earned) 102,198 Automatic Dividend Reinvestment and Share Purchase Plan: Dividends Reinvested 29,599 Cash Invested 23,740 Vesting of Restricted Stock Units 29,100 Restricted Stock Issued to Directors 15,700 Directors Deferred Compensation 594 Retirements: Shares Withheld for Individual Income Tax Requirements (55,224 ) Common Shares Outstanding, December 31, 2019 40,157,591
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]	2019 2018 2017 Weighted Average Common Shares Outstanding – Basic 39,720,847 39,599,944 39,457,261 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 147,001 212,043 210,784 Underlying Shares Related to Nonvested Restricted Stock Units Granted to Employees 65,421 59,980 56,952 Nonvested Restricted Shares 15,377 17,751 20,380 Shares Expected to be Issued Under the Employee Stock Purchase Plan 3,228 - - Shares Expected to be Issued Under the Deferred Compensation Program for Directors 1,952 2,478 2,970 Total Dilutive Shares 232,979 292,252 291,086 Weighted Average Common Shares Outstanding – Diluted 39,953,826 39,892,196 39,748,347

Note 6 - Share-based Payments (

Note 6 - Share-based Payments (Tables)	12 Months Ended
	Dec. 31, 2019
Notes Tables
Share-based Payment Arrangement, Nonemployee Director Award Plan, Activity [Table Text Block]	Directors’ Restricted Stock Awards 2019 2018 2017 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 43,225 $ 37.53 46,800 $ 32.65 46,334 $ 29.71 Granted 15,700 49.73 18,200 43.40 17,600 37.75 Vested 18,320 36.02 21,775 31.94 17,134 29.93 Forfeited - - - Nonvested, End of Year 40,605 42.93 43,225 37.53 46,800 32.65 Compensation Expense Recognized $ 776,000 $ 661,000 $ 658,000 Fair Value of Shares Vested in Year $ 660,000 $ 696,000 $ 513,000
Schedule of Share-based Compensation, Restricted Stock Activity [Table Text Block]	Employees’ Restricted Stock Awards 2019 2018 2017 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 Granted - - - Vested - - 2,895 29.41 4,285 29.94 Forfeited - - - Nonvested, End of Year - - - - 2,895 29.41 Compensation Expense Recognized $ - $ 16,000 $ 70,000 Fair Value of Awards Vested $ - $ 85,000 $ 128,000
Share-based Payment Arrangement, Restricted Stock Unit, Activity [Table Text Block]	Employees’ Restricted Stock Unit Awards 2019 2018 2017 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 49,470 $ 31.03 46,440 $ 27.07 47,370 $ 25.19 Granted 13,270 44.45 14,780 38.99 10,995 33.28 Vested 13,250 27.62 8,925 25.23 11,550 25.30 Forfeited 1,900 34.57 2,825 25.86 375 26.92 Nonvested, End of Year 47,590 35.58 49,470 31.03 46,440 27.07 Compensation Expense Recognized $ 427,000 $ 351,000 $ 331,000 Fair Value of Awards Vested $ 366,000 $ 225,000 $ 292,000
Executive Officers and Key Employees [Member]
Notes Tables
Schedule of Share-based Compensation, Restricted Stock Activity [Table Text Block]	Dividend Equivalent Restricted Stock Unit Awards 2019 2018 2017 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 45,300 $ 35.70 47,750 $ 32.71 41,825 $ 30.23 Granted 16,200 49.72 15,200 41.325 15,900 37.65 Vested 15,850 34.46 17,650 32.462 9,975 30.16 Forfeited - - - Nonvested, End of Year 45,650 41.10 45,300 35.70 47,750 32.71 Compensation Expense Recognized $ 1,055,000 $ 769,000 $ 576,000 Fair Value of Awards Vested $ 546,000 $ 573,000 $ 301,000
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 2019 2018 2017 2019-2021 83,400 55,600 $ 2,168,000 2018-2020 81,000 54,000 897,000 $ 1,121,000 2017-2019 89,250 59,500 524,000 729,000 $ 854,000 69,997 2016-2018 122,250 81,500 8,000 772,000 580,000 113,298 2015-2017 126,450 84,300 - 23,000 573,000 114,648 Total $ 3,597,000 $ 2,645,000 $ 2,007,000 297,943

Note 8 - Leases (Tables)

Note 8 - Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Notes Tables
Leases, Right-of-use Assets and Lease Liabilities By Business Segment [Table Text Block]	(in thousands) Electric Manufacturing Plastics Corporate Total Right of Use Assets – Operating Leases: Gross $ 4,137 $ 20,347 $ 666 $ 769 $ 25,919 Accumulated Amortization (1,166 ) (2,375 ) (395 ) (132 ) (4,068 ) Net of Accumulated Amortization $ 2,971 $ 17,972 $ 271 $ 637 $ 21,851 Obligations: Current Operating Lease Liabilities $ 1,116 $ 2,609 $ 256 $ 155 $ 4,136 Long-Term Operating Lease Liabilities 2,176 15,470 15 532 18,193 Total Lease Liabilities $ 3,292 $ 18,079 $ 271 $ 687 $ 22,329
Lessee, Operating Lease, Liability, Maturity [Table Text Block]	Right-of-Use Operating Leases (in thousands) OTP Nonelectric Total 2020 $ 1,243 $ 3,969 $ 5,212 2021 1,228 3,717 4,945 2022 326 3,588 3,914 2023 268 3,303 3,571 2024 217 2,870 3,087 Beyond 2024 283 5,304 5,587 Total Minimum Obligations $ 3,565 $ 22,751 $ 26,316 Interest Component of Obligations (273 ) (3,714 ) (3,987 ) Present Value of Minimum Obligations, December 31, 2019 $ 3,292 $ 19,037 $ 22,329
Lessee, Operating Lease, Lease Obligation Activity [Table Text Block]	(in thousands) OTP Nonelectric Total Operating Lease Obligations, January 1, 2019 $ 3,609 $ 16,760 $ 20,369 Non-cash Acquisition of Right-of-Use Assets 758 5,560 6,318 Lease Adjustments/Modifications 71 (187 ) (116 ) Lease Obligation Payments (1,316 ) (4,055 ) (5,371 ) Interest Component of Lease Obligation Payment 170 959 1,129 Operating Lease Obligations, December 31, 2019 $ 3,292 $ 19,037 $ 22,329
Lease, Cost [Table Text Block]	Operating Lease Cost Variable Lease Cost Total Lease Cost Plant in Service or CWIP $ 29 $ - $ 29 Inventory 952 - 952 Cost of Products Sold 3,848 1,068 4,916 Electric Operation and Maintenance Expenses 334 - 334 Other Nonelectric Expenses 208 - 208 Total $ 5,371 $ 1,068 $ 6,439

Note 9 - Commitments and Cont_2

Note 9 - Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2019
Notes Tables
Contractual Obligation, Fiscal Year Maturity [Table Text Block]	(in thousands) Construction Program and Other Commitments Capacity and Energy Requirements Coal Purchase Commitments OTP Land Easement Payments 2020 $ 269,774 $ 24,844 $ 22,644 $ 630 2021 47,230 12,988 22,935 642 2022 22 11,827 22,793 655 2023 11 11,827 23,955 668 2024 6 11,801 24,369 682 Beyond 2024 - 131,913 479,123 6,931 Total $ 317,043 $ 205,200 $ 595,819 $ 10,208

Note 10 - Short-term and Long_2

Note 10 - Short-term and Long-term Borrowings (Tables)	12 Months Ended
	Dec. 31, 2019
Notes Tables
Schedule of Line of Credit Facilities [Table Text Block]	(in thousands) Line Limit In Use on December 31, 2019 Restricted due to Outstanding Letters of Credit Available on December 31, 2019 Available on December 31, 2018 Otter Tail Corporation Credit Agreement $ 170,000 $ 6,000 $ - $ 164,000 $ 120,785 OTP Credit Agreement 170,000 - 15,476 154,524 160,316 Total $ 340,000 $ 6,000 $ 15,476 $ 318,524 $ 281,101
Schedule of Debt [Table Text Block]	December 31, 2019 (in thousands) OTP Otter Tail Corporation Otter Tail Corporation Consolidated Short-Term Debt $ - $ 6,000 $ 6,000 Long-Term Debt: 3.55% Guaranteed Senior Notes, due December 15, 2026 $ 80,000 $ 80,000 Senior Unsecured Notes 4.63%, Series 2011A, due December 1, 2021 $ 140,000 140,000 Senior Unsecured Notes 6.15%, Series 2007B, due August 20, 2022 30,000 30,000 Senior Unsecured Notes 6.37%, Series 2007C, due August 20, 2027 42,000 42,000 Senior Unsecured Notes 4.68%, Series 2013A, due February 27, 2029 60,000 60,000 Senior Unsecured Notes 3.07%, Series 2019A, due October 10, 2029 1 10,000 10,000 Senior Unsecured Notes 6.47%, Series 2007D, due August 20, 2037 50,000 50,000 Senior Unsecured Notes 3.52%, Series 2019B, due October 10, 2039 26,000 26,000 Senior Unsecured Notes 5.47%, Series 2013B, due February 27, 2044 90,000 90,000 Senior Unsecured Notes 4.07%, Series 2018A, due February 7, 2048 100,000 100,000 Senior Unsecured Notes 3.82%, Series 2019C, due October 10, 2049 64,000 64,000 PACE Note, 2.54%, due March 18, 2021 351 351 Total $ 612,000 $ 80,351 $ 692,351 Less: Current Maturities net of Unamortized Debt Issuance Costs - 183 183 Unamortized Long-Term Debt Issuance Costs 2,231 356 2,587 Total Long-Term Debt net of Unamortized Debt Issuance Costs $ 609,769 $ 79,812 $ 689,581 Total Short-Term and Long-Term Debt (with current maturities) $ 609,769 $ 85,995 $ 695,764 December 31, 2018 (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%, Series 2011A, due December 1, 2021 $ 140,000 140,000 Senior Unsecured Notes 6.15%, Series 2007B, due August 20, 2022 30,000 30,000 Senior Unsecured Notes 6.37%, Series 2007C, due August 20, 2027 42,000 42,000 Senior Unsecured Notes 4.68%, Series 2013A, due February 27, 2029 60,000 60,000 Senior Unsecured Notes 6.47%, Series 2007D, due August 20, 2037 50,000 50,000 Senior Unsecured Notes 5.47%, Series 2013B, 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
Schedule of Maturities of Long-term Debt [Table Text Block]	(in thousands) 2020 2021 2022 2023 2024 Aggregate Amounts of Debt Maturities $ 183 $ 140,168 $ 30,000 $ - $ -

Note 11 - Pension Plan and Ot_2

Note 11 - Pension Plan and Other Postretirement Benefits (Tables)	12 Months Ended
	Dec. 31, 2019
Notes Tables
Schedule of Net Benefit Costs [Table Text Block]	(in thousands) 2019 2018 2017 Service Cost–Benefit Earned During the Period $ 5,491 $ 6,459 $ 5,629 Interest Cost on Projected Benefit Obligation 14,412 13,452 14,139 Expected Return on Assets (21,297 ) (21,199 ) (19,229 ) Amortization of Prior Service Cost: From Regulatory Asset 5 16 120 From Other Comprehensive Income 1 9 - 3 Amortization of Net Actuarial Loss: From Regulatory Asset 4,642 7,135 5,090 From Other Comprehensive Income 1 114 183 125 Net Periodic Pension Cost 2 $ 3,376 $ 6,046 $ 5,877 2 Allocation of costs: 2019 2018 2017 Service c osts included in OTP capital expenditures $ 1,365 $ 1,542 $ 1,094 Service costs included in electric operation and maintenance expenses 3,994 4,756 4,400 Service costs included in other nonelectric expenses 132 161 135 Nonservice costs capitalized (526 ) (99 ) 48 Nonservice costs included in n onservice cost components of postretirement benefits (1,589 ) (314 ) 200 (in thousands) 2019 2018 2017 Service Cost–Benefit Earned During the Period $ 418 $ 408 $ 290 Interest Cost on Projected Benefit Obligation 1,735 1,589 1,686 Amortization of Prior Service Cost: From Regulatory Asset 5 20 16 From Other Comprehensive Income 1 17 34 38 Amortization of Net Actuarial Loss: From Regulatory Asset 124 206 285 From Other Comprehensive Income 1 348 722 440 Net Periodic Pension Cost 2 $ 2,647 $ 2,979 $ 2,755 2 Allocation of costs: 2019 2018 2017 Service costs included in electric operation and maintenance expenses $ 104 $ 99 $ 94 Service costs included in other nonelectric expenses 314 309 196 Nonservice costs included in n onservice cost components of postretirement benefits 2,229 2,571 2,465 (in thousands) 2019 2018 2017 Service Cost–Benefit Earned During the Period $ 1,286 $ 1,526 $ 1,425 Interest Cost on Projected Benefit Obligation 3,083 2,583 2,712 Amortization of Prior Service Cost From Regulatory Asset - - (4 ) From Other Comprehensive Income 1 - - 4 Amortization of Net Actuarial Loss From Regulatory Asset 1,571 1,648 936 From Other Comprehensive Income 1 38 42 19 Net Periodic Postretirement Benefit Cost 2 $ 5,978 $ 5,799 $ 5,092 Effect of Medicare Part D Subsidy $ (179 ) $ (470 ) $ (561 ) 2 Allocation of cost: 2019 2018 2017 Service costs included in OTP capital expenditures $ 320 $ 364 $ 277 Service costs included in electric operation and maintenance expenses 935 1,124 1,114 Service costs included in other nonelectric expenses 31 38 34 Nonservice costs capitalized 1,167 1,020 712 Nonservice costs included in n onservice cost components of postretirement benefits 3,525 3,253 2,955
Schedule of Amounts Recognized in Balance Sheet [Table Text Block]	(in thousands) 2019 2018 Regulatory Assets: Unrecognized Prior Service Cost $ - $ 5 Unrecognized Actuarial Loss 120,592 104,891 Total Regulatory Assets $ 120,592 $ 104,896 Accumulated Other Comprehensive Loss: Unrecognized Prior Service Cost $ - $ 9 Unrecognized Actuarial (Gain) Loss (82 ) 137 Total Accumulated Other Comprehensive Loss $ (82 ) $ 146 Noncurrent Liability $ 55,004 $ 58,659 (in thousands) 2019 2018 Regulatory Assets: Unrecognized Prior Service Cost $ - $ 20 Unrecognized Actuarial Loss 2,170 1,768 Total Regulatory Assets $ 2,170 $ 1,788 Projected Benefit Obligation Liability – Net Amount Recognized $ (43,966 ) $ (39,699 ) Accumulated Other Comprehensive Loss: Unrecognized Prior Service Cost $ 1 $ 64 Unrecognized Actuarial Loss 9,170 6,455 Total Accumulated Other Comprehensive Loss $ 9,171 $ 6,519 (in thousands) 2019 2018 Regulatory Asset: Unrecognized Prior Service Credit $ (20,363 ) - Unrecognized Net Actuarial Loss (Gain) 35,322 $ 18,094 Net Regulatory Asset $ 14,959 $ 18,094 Projected Benefit Obligation Liability – Net Amount Recognized $ (71,437 ) $ (71,561 ) Accumulated Other Comprehensive (Income) Loss: Unrecognized Prior Service Credit $ (501 ) - Unrecognized Net Actuarial Loss (Gain) 184 $ (107 ) Accumulated Other Comprehensive (Income) Loss: $ (317 ) $ (107 )
Schedule of Net Funded Status [Table Text Block]	(in thousands) 2019 2018 Accumulated Benefit Obligation $ (346,723 ) $ (297,972 ) Projected Benefit Obligation $ (384,785 ) $ (328,442 ) Fair Value of Plan Assets 329,781 269,783 Funded Status $ (55,004 ) $ (58,659 )
Schedule of Changes in Projected Benefit Obligations and Changes in Plan Assets [Table Text Block]	(in thousands) 2019 2018 Reconciliation of Fair Value of Plan Assets: Fair Value of Plan Assets at January 1 $ 269,783 $ 285,319 Actual Return on Plan Assets 52,640 (21,334 ) Discretionary Company Contributions 22,500 20,000 Benefit Payments (15,142 ) (14,202 ) Fair Value of Plan Assets at December 31 $ 329,781 $ 269,783 Estimated Asset Return 19.3 % (7.3 %) Reconciliation of Projected Benefit Obligation: Projected Benefit Obligation at January 1 $ 328,442 $ 352,718 Service Cost 5,491 6,459 Interest Cost 14,412 13,452 Benefit Payments (15,142 ) (14,202 ) Actuarial Loss (Gain) 51,582 (29,985 ) Projected Benefit Obligation at December 31 $ 384,785 $ 328,442 (in thousands) 2019 2018 Reconciliation of Fair Value of Plan Assets: Fair Value of Plan Assets at January 1 $ - $ - Actual Return on Plan Assets - - Employer Contributions 1,475 1,505 Benefit Payments (1,475 ) (1,505 ) Fair Value of Plan Assets at December 31 $ - $ - Reconciliation of Projected Benefit Obligation: Projected Benefit Obligation at January 1 $ 39,699 $ 42,308 Service Cost 418 408 Interest Cost 1,735 1,589 Benefit Payments (1,475 ) (1,505 ) Curtailments (1,671 ) - Actuarial Loss (Gain) 5,260 (3,101 ) Projected Benefit Obligation at December 31 $ 43,966 $ 39,699 (in thousands) 2019 2018 Reconciliation of Fair Value of Plan Assets: Fair Value of Plan Assets at January 1 $ - $ - Actual Return on Plan Assets - - Company Contributions 2,757 3,183 Benefit Payments (Net of Medicare Part D Subsidy) (7,164 ) (6,684 ) Participant Premium Payments 4,407 3,501 Fair Value of Plan Assets at December 31 $ - $ - Reconciliation of Projected Benefit Obligation: Projected Benefit Obligation at January 1 $ 71,561 $ 69,774 Service Cost (Net of Medicare Part D Subsidy) 1,286 1,526 Interest Cost (Net of Medicare Part D Subsidy) 3,083 2,583 Benefit Payments (Net of Medicare Part D Subsidy) (7,164 ) (6,684 ) Participant Premium Payments 4,407 3,501 Plan Amendments (20,864 ) - Actuarial Loss 19,128 861 Projected Benefit Obligation at December 31 $ 71,437 $ 71,561 Reconciliation of Accrued Postretirement Cost: Accrued Postretirement Cost at January 1 $ (53,574 ) $ (50,958 ) Expense (5,978 ) (5,799 ) Net Company Contribution 2,757 3,183 Accrued Postretirement Cost at December 31 $ (56,795 ) $ (53,574 )
Defined Benefit Plan, Assumptions [Table Text Block]	2019 2018 Discount Rate 3.47 % 4.50 % 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 % 2019 2018 Discount Rate 3.36 % 4.46 % Rate of Increase in Future Compensation Level: 3.50 % 3.40 % 2019 2018 Discount Rate 3.43 % 4.44 %
Schedule of Measurement Dates [Table Text Block]	Measurement Dates: 2019 2018 Net Periodic Pension Cost January 1, 2019 January 1, 2018 End of Year Benefit Obligations January 1, 2019 projected to December 31, 2019 January 1, 2018 projected to December 31, 2018 Market Value of Assets December 31, 2019 December 31, 2018 Measurement Dates: 2019 2018 Net Periodic Postretirement Benefit Cost January 1, 2019 January 1, 2018 End of Year Benefit Obligations January 1, 2019 projected to December 31, 2019 January 1, 2018 projected to December 31, 2018
Schedule of Expected Benefit Payments [Table Text Block]	(in thousands) 2020 2021 2022 2023 2024 Years 2025-2029 $ 15,908 $ 16,477 $ 17,116 $ 17,768 $ 18,374 $ 98,994 Years (in thousands) 2020 2021 2022 2023 2024 2025-2029 $ 1,571 $ 1,681 $ 2,279 $ 2,680 $ 2,627 $ 13,976 Years (in thousands) 2020 2021 2022 2023 2024 2025-2029 $ 3,323 $ 3,470 $ 3,653 $ 3,720 $ 3,778 $ 19,852
Schedule of Weighted Average Asset Allocation [Table Text Block]	Asset Allocation 2019 2018 Global MGD Volatility Fund ( mixed equities fund 20.4 % - Large Capitalization Equity Securities 11.3 % 17.5 % International Equity Securities 9.3 % 17.0 % Emerging Markets Equity Fund 4.2 % 3.4 % Small and Mid-Capitalization Equity Securities 4.1 % 6.7 % SEI Dynamic Asset Allocation Fund 3.1 % 4.0 % Equity Securities 52.4 % 48.6 % Fixed-Income Securities and Cash 44.7 % 47.1 % Other – SEI Energy Debt Collective Fund 2.9 % 4.3 % 100.0 % 100.0 %
Schedule of Health Care Cost Trend Rates [Table Text Block]	2019 2018 Healthcare Cost-Trend Rate Assumed for Next Year 6.72 % 7.00 % 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
Pension Plan [Member]
Notes Tables
Schedule of Assumptions Used in Calculating Net Periodic Benefit Cost [Table Text Block]	2019 2018 2017 Discount Rate 4.50 % 3.90 % 4.60 % Long-Term Rate of Return on Plan Assets 7.25 % 7.50 % 7.50 % Rate of Increase in Future Compensation Level See below See below 3.00 % 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 % 2019 2018 2017 Discount Rate 4.46 % 3.85 % 4.60 % Rate of Increase in Future Compensation Level 3.40 % 2.92 % 3.00 % 2019 2018 2017 Discount Rate 4.44 % 3.81 % 4.46 %
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) 2019 2018 Assets in Level 1 of the Fair Value Hierarchy $ 320,241 $ 258,307 SEI Energy Debt Collective Fund at NAV 9,540 11,476 Total Assets $ 329,781 $ 269,783
Fair Value Measurement Inputs and Valuation Techniques [Table Text Block]	(in thousands) 2019 2018 Global MGD Volatility Fund ( mixed equities fund $ 67,184 $ - Large Capitalization Equity Securities Mutual Fund 37,357 47,198 International Equity Securities Mutual Funds 30,653 45,912 Small and Mid-Capitalization Equity Securities Mutual Fund 13,447 17,971 SEI Dynamic Asset Allocation Mutual Fund 10,168 10,929 Emerging Markets Equity Fund 13,792 9,197 Fixed Income Securities Mutual Funds 147,639 127,098 Cash Management – Money Market Fund 1 2 Total Assets $ 320,241 $ 258,307

Note 12 - Fair Value of Finan_2

Note 12 - Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2019
Notes Tables
Fair Value, by Balance Sheet Grouping [Table Text Block]	December 31, 2019 December 31, 2018 (in thousands) Carrying Amount Fair Value Carrying Amount Fair Value Cash and Cash Equivalents $ 21,199 $ 21,199 $ 861 $ 861 Short-Term Debt (6,000 ) (6,000 ) (18,599 ) (18,599 ) Long-Term Debt including Current Maturities (689,764 ) (742,279 ) (590,174 ) (601,513 )

Note 13 - Property, Plant and_2

Note 13 - Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2019
Notes Tables
Property, Plant and Equipment [Table Text Block]	(in thousands) December 31, 2019 December 31, 2018 Electric Plant in Service Production $ 915,996 $ 905,224 Transmission 647,474 512,832 Distribution 526,146 502,261 General 123,268 99,404 Electric Plant in Service 2,212,884 2,019,721 Construction Work in Progress 177,584 170,090 Total Gross Electric Plant 2,390,468 2,189,811 Less Accumulated Depreciation and Amortization 731,110 699,642 Net Electric Plant $ 1,659,358 $ 1,490,169 Nonelectric Operations Plant Equipment $ 187,904 $ 170,634 Buildings and Leasehold Improvements 53,412 53,011 Land 6,040 4,475 Nonelectric Operations Plant 247,356 228,120 Construction Work in Progress 7,654 11,536 Total Gross Nonelectric Plant 255,010 239,656 Less Accumulated Depreciation and Amortization 160,574 148,727 Net Nonelectric Operations Plant $ 94,436 $ 90,929 Net Plant $ 1,753,794 $ 1,581,098
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 51 75 Distribution Plant 15 70 General Plant 5 57 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, 2019
Notes Tables
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]	(in thousands) 2019 2018 2017 Tax Computed at Federal Statutory Rate $ 21,901 $ 20,356 $ 34,893 Increases (Decreases) in Tax from: State Income Taxes Net of Federal Income Tax Expense 3,561 5,210 4,368 Differences Reversing in Excess of Federal Rates (3,357 ) (3,432 ) 551 Permanent Differences, R&D Tax Credits, Unitary Tax and Other Adjustments (1,315 ) (1,864 ) (1,873 ) North Dakota Wind Tax Credit Amortization – Net of Federal Taxes (1,033 ) (1,033 ) (850 ) Corporate-owned Life Insurance (749 ) (3 ) (845 ) Excess Tax deduction – Equity Method Stock Awards (744 ) (708 ) (751 ) Allowance for Funds Used During Construction – Equity (501 ) (431 ) (322 ) Employee Stock Ownership Plan Dividend Deduction (281 ) (298 ) (509 ) Investment Tax Credit Amortization (41 ) (98 ) (164 ) Federal Production Tax Credits (PTCs) - (3,111 ) (7,527 ) Section 199 Domestic Production Activities Deduction - - (1,471 ) Effect of TCJA Tax Rate Reduction on Value of Net Deferred Tax Assets - - 1,756 Income Tax Expense $ 17,441 $ 14,588 $ 27,256 Overall Effective Federal and State Income Tax Rate 16.7 % 15.0 % 27.3 % Income Tax Expense Includes the Following: Current Federal Income Taxes $ 5,156 $ 4,960 $ 4,434 Current State Income Taxes 1,333 1,395 1,128 Deferred Federal Income Taxes 8,859 8,065 25,648 Deferred State Income Taxes 3,167 4,410 4,587 Federal PTCs - (3,111 ) (7,527 ) North Dakota Wind Tax Credit Amortization – Net of Federal Taxes (1,033 ) (1,033 ) (850 ) Investment Tax Credit Amortization (41 ) (98 ) (164 ) Total $ 17,441 $ 14,588 $ 27,256 Total Income Before Income Taxes $ 104,288 $ 96,933 $ 99,695
Schedule of Deferred Tax Assets and Liabilities [Table Text Block]	(in thousands) 2019 2018 Deferred Tax Assets Benefit Liabilities $ 36,246 $ 33,967 Retirement Benefits Liabilities 36,206 32,664 Regulatory Tax Liability 35,700 33,228 North Dakota Wind Tax Credits 31,611 32,570 Cost of Removal 25,604 21,787 Federal PTCs 20,017 32,101 Differences Related to Property 6,979 6,842 Lease Liability 5,733 - Vacation Accrual 1,884 1,919 Net Operating Loss Carryforward 1,860 2,489 Investment Tax Credits 408 449 Other 344 3,218 Valuation Allowance (800 ) (600 ) Total Deferred Tax Assets $ 201,792 $ 200,634 Deferred Tax Liabilities Differences Related to Property $ (268,495 ) $ (261,396 ) Retirement Benefits Regulatory Asset (36,206 ) (32,664 ) Excess Tax over Book Pension (17,556 ) (15,145 ) Right of Use Asset (5,705 ) - North Dakota Wind Tax Credits (3,126 ) (4,386 ) Impact of State Net Operating Losses on Federal Taxes (385 ) (523 ) Other (2,260 ) (7,496 ) Total Deferred Tax Liabilities $ (333,733 ) $ (321,610 ) Deferred Income Taxes $ (131,941 ) $ (120,976 )
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 $ 23,002 $ - $ 22,220 $ 782 State Net Operating Losses 1,860 1,833 27 - State Tax Credits 32,177 - 2,643 29,534
Summary of Income Tax Contingencies [Table Text Block]	(in thousands) 2019 2018 2017 Balance on January 1 $ 1,282 $ 684 $ 891 Increases Related to Tax Positions for Prior Years 37 6 28 Decreases Related to Tax Positions for Prior Years - - (172 ) Increases Related to Tax Positions for Current Year 339 778 143 Uncertain Positions Resolved During Year (170 ) (186 ) (206 ) Balance on December 31 $ 1,488 $ 1,282 $ 684

Note 15 - Asset Retirement Ob_2

Note 15 - Asset Retirement Obligations (AROs) (Tables)	12 Months Ended
	Dec. 31, 2019
Notes Tables
Schedule of Change in Asset Retirement Obligation [Table Text Block]	(in thousands) 2019 2018 Asset Retirement Obligations Beginning Balance $ 9,117 $ 8,719 New Obligations Recognized - - Adjustments Due to Revisions in Cash Flow Estimates 3,099 - Accrued Accretion 440 398 Settlements - - Ending Balance $ 12,656 $ 9,117 Asset Retirement Costs Capitalized Beginning Balance $ 2,983 $ 2,983 New Obligations Recognized - - Adjustments Due to Revisions in Cash Flow Estimates 3,099 - Settlements - - Ending Balance $ 6,082 $ 2,983 Accumulated Depreciation – Asset Retirement Costs Capitalized Beginning Balance $ 1,034 $ 915 New Obligations Recognized - - Adjustments Due to Revisions in Cash Flow Estimates - - Depreciation Expense 163 119 Settlements - - Ending Balance $ 1,197 $ 1,034 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, 2019
Notes Tables
Schedule of Subsequent Events [Table Text Block]	Award Shares/Units Granted Vesting Restricted Stock Units Granted 15,300 25% per year through February 6, 2024 Stock Performance Awards Granted: Under Executive Agreement 47,600 December 31, 2022 Under Legacy Agreement 7,400 December 31, 2022

Schedule 1 - Condensed Financ_2

Schedule 1 - Condensed Financial Information of Registrant (Tables)	12 Months Ended
	Dec. 31, 2019
Notes Tables
Schedule of Related Party Transactions [Table Text Block]	As of December 31, 2019: (in thousands) Accounts Receivable Interest Receivable Current Notes Receivable Long-Term Notes Receivable Accounts Payable Current Notes Payable Otter Tail Power Company $ 2,056 $ - $ - $ - $ 7 $ - Northern Pipe Products, Inc. - 8 - 5,351 - 3,056 Vinyltech Corporation 4 17 - 11,500 - 15,099 BTD Manufacturing, Inc. - 77 - 52,000 - 18,474 T.O. Plastics, Inc. - 15 - 10,400 - 3,099 Varistar Corporation - - - - - 49,883 Otter Tail Assurance Limited 84 - - - - - $ 2,144 $ 117 $ - $ 79,251 $ 7 $ 89,611 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 $ - Northern Pipe Products, Inc. - 8 - 5,522 - 5,623 Vinyltech Corporation 4 17 - 11,500 - 15,305 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
Cash Dividends Paid to Parent by Subsidiaries [Table Text Block]	(in thousands) 2019 2018 2017 Cash Dividends Paid to Parent by Subsidiaries $ 55,660 $ 53,134 $ 50,571
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) 2019 2018 Assets Current Assets Cash and Cash Equivalents $ 4,959 $ - Accounts Receivable - - Accounts Receivable from Subsidiaries 2,144 1,931 Interest Receivable from Subsidiaries 117 117 Notes Receivable from Subsidiaries - 1,167 Income Taxes Receivable 1,487 - Other 1,050 3,482 Total Current Assets 9,757 6,697 Investments in Subsidiaries 860,646 787,869 Notes Receivable from Subsidiaries 79,251 79,422 Deferred Income Taxes 25,505 21,100 Right of Use Assets - Operating 637 - Other Assets 35,503 31,547 Total Assets $ 1,011,299 $ 926,635 Liabilities and Equity Current Liabilities Short-Term Debt $ 6,000 $ 9,215 Current Maturities of Long-Term Debt 183 172 Accounts Payable to Subsidiaries 7 7 Notes Payable to Subsidiaries 89,611 60,626 Current Operating Lease Liabilities 156 - Other 9,473 9,994 Total Current Liabilities 105,430 80,014 Long Term Operating Lease Liabilities 533 - Other Noncurrent Liabilities 44,042 37,814 Commitments and Contingencies Capitalization Long-Term Debt, Net of Current Maturities 79,812 79,944 Common Shareholder Equity 781,482 728,863 Total Capitalization 861,294 808,807 Total Liabilities and Equity $ 1,011,299 $ 926,635
Condensed Income Statement [Table Text Block]	Otter Tail Corporation (PARENT COMPANY) Condensed Statements of Income--For the Years Ended December 31 (in thousands) 2019 2018 2017 Operating Loss Revenue from Contracts with Customers $ - $ - $ - Operating Expenses 10,529 9,916 7,138 Operating Loss (10,529 ) (9,916 ) (7,138 ) Other Income (Expense) Equity Income in Earnings of Subsidiaries 93,731 91,446 82,715 Interest Charges (4,863 ) (4,043 ) (4,270 ) Interest Charges to Subsidiaries (306 ) (387 ) (244 ) Interest Income from Subsidiaries 3,063 2,839 2,848 Nonservice Cost Components of Postretirement Benefits (1,297 ) (1,422 ) (1,215 ) Other Income 1,566 550 1,054 Total Other Income 91,894 88,983 80,888 Income Before Income Taxes 81,365 79,067 73,750 Income Tax (Benefit) Expense (5,482 ) (3,278 ) 1,311 Net Income $ 86,847 $ 82,345 $ 72,439
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) 2019 2018 2017 Cash Flows from Operating Activities Net Cash Provided by Operating Activities $ 52,263 $ 56,947 $ 50,205 Cash Flows from Investing Activities Investment in Subsidiaries (34,990 ) (24,764 ) - Debt Repaid by Subsidiaries 1,338 774 151 Cash Used in Investing Activities (257 ) (623 ) (121 ) Net Cash (Used in) Provided by Investing Activities (33,909 ) (24,613 ) 30 Cash Flows from Financing Activities Change in Checks Written in Excess of Cash (31 ) 31 - Net Short-Term (Repayments) Borrowings (3,215 ) 9,215 - Borrowings from (Repayments to) Subsidiaries 28,985 (1,281 ) 23,389 Proceeds from Issuance of Common Stock 20,338 - 4,349 Common Stock Issuance Expenses (577 ) (108 ) - Payments for Retirement of Capital Stock (2,730 ) (3,011 ) (1,799 ) Short-Term and Long-Term Debt Issuance Expenses (270 ) (164 ) (158 ) Payments for Retirement of Long-Term Debt (172 ) (189 ) (15,231 ) Dividends Paid (55,723 ) (53,198 ) (50,632 ) Net Cash Used in Financing Activities (13,395 ) (48,705 ) (40,082 ) Net Change in Cash and Cash Equivalents 4,959 (16,371 ) 10,153 Cash and Cash Equivalents at Beginning of Period - 16,371 6,218 Cash and Cash Equivalents at End of Period $ 4,959 $ - $ 16,371

Note 1 - Summary of Significa_3

Note 1 - Summary of Significant Accounting Policies (Details Textual) - USD ($)	Jan. 01, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Standard Product Warranty Term		1 year
Standard Product Warranty Accrual, Decrease for Payments		$ 0	$ 0	$ 0
Accounting Standards Update 2018-02 [Member]
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	$ 784,000
Otter Tail Power Company [Member] | Coyote Creek Mining Company, L.L.C. (CCMC) [Member] | Lignite Sales Agreement [Member]
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount		$ 50,400,000
Variable Interest Entity Reporting Entity Involvement, Maximum Loss Exposure, Percentage		35.00%
Electric Plant [Member]
Interest Costs Capitalized		$ 1,728,000	$ 1,206,000	$ 741,000
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service		2.81%	2.76%	2.74%
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

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, 2019	Dec. 31, 2018
Big Stone Plant [Member]
Electric Plant in Service		$ 337,197	$ 336,051
Construction Work in Progress		384	361
Accumulated Depreciation		(98,654)	(92,007)
Net Plant		$ 238,927	$ 244,405
Big Stone Plant [Member] | Otter Tail Power Company [Member]
Ownership Percentage		53.90%	53.90%
Coyote Station [Member]
Electric Plant in Service		$ 184,493	$ 177,713
Construction Work in Progress		83	2,588
Accumulated Depreciation		(108,248)	(100,997)
Net Plant		$ 76,328	$ 79,304
Coyote Station [Member] | Otter Tail Power Company [Member]
Ownership Percentage		35.00%	35.00%
Big Stone South Ellendale Multi Value Project [Member]
Electric Plant in Service	[1]	$ 106,343	$ 0
Construction Work in Progress	[1]	0	106,490
Accumulated Depreciation	[1]	(819)	0
Net Plant	[1]	$ 105,524	$ 106,490
Big Stone South Ellendale Multi Value Project [Member] | Otter Tail Power Company [Member]
Ownership Percentage	[1]	50.00%	50.00%
Fargo Project [Member]
Electric Plant in Service		$ 78,184	$ 78,184
Construction Work in Progress		0	0
Accumulated Depreciation		(7,011)	(5,891)	[1]
Net Plant		$ 71,173	$ 72,293
Fargo Project [Member] | Otter Tail Power Company [Member]
Ownership Percentage		14.20%	14.20%
Big Stone South Brookings Multi Value Project [Member]
Electric Plant in Service		$ 53,036	$ 53,235
Construction Work in Progress		0	(150)
Accumulated Depreciation		(2,016)	(1,264)
Net Plant		$ 51,020	$ 51,821
Big Stone South Brookings Multi Value Project [Member] | Otter Tail Power Company [Member]
Ownership Percentage		50.00%	50.00%
Brookings Project [Member]
Electric Plant in Service		$ 26,286	$ 26,281
Construction Work in Progress		0	0
Accumulated Depreciation		(2,086)	(1,713)
Net Plant		$ 24,200	$ 24,568
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		0	0
Accumulated Depreciation		(233)	(2,091)
Net Plant		$ 16,098	$ 14,240
Bemidji Project [Member] | Otter Tail Power Company [Member]
Ownership Percentage		14.80%	14.80%
[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, 2019	Dec. 31, 2018
Total Investments	$ 9,894	$ 8,961
Economic Development Loan Pools [Member]
Cost Method	24	34
Other Cost Method [Member]
Cost Method	73	123
Partnerships [Member]
Equity Method Partnerships	27	26
Debt Securities [Member]
Marketable Securities Classified as Available-for-Sale	8,184	7,484
Equity Securities [Member]
Marketable Securities Classified as Available-for-Sale	$ 1,586	$ 1,294

Note 1 - Summary of Significa_6

Note 1 - Summary of Significant Accounting Policies - Inventories (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Finished Goods	$ 31,863	$ 37,130
Work in Process	16,508	20,393
Raw Material, Fuel and Supplies	49,480	48,747
Total Inventories	$ 97,851	$ 106,270

Note 1 - Summary of Significa_7

Note 1 - Summary of Significant Accounting Policies - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Inputs, Level 1 [Member]
Total Assets	$ 3,949	$ 2,132
Fair Value, Inputs, Level 1 [Member] | Equity Funds [Member]
Investments	1,586	1,294
Fair Value, Inputs, Level 1 [Member] | Money Market and Mutual Funds [Member]
Other Assets	2,363	838
Fair Value, Inputs, Level 2 [Member]
Total Assets	8,184	7,484
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Investments	2,124	5,898
Fair Value, Inputs, Level 2 [Member] | Government-backed and Government-sponsored Enterprises' Debt Securities [Member]
Investments	$ 6,060	$ 1,586

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Gross Balance		$ 37,572	$ 37,572
Accumulated Impairments		0	0
Balance	$ 37,572	37,572	37,572
Adjustments to Goodwill	0	0
Manufacturing [Member]
Gross Balance		18,270	18,270
Accumulated Impairments		0	0
Balance	18,270	18,270	18,270
Adjustments to Goodwill	0	0
Plastics [Member]
Gross Balance		19,302	19,302
Accumulated Impairments		0	0
Balance	19,302	19,302	$ 19,302
Adjustments to Goodwill	$ 0	$ 0

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, 2019	Dec. 31, 2018
Gross Carrying Amount	$ 22,670	$ 22,645
Accumulated Amortization	11,380	10,195
Net Carrying Amount	11,290	12,450
Customer Relationships [Member]
Gross Carrying Amount	22,491	22,491
Accumulated Amortization	11,259	10,127
Net Carrying Amount	$ 11,232	$ 12,364
Customer Relationships [Member] | Minimum [Member]
Remaining Amortization Periods (Month)	88 months	12 months
Customer Relationships [Member] | Maximum [Member]
Remaining Amortization Periods (Month)	188 months	200 months
Other Intangible Assets [Member]
Gross Carrying Amount	$ 179	$ 154
Accumulated Amortization	121	68
Net Carrying Amount	$ 58	$ 86
Remaining Amortization Periods (Month)		20 months
Other Intangible Assets [Member] | Minimum [Member]
Remaining Amortization Periods (Month)	8 months
Other Intangible Assets [Member] | Maximum [Member]
Remaining Amortization Periods (Month)	45 months

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Amortization Expense – Intangible Assets	$ 1,186	$ 1,315	$ 1,347

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, 2019USD ($)
2020	$ 1,140
2021	1,105
2022	1,105
2023	1,104
2024	$ 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Transactions Related to Capital Additions not Settled in Cash	$ 37,429	$ 13,757
Cash Paid During the Year for:
Interest (net of amount capitalized)	30,132	28,109	$ 29,791
Income Taxes	$ 4,797	$ 6,109	$ 5,064

Note 1 - Summary of Signific_13

Note 1 - Summary of Significant Accounting Policies - Effect of Stranded Tax Effect (Details) - USD ($) $ in Thousands	Jan. 01, 2019	Dec. 22, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Balance			$ (1,509)	$ 1,487	$ (1,831)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Balance		$ 71
Effect of TCJA 14% Federal Tax Rate Reduction on Gross Deferred Tax Amounts	$ 10
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member]
Balance		(5,672)
Effect of TCJA 14% Federal Tax Rate Reduction on Gross Deferred Tax Amounts	(794)
AOCI Attributable to Parent [Member]
Balance		$ (5,601)	(1,509)	$ 1,487	$ (1,831)
Effect of TCJA 14% Federal Tax Rate Reduction on Gross Deferred Tax Amounts	$ (784)		$ (784)

Note 2 - Business Segment Inf_3

Note 2 - Business Segment Information (Details Textual)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Reportable Segments	3
Revenue, Segment Benchmark [Member] | Customer Concentration Risk [Member]
Number of Customers	0	0	0
Revenue, Segment Benchmark [Member] | Customer Concentration Risk [Member] | Electric [Member]
Number of Customers	1
Concentration Risk, Percentage	11.90%
Revenue, Segment Benchmark [Member] | Customer Concentration Risk [Member] | Manufacturing [Member]
Number of Customers	5
Concentration Risk, Percentage	54.00%
Revenue, Segment Benchmark [Member] | Customer Concentration Risk [Member] | Manufacturing [Member] | Customer that Manufactures and Sells Recreational Vehicles [Member]
Number of Customers	1
Concentration Risk, Percentage	23.80%
Revenue, Segment Benchmark [Member] | Customer Concentration Risk [Member] | Manufacturing [Member] | Customer that Manufactures and Sells Lawn and Garden Equipment [Member]
Number of Customers	1
Concentration Risk, Percentage	11.10%
Revenue, Segment Benchmark [Member] | Customer Concentration Risk [Member] | Plastics [Member]
Number of Customers	2
Revenue, Segment Benchmark [Member] | Customer Concentration Risk [Member] | Plastics [Member] | Customer One [Member]
Concentration Risk, Percentage	25.30%
Revenue, Segment Benchmark [Member] | Customer Concentration Risk [Member] | Plastics [Member] | Customer Two [Member]
Concentration Risk, Percentage	20.40%
Revenue Benchmark [Member] | UNITED STATES
Concentration Risk, Percentage	98.80%	98.40%	98.20%

Note 2 - Business Segment Inf_4

Note 2 - Business Segment Information - Business Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Total	$ 919,503	$ 916,447	$ 849,350
Cost of Products Sold	355,119	354,559	316,562
Other Nonelectric Expenses	50,782	51,544	41,492
Depreciation and amortization	78,086	74,666	72,545
Operating income (loss)	134,880	129,389	132,287
Interest charges	31,411	30,408	29,604
Total	17,441	14,588	27,256
Net income (loss)	86,847	82,345	72,439
Capital expenditures	207,365	105,425	132,913
Identifiable Assets	2,273,595	2,052,517	2,004,278
Corporate, Non-Segment [Member]
Other Nonelectric Expenses	9,515	9,607	6,182
Depreciation and amortization	330	218	73
Operating income (loss)	(9,845)	(9,825)	(6,255)
Interest charges	1,800	1,204	1,422
Total	(5,519)	(3,218)	1,806
Net income (loss)	(5,670)	(8,744)	(9,753)
Capital expenditures	283	623	121
Identifiable Assets	54,279	44,797	59,801
Intersegment Eliminations [Member]
Regulated operating revenues	(55)	(57)	(57)
Cost of Products Sold	(34)	(21)	(18)
Other Nonelectric Expenses	(21)	(36)	(39)
Electric [Member] | Operating Segments [Member]
Regulated operating revenues	459,097	450,255	434,537
Depreciation and amortization	60,044	55,935	53,276
Operating income (loss)	98,417	88,031	94,797
Interest charges	26,548	26,365	25,334
Total	12,867	5,685	17,013
Net income (loss)	59,046	54,431	49,446
Capital expenditures	187,362	87,287	118,444
Identifiable Assets	1,931,525	1,728,534	1,690,224
Electric [Member] | Operating Segments [Member] | Retail [Member]
Revenue from Contracts with Customers	405,446	388,690	376,902
Changes in Accrued ARP Revenues	1,032	(439)	(1,971)
Regulated operating revenues	406,478	388,251	374,931
Electric [Member] | Operating Segments [Member] | Electric Transmission [Member]
Revenue from Contracts with Customers	40,542	46,947	46,664
Electric [Member] | Operating Segments [Member] | Wholesale [Member]
Revenue from Contracts with Customers	5,007	7,735	5,173
Electric [Member] | Operating Segments [Member] | Product and Service, Other [Member]
Revenue from Contracts with Customers	7,070	7,322	7,769
Manufacturing [Member] | Operating Segments [Member]
Revenue from Contracts with Customers	277,204	268,409	229,738
Cost of Products Sold	215,179	205,699	176,473
Other Nonelectric Expenses	29,895	29,650	23,785
Depreciation and amortization	14,261	14,794	15,379
Operating income (loss)	17,869	18,266	14,101
Interest charges	2,345	2,230	2,215
Total	2,784	3,393	989
Net income (loss)	12,899	12,839	11,050
Capital expenditures	14,268	13,316	9,916
Identifiable Assets	195,742	187,556	167,023
Manufacturing [Member] | Operating Segments [Member] | Metal Parts and Tooling [Member]
Revenue from Contracts with Customers	236,032	223,765	189,242
Manufacturing [Member] | Operating Segments [Member] | Plastic Products [Member]
Revenue from Contracts with Customers	35,173	35,836	33,939
Manufacturing [Member] | Operating Segments [Member] | Manufactured Product, Other [Member]
Revenue from Contracts with Customers	5,999	8,808	6,557
Plastics [Member] | Operating Segments [Member]
Revenue from Contracts with Customers	183,257	197,840	185,132
Cost of Products Sold	139,974	148,881	140,107
Other Nonelectric Expenses	11,393	12,323	11,564
Depreciation and amortization	3,451	3,719	3,817
Operating income (loss)	28,439	32,917	29,644
Interest charges	718	609	633
Total	7,309	8,728	7,448
Net income (loss)	20,572	23,819	21,696
Capital expenditures	5,452	4,199	4,432
Identifiable Assets	$ 92,049	$ 91,630	$ 87,230

Note 3 - Rate and Regulatory _3

Note 3 - Rate and Regulatory Matters (Details Textual) - USD ($)	Nov. 21, 2019	Jul. 16, 2019	Jul. 09, 2019	Jun. 11, 2019	May 30, 2019	Sep. 26, 2018	Apr. 20, 2018	Mar. 23, 2018	Mar. 22, 2018	Nov. 17, 2017	Nov. 16, 2016	Sep. 28, 2016	May 25, 2016	Feb. 12, 2015	Jan. 05, 2015	Nov. 06, 2014	Nov. 12, 2013	Apr. 30, 2019	Jun. 30, 2016	Dec. 22, 2015	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 01, 2018	Jan. 01, 2018	Nov. 02, 2017	Oct. 31, 2017	Dec. 31, 2016	Apr. 16, 2016
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent																							21.00%	21.00%	35.00%
Regulatory Liabilities, Total																							$ 247,386,000	$ 227,207,000
Minnesota1 [Member]
Effect of Tax Cuts and Jobs Act, Refund to Customers																					$ 11,500,000
Federal Energy Regulatory Commission [Member]
Additional Refund from Reducing ROE																							200,000
Otter Tail Power Company [Member]
Environmental Cost Recovery Rider Rate																								0.00%
Otter Tail Power Company [Member] | Litigation Case with Minnesota Customers [Member]
Loss Contingency, Estimate of Possible Loss																							$ 2,600,000
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member]
Current Return on Equity Used in Transmission Rates												10.32%					12.38%			10.32%			12.38%
Proposed Reduced Return on Equity Used in Transmission Rates												9.88%		8.67%			9.15%		9.70%				9.88%
Additional Incentive Basis Point																0.50%
Expected Percentage of Return on Equity												10.82%
Regulatory Liabilities, Total																					$ 1,600,000									$ 2,700,000
Contract with Customer, Refund Liability, Total																							$ 3,000,000	$ 1,600,000
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member] | First Complaint Period [member]
Current Return on Equity Used in Transmission Rates												10.82%											10.38%
Additional Incentive Basis Point															0.50%
Contract with Customer, Refund Liability, Total																							$ 1,300,000
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member] | Second Complaint Period [Member]
Current Return on Equity Used in Transmission Rates	9.88%											12.38%
Additional Incentive Basis Point															0.50%
Contract with Customer, Refund Liability, Total																							1,500,000
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%
Percentage of Allowed Rate of Return on Equity																															9.41%
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																									12 months
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																							2,700,000	$ 3,000,000	$ 2,900,000
Financial Incentives Reserve Increase (Decrease) During Period																							(300,000)		300,000
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Conservation Improvement Program [Member] | Recognized in 2017 [Member]
Financial Incentives Recognized During Period																									$ 2,600,000
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%
Percentage of Allowed Rate of Return on Equity																												10.30%
General Rate Revenue Increase Requested																											$ 12,800,000	$ 13,100,000
Percentage of Increase in Base Rate Revenue Requested																												8.72%
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]
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%
Refunds of Excess Revenues Collected, Amounts Collected Under Higher Interim Tax Rates																		$ 800,000
Otter Tail Power Company [Member] | South Dakota Public Utilities Commission [Member] | The 2018 General Rate Case [Member]
Percentage of Allowed Rate of Return on Equity					8.75%
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%
Contract with Customer, Liability, Revenue Recognized					$ 2,200,000																	$ 1,000,000
Understated Amount of OTP's Electric Transmission Plant in Service			$ 4,100,000
Annual Revenue Requirement Shortfall Resulted from Understatement			341,000
Increase in Non-fuel Annual Revenue Resulted from Increased in General Rate Case			$ 2,600,000
Increase in Non-fuel Annual Revenue Resulted from Increased in General Rate Case, Percentage			7.70%
Non-fuel Annual Revenue Increased, Perecenage of Adjusted Requsted Annual Revenue			69.00%
Adjusted Requested Annual Revenue Increased, Amount			$ 3,700,000
Adjusted Requested Annual Revenue Increased, Percentage			11.10%
Authorized Return of Equity			8.75%
Percentage of Excess Weather-normalized Revenue, Refund to Customer			50.00%
Maximum ROE, Input for Refund to Customer			9.50%
Percentage of Any Earnings Above Maximum ROE			100.00%
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
Percentage of Assets of Project																							100.00%
Otter Tail Power Company [Member] | Astoria Station Project [Member]
Expanded Capacity of Projects (MW)																							245
Current Project Capitalized Cost																							$ 58,700,000
Otter Tail Power Company [Member] | EDF [Member] | Merricourt Project [Member]
Asset Purchase Agreement, Purchase Price		$ 37,700,000		$ 37,700,000							$ 34,700,000
Turnkey Engineering, Procurement and Construction Services Agreement, Costs											$ 200,500,000
Current Project Cost																							$ 81,700,000

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]	12 Months Ended
	Dec. 31, 2019USD ($)kWhBoe	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Annual Revenue	$ 26,092,000	$ 35,962,000	$ 47,850,000
Minnesota1 [Member] | Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue	$ 8,271,000	8,127,000	6,008,000
Minnesota1 [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2018 Incentive and Cost Recovery [Member]
Approval date	Dec. 27, 2019
Effective Date Requested or Approved	Jan. 1, 2020
Annual Revenue	$ 11,926,000
Rate rider rate (Kilowatt-Hour) | kWh	0.00710
Minnesota1 [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2017 Incentive and Cost Recovery [Member]
Approval date	Oct. 4, 2018
Effective Date Requested or Approved	Nov. 1, 2018
Annual Revenue	$ 10,283,000
Rate rider rate (Kilowatt-Hour) | kWh	0.00600
Minnesota1 [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2016 Incentive and Cost Recovery [Member]
Approval date	Sep. 15, 2017
Effective Date Requested or Approved	Oct. 1, 2017
Annual Revenue	$ 9,868,000
Rate rider rate (Kilowatt-Hour) | kWh	0.00536
Minnesota1 [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2015 Incentive and Cost Recovery [Member]
Approval date	Jul. 19, 2016
Effective Date Requested or Approved	Oct. 1, 2016
Annual Revenue	$ 8,590,000
Rate rider rate (Kilowatt-Hour) | kWh	0.00275
Minnesota1 [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue	$ 2,497,000	(2,039,000)	2,973,000
Minnesota1 [Member] | Transmission Cost Recovery Rider [Member] | The 2018 Annual Update - Scenario A [Member]
Effective Date Requested or Approved	Jun. 1, 2019
Annual Revenue	$ 6,475,000
Request date	Nov. 30, 2018
Minnesota1 [Member] | Transmission Cost Recovery Rider [Member] | The 2018 Annual Update - Scenario B [Member]
Annual Revenue	$ 2,708,000
Minnesota1 [Member] | Transmission Cost Recovery Rider [Member] | The 2017 Rate Reset [Member]
Approval date	Oct. 30, 2017
Effective Date Requested or Approved	Nov. 1, 2017
Annual Revenue	$ (3,311,000)
Minnesota1 [Member] | Transmission Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
Approval date	Jul. 5, 2016
Effective Date Requested or Approved	Sep. 1, 2016
Annual Revenue	$ 4,736,000
Minnesota1 [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue	$ (1,000)	(24,000)	8,148,000
Minnesota1 [Member] | Environmental Cost Recovery Rider [Member] | The 2017 Rate Reset [Member]
Approval date	Oct. 30, 2017
Effective Date Requested or Approved	Nov. 1, 2017
Annual Revenue	$ (1,943,000)
Rate of base	(0.935%)
Minnesota1 [Member] | Environmental Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
Approval date	Jul. 5, 2016
Effective Date Requested or Approved	Sep. 1, 2016
Annual Revenue	$ 11,884,000
Rate of base	6.927%
Minnesota1 [Member] | Environmental Cost Recovery Rider [Member] | The 2018 Annual Update [Member]
Approval date	Nov. 29, 2018
Effective Date Requested or Approved	Dec. 1, 2018
Annual Revenue	$ 0
Rate of base	0.00%
Minnesota1 [Member] | Renewable Resource Adjustment [Member]
Annual Revenue	$ 5,513,000	3,067,000	(196,000)
Minnesota1 [Member] | Renewable Resource Adjustment [Member] | The 2017 Rate Reset [Member]
Approval date	Oct. 30, 2017
Effective Date Requested or Approved	Nov. 1, 2017
Annual Revenue	$ 1,279,000
Rate rider rate (Kilowatt-Hour) | kWh	0.00049
Minnesota1 [Member] | Renewable Resource Adjustment [Member] | The 2018 Annual Update [Member]
Approval date	Aug. 29, 2018
Effective Date Requested or Approved	Nov. 1, 2018
Annual Revenue	$ 5,886,000
Rate rider rate (Kilowatt-Hour) | Boe	0.00219
Minnesota1 [Member] | Renewable Resource Adjustment [Member] | The 2019 Annual Update - Revised [Member]
Approval date	Dec. 19, 2019
Effective Date Requested or Approved	Jan. 1, 2020
Annual Revenue	$ 12,506,000
Rate rider rate (Kilowatt-Hour) | Boe	0.00467
North Dakota 1 [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue	$ 5,292,000	7,016,000	8,729,000
North Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
Approval date	Dec. 14, 2016
Effective Date Requested or Approved	Jan. 1, 2017
Annual Revenue	$ 6,916,000
North Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2019 Annual Update [Member]
Approval date	Dec. 18, 2019
Effective Date Requested or Approved	Jan. 1, 2020
Annual Revenue	$ 5,739,000
North Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2018 Rate Reset for Effect of TCJA [Member]
Approval date	Feb. 27, 2018
Effective Date Requested or Approved	Mar. 1, 2018
Annual Revenue	$ 7,469,000
North Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2018 Supplemental Update [Member]
Approval date	Dec. 6, 2018
Effective Date Requested or Approved	Feb. 1, 2019
Annual Revenue	$ 4,801,000
North Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2017 Annual Update [Member]
Approval date	Nov. 29, 2017
Effective Date Requested or Approved	Jan. 1, 2018
Annual Revenue	$ 7,959,000
North Dakota 1 [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue	$ 550,000	7,318,000	9,782,000
North Dakota 1 [Member] | Environmental Cost Recovery Rider [Member] | The 2017 Rate Reset [Member]
Approval date	Dec. 20, 2017
Effective Date Requested or Approved	Jan. 1, 2018
Annual Revenue	$ 8,537,000
Rate of base	6.629%
North Dakota 1 [Member] | Environmental Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
Approval date	Jun. 22, 2016
Effective Date Requested or Approved	Jul. 1, 2016
Annual Revenue	$ 10,359,000
Rate of base	7.904%
North Dakota 1 [Member] | Environmental Cost Recovery Rider [Member] | The 2018 Annual Update [Member]
Approval date	Dec. 19, 2018
Effective Date Requested or Approved	Feb. 1, 2019
Annual Revenue	$ (378,000)
Rate of base	(0.31%)
North Dakota 1 [Member] | Environmental Cost Recovery Rider [Member] | The 2019 Annual Update [Member]
Approval date	Oct. 22, 2019
Effective Date Requested or Approved	Nov. 1, 2019
Annual Revenue	$ 0
Rate of base	0.00%
North Dakota 1 [Member] | Environmental Cost Recovery Rider [Member] | The 2018 Rate Reset for Effect of TCJA [Member]
Approval date	Feb. 27, 2018
Effective Date Requested or Approved	Mar. 1, 2018
Annual Revenue	$ 7,718,000
Rate of base	5.593%
North Dakota 1 [Member] | Environmental Cost Recovery Rider [Member] | The 2017 Annual Update [Member]
Approval date	Jul. 12, 2017
Effective Date Requested or Approved	Aug. 1, 2017
Annual Revenue	$ 9,917,000
Rate of base	7.633%
North Dakota 1 [Member] | Renewable Resource Adjustment [Member]
Annual Revenue	$ 230,000	8,529,000	7,620,000
North Dakota 1 [Member] | Renewable Resource Adjustment [Member] | The 2017 Rate Reset [Member]
Approval date	Dec. 20, 2017
Effective Date Requested or Approved	Jan. 1, 2018
Annual Revenue	$ 9,989,000
Rate of base	7.756%
North Dakota 1 [Member] | Renewable Resource Adjustment [Member] | The 2016 Annual Update [Member]
Approval date	Mar. 15, 2017
Effective Date Requested or Approved	Apr. 1, 2017
Annual Revenue	$ 9,156,000
Rate of base	7.005%
North Dakota 1 [Member] | Renewable Resource Adjustment [Member] | The 2020 Annual Update [Member]
Effective Date Requested or Approved	Apr. 1, 2020
Annual Revenue	$ 3,828,000
Request date	Dec. 31, 2019
Rate of base	3.744%
North Dakota 1 [Member] | Renewable Resource Adjustment [Member] | The 2019 Annual Update [Member]
Approval date	May 1, 2019
Effective Date Requested or Approved	Jun. 1, 2019
Annual Revenue	$ (235,000)
Rate of base	(0.224%)
North Dakota 1 [Member] | Renewable Resource Adjustment [Member] | The 2018 Rate Reset for Effect of TCJA [Member]
Approval date	Feb. 27, 2018
Effective Date Requested or Approved	Mar. 1, 2018
Annual Revenue	$ 9,650,000
Rate of base	7.493%
North Dakota 1 [Member] | Renewable Resource Adjustment [Member] | The 2015 Annual Update [Member]
Approval date	Jun. 22, 2016
Effective Date Requested or Approved	Jul. 1, 2016
Annual Revenue	$ 9,262,000
Rate of base	7.573%
North Dakota 1 [Member] | Generation Cost Recovery [Member]
Annual Revenue	$ 878,000	0	0
North Dakota 1 [Member] | Generation Cost Recovery [Member] | The 2019 Initial Request [Member]
Approval date	May 15, 2019
Effective Date Requested or Approved	Jul. 1, 2019
Annual Revenue	$ 2,720,000
Rate of base	2.547%
South Dakota 1 [Member] | Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue	$ 851,000	628,000	598,000
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue	$ 2,165,000	1,664,000	1,843,000
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
Approval date	Feb. 17, 2017
Effective Date Requested or Approved	Mar. 1, 2017
Annual Revenue	$ 2,053,000
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2020 Annual Update [Member]
Effective Date Requested or Approved	Mar. 1, 2020
Annual Revenue	$ 2,407,000
Request date	Oct. 31, 2019
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2019 Annual Update [Member]
Approval date	Feb. 20, 2019
Effective Date Requested or Approved	Mar. 1, 2019
Annual Revenue	$ 1,638,000
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2015 Annual Update [Member]
Approval date	Feb. 12, 2016
Effective Date Requested or Approved	Mar. 1, 2016
Annual Revenue	$ 1,895,000
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2017 Annual Update [Member]
Approval date	Feb. 28, 2018
Effective Date Requested or Approved	Mar. 1, 2018
Annual Revenue	$ 1,779,000
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2019 Rate Reset [Member]
Approval date	Sep. 17, 2019
Effective Date Requested or Approved	Oct. 1, 2019
Annual Revenue	$ 2,046,000
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2018 Interim Rate Reset [Member]
Approval date	Oct. 18, 2018
Effective Date Requested or Approved	Oct. 18, 2018
Annual Revenue	$ 1,171,000
South Dakota 1 [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue	$ (29,000)	1,676,000	2,345,000
South Dakota 1 [Member] | Environmental Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
Approval date	Oct. 26, 2016
Effective Date Requested or Approved	Nov. 1, 2016
Annual Revenue	$ 2,238,000
Rate rider rate (Kilowatt-Hour) | Boe	0.00536
South Dakota 1 [Member] | Environmental Cost Recovery Rider [Member] | The 2017 Annual Update [Member]
Approval date	Oct. 13, 2017
Effective Date Requested or Approved	Nov. 1, 2017
Annual Revenue	$ 2,082,000
Rate rider rate (Kilowatt-Hour) | Boe	0.00483
South Dakota 1 [Member] | Environmental Cost Recovery Rider [Member] | The 2018 Interim Rate Reset [Member]
Approval date	Oct. 18, 2018
Effective Date Requested or Approved	Oct. 18, 2018
Annual Revenue	$ (189,000)
Rate rider rate (Kilowatt-Hour) | Boe	(0.00075)
South Dakota 1 [Member] | Phase-In Rate Plan [Member]
Annual Revenue	$ (125,000)	$ 0	$ 0
South Dakota 1 [Member] | Phase-In Rate Plan [Member] | The 2019 Initial Request [Member]
Approval date	Aug. 21, 2019
Effective Date Requested or Approved	Sep. 1, 2019
Annual Revenue	$ 864,000
Rate of base	3.345%

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues recorded under rate riders	$ 26,092	$ 35,962	$ 47,850
Conservation Improvement Program Costs and Incentives [Member] | Minnesota1 [Member]
Revenues recorded under rate riders	8,271	8,127	6,008
Conservation Improvement Program Costs and Incentives [Member] | South Dakota 1 [Member]
Revenues recorded under rate riders	851	628	598
Renewable Resource Adjustment [Member] | Minnesota1 [Member]
Revenues recorded under rate riders	5,513	3,067	(196)
Renewable Resource Adjustment [Member] | North Dakota 1 [Member]
Revenues recorded under rate riders	230	8,529	7,620
Transmission Cost Recovery Rider [Member] | Minnesota1 [Member]
Revenues recorded under rate riders	2,497	(2,039)	2,973
Transmission Cost Recovery Rider [Member] | North Dakota 1 [Member]
Revenues recorded under rate riders	5,292	7,016	8,729
Transmission Cost Recovery Rider [Member] | South Dakota 1 [Member]
Revenues recorded under rate riders	2,165	1,664	1,843
Environmental Cost Recovery Rider [Member] | Minnesota1 [Member]
Revenues recorded under rate riders	(1)	(24)	8,148
Environmental Cost Recovery Rider [Member] | North Dakota 1 [Member]
Revenues recorded under rate riders	550	7,318	9,782
Environmental Cost Recovery Rider [Member] | South Dakota 1 [Member]
Revenues recorded under rate riders	(29)	1,676	2,345
Generation Cost Recovery [Member] | North Dakota 1 [Member]
Revenues recorded under rate riders	878	0	0
Phase-In Rate Plan [Member] | South Dakota 1 [Member]
Revenues recorded under rate riders	$ (125)	$ 0	$ 0

Note 4 - Regulatory Assets an_3

Note 4 - Regulatory Assets and Liabilities - Amount of Regulatory Assets and Liabilities Recorded on Consolidated Balance Sheets (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets - Current		$ 21,650	$ 17,225
Regulatory Assets - Long -Term		144,138	135,257
Regulatory Assets - Total		165,788	152,482
Regulatory Assets - Long -Term		144,138	135,257
Regulatory Liabilities - Current		7,480	738
Regulatory Liabilities - Long -Term		239,906	226,469
Regulatory Liabilities, Total		247,386	227,207
Net Regulatory Asset Position - Current		14,170	16,487
Net Regulatory Asset Position - Long-Term		(95,768)	(91,212)
Net Regulatory Asset/(Liability) Position		(81,598)	(74,725)
Deferred Income Taxes [Member]
Regulatory Liabilities - Current		0	0
Regulatory Liabilities - Long -Term		141,707	142,779
Regulatory Liabilities, Total		141,707	142,779
Accumulated Reserve for Estimated Removal Costs - Net of Salvage [Member]
Regulatory Liabilities - Current		0	0
Regulatory Liabilities - Long -Term		97,726	83,229
Regulatory Liabilities, Total		97,726	83,229
Refundable Fuel Clause Adjustment Revenues [Member]
Regulatory Liabilities - Current		3,982	121
Regulatory Liabilities - Long -Term		0	0
Regulatory Liabilities, Total		$ 3,982	$ 121
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		12 months	12 months
North Dakota Renewable Resource Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current		$ 1,515	$ 177
Regulatory Liabilities - Long -Term		0	0
Regulatory Liabilities, Total		$ 1,515	$ 177
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		12 months	12 months
North Dakota Transmission Cost Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current		$ 700	$ 60
Regulatory Liabilities - Long -Term		0	0
Regulatory Liabilities, Total		$ 700	$ 60
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		12 months	12 months
Prior Service Costs and Actuarial Losses on Pensions and Other Postretirement Benefits [Member]
Regulatory Liabilities - Current		$ 471
Regulatory Liabilities - Long -Term		0
Regulatory Liabilities, Total		$ 471
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		12 months
Revenue for Rate Case Expenses Subject to Refund - Minnesota [Member]
Regulatory Liabilities - Current		$ 0	$ 0
Regulatory Liabilities - Long -Term		401	166
Regulatory Liabilities, Total		401	166
South Dakota Phase-in Rate Plan Rider Accrued Refund [Member]
Regulatory Liabilities - Current		355
Regulatory Liabilities - Long -Term		0
Regulatory Liabilities, Total		$ 355
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		9 months
MISO Schedule 26/26A Transmission Cost Recovery Rider True-up [Member]
Regulatory Liabilities - Current			0
Regulatory Liabilities - Long -Term			187
Regulatory Liabilities, Total			$ 187
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)			24 months
North Dakota Generation Cost Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current		$ 287
Regulatory Liabilities - Long -Term		0
Regulatory Liabilities, Total		$ 287
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		6 months
South Dakota Transmission Cost Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current			$ 168
Regulatory Liabilities - Long -Term			0
Regulatory Liabilities, Total			$ 168
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)			12 months
Minnesota Energy Intensive Trade Exposed Rider Accrued Refund [Member]
Regulatory Liabilities - Current		$ 164
Regulatory Liabilities - Long -Term		0
Regulatory Liabilities, Total		$ 164
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		12 months
South Dakota Environmental Cost Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current			$ 207
Regulatory Liabilities - Long -Term			0
Regulatory Liabilities, Total			$ 207
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)			12 months
Other [Member]
Regulatory Liabilities - Current		$ 6	$ 5
Regulatory Liabilities - Long -Term		72	108
Regulatory Liabilities, Total		$ 78	$ 113
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		168 months	180 months
Prior Service Costs and Actuarial Losses on Pensions and Other Postretirement Benefits [Member]
Regulatory Assets - Current	[1]	$ 9,090	$ 6,346
Regulatory Assets - Long -Term	[1]	129,102	118,433
Regulatory Assets - Total	[1]	138,192	124,779
Regulatory Assets - Long -Term	[1]	129,102	118,433
Accumulated Asset Retirement Obligation (ARO) Accretion/Depreciation Adjustment [Member]
Regulatory Assets - Current	[1]	0
Regulatory Assets - Long -Term	[1]	7,772
Regulatory Assets - Total	[2]	7,772
Regulatory Assets - Long -Term	[1]	7,772
Accumulated ARO Accretion/Depreciation Adjustment [Member]
Regulatory Assets - Current	[1]		0
Regulatory Assets - Long -Term	[1]		7,169
Regulatory Assets - Total	[1]		7,169
Regulatory Assets - Long -Term	[1]		7,169
Conservation Improvement Program Costs and Incentives [Member]
Regulatory Assets - Current	[2]	4,024	5,995
Regulatory Assets - Long -Term	[2]	2,844	3,285
Regulatory Assets - Total	[2]	6,868	9,280
Regulatory Assets - Long -Term	[2]	$ 2,844	$ 3,285
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]	21 months	21 months
Minnesota Transmission Cost Recovery Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]	$ 4,208	$ 444
Regulatory Assets - Long -Term	[2]	0	0
Regulatory Assets - Total	[2]	4,208	444
Regulatory Assets - Long -Term	[2]	$ 0	$ 0
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]	12 months	12 months
MISO Schedule 26/26A Transmission Cost Recovery Rider True-up [Member]
Regulatory Assets - Current	[1]	$ 2,033	$ 240
Regulatory Assets - Long -Term	[1]	968	0
Regulatory Assets - Total	[1]	3,001	240
Regulatory Assets - Long -Term	[1]	$ 968	$ 0
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	24 months	12 months
Nonservice Costs Components of Postretirement Benefits Capitalized for Ratemaking Purposes and Subject to Deferred Recovery [Member]
Regulatory Assets - Current	[1]	$ 0	$ 0
Regulatory Assets - Long -Term	[1]	1,681	986
Regulatory Assets - Total	[1]	1,681	986
Regulatory Assets - Long -Term	[1]	1,681	986
Big Stone II Unrecovered Project Costs - Minnesota [Member]
Regulatory Assets - Current	[1]	715	681
Regulatory Assets - Long -Term	[1]	225	947
Regulatory Assets - Total	[1]	940	1,628
Regulatory Assets - Long -Term	[1]	$ 225	$ 947
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	16 months	28 months
Debt Reacquisition Premiums [Member]
Regulatory Assets - Current	[1]	$ 201	$ 207
Regulatory Assets - Long -Term	[1]	548	753
Regulatory Assets - Total	[1]	749	960
Regulatory Assets - Long -Term	[1]	$ 548	$ 753
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	153 months	165 months
Deferred Marked-to-Market Losses [Member]
Regulatory Assets - Current	[1]	$ 743	$ 1,661
Regulatory Assets - Long -Term	[1]		743
Regulatory Assets - Total	[1]	$ 743	2,404
Regulatory Assets - Long -Term	[1]		$ 743
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	12 months	24 months
Big Stone II Unrecovered Project Costs - South Dakota [Member]
Regulatory Assets - Current	[1]	$ 144	$ 100
Regulatory Assets - Long -Term	[1]	253	342
Regulatory Assets - Total	[1]	397	442
Regulatory Assets - Long -Term	[1]	$ 253	$ 342
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	33 months	53 months
South Dakota Deferred Rate Case Expenses Subject to Recovery [Member]
Regulatory Assets - Current	[1]	$ 138	$ 178
Regulatory Assets - Long -Term	[1]	245	0
Regulatory Assets - Total	[1]	383	178
Regulatory Assets - Long -Term	[1]	$ 245	$ 0
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	34 months	12 months
North Dakota Deferred Rate Case Expenses Subject to Recovery [Member]
Regulatory Assets - Current	[1]	$ 122	$ 455
Regulatory Assets - Long -Term	[1]	244	0
Regulatory Assets - Total	[1]	366	455
Regulatory Assets - Long -Term	[1]	$ 244	$ 0
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	36 months	12 months
Minnesota SPP Transmission Cost Recovery Tracker [Member]
Regulatory Assets - Current	[1]	$ 0	$ 0
Regulatory Assets - Long -Term	[1]	202	176
Regulatory Assets - Total	[1]	202	176
Regulatory Assets - Long -Term	[1]	202	176
Minnesota Renewable Resource Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]	131
Regulatory Assets - Long -Term	[2]	0
Regulatory Assets - Total	[2]	131
Regulatory Assets - Long -Term	[2]	$ 0
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	12 months
Minnesota Renewable Resource Recovery Rider, Accrued Revenues [Member]
Regulatory Assets - Current	[2]		452
Regulatory Assets - Long -Term	[2]		0
Regulatory Assets - Total	[2]		452
Regulatory Assets - Long -Term	[2]		$ 0
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]		12 months
South Dakota Transmission Cost Recovery Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]	$ 97
Regulatory Assets - Long -Term	[2]	0
Regulatory Assets - Total	[2]	97
Regulatory Assets - Long -Term	[2]	$ 0
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]	2 months
Minnesota Environmental Cost Recovery Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]	$ 4	$ 121
Regulatory Assets - Long -Term	[2]	0	0
Regulatory Assets - Total	[2]	4	121
Regulatory Assets - Long -Term	[2]	$ 0	$ 0
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]	12 months	12 months
Deferred Lease Expenses [Member]
Regulatory Assets - Current	[1]	$ 0
Regulatory Assets - Long -Term	[1]	54
Regulatory Assets - Total	[1]	54
Regulatory Assets - Long -Term	[1]	$ 54
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	39 months
Deferred Income Taxes [Member]
Regulatory Assets - Current	[1]		$ 0
Regulatory Assets - Long -Term	[1]		2,423
Regulatory Assets - Total	[1]		2,423
Regulatory Assets - Long -Term	[1]		2,423
Minnesota Energy Intensive Trade Exposed Rider Accrued Revenues [Member]
Regulatory Assets - Current	[1]		328
Regulatory Assets - Long -Term	[1]		0
Regulatory Assets - Total	[1]		328
Regulatory Assets - Long -Term	[1]		$ 0
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]		4 months
North Dakota Environmental Cost Recovery Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]		$ 17
Regulatory Assets - Long -Term	[2]		0
Regulatory Assets - Total	[2]		17
Regulatory Assets - Long -Term	[2]		$ 0
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]		12 months
[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) - USD ($)	6 Months Ended	12 Months Ended			24 Months Ended	30 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2019	Jun. 30, 2019	Nov. 08, 2019	May 03, 2018	Oct. 13, 2015
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	1,900,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	1,010,110	1,010,110
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
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	349,763	349,763
Share-based Compensation Arrangements by Share-based Payment Award, Percentage of Market Price at Eligible Employees Granted Options to Purchase Shares	85.00%	100.00%			100.00%	15.00%
Stock Issued During Period, Shares for Cash Invested under Employee Stock Purchase Plan		13,432		5,284
Open Market Share Purchases		3,672	7,757	4,202
Dividend Reinvestment and Share Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	1,220,032	1,220,032
Stock Issued During Period, Shares for Cash Invested under Employee Stock Purchase Plan		53,339
Open Market Share Purchases		109,807	116,822
Shelf Registration, Shares								1,500,000
Prior Shelf Registration for Dividend Reinvestment and Share Purchase Plan [Member]
Stock Issued During Period, Shares for Cash Invested under Employee Stock Purchase Plan				97,698
Open Market Share Purchases			53,853	87,634
Shelf Registration, Shares									1,500,000
Distribution Agreement With KeyBanc [Member]
Sale of Stock, Maximum Aggregate Sales Price							$ 75,000,000
Sale of Stock, Commission Percent							2.00%

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, 2019shares
Common Shares Outstanding, beginning balance (in shares)	39,664,884
Dividends Reinvested (in shares)	29,599
Cash Invested (in shares)	23,740
Vesting of Restricted Stock Units (in shares)	29,100
Restricted Stock Issued to Directors (in shares)	15,700
Directors Deferred Compensation (in shares)	594
Shares Withheld for Individual Income Tax Requirements (in shares)	(55,224)
Common Shares Outstanding, ending balance (in shares)	40,157,591
Performance Awards 2016 [Member]
Executive Stock Performance Awards (2016 awards earned) (in shares)	102,198
At the Market Offering [Member]
At-the-Market Offering (in shares)	347,000

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Weighted Average Common Shares Outstanding – Basic (in shares)	39,720,847	39,599,944	39,457,261
Shares Expected to be Awarded for Stock Performance Awards Granted to Executive Officers based on Measurement Period-to-Date Performance (in shares)	147,001	212,043	210,784
Underlying Shares Related to Nonvested Restricted Stock Units Granted to Employees (in shares)	65,421	59,980	56,952
Nonvested Restricted Shares (in shares)	15,377	17,751	20,380
Shares Expected to be Issued Under the Employee Stock Purchase Plan (in shares)	3,228	0	0
Shares Expected to be Issued Under the Deferred Compensation Program for Directors (in shares)	1,952	2,478	2,970
Total Dilutive Shares (in shares)	232,979	292,252	291,086
Weighted Average Common Shares Outstanding – Diluted (in shares)	39,953,826	39,892,196	39,748,347

Note 6 - Share-based Payments_2

Note 6 - Share-based Payments (Details Textual) - USD ($)	Apr. 08, 2023	Dec. 17, 2019	Apr. 08, 2019	Feb. 13, 2019	Feb. 06, 2019	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total						$ 3,500,000	$ 3,500,000
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition							1 year 10 months 24 days
Restricted Stock [Member] | Director [Member]
Share-based Payment Arrangement, Expense							$ 776,000	$ 661,000	$ 658,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period			15,700				15,700	18,200	17,600
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value			$ 49.73				$ 49.73	$ 43.40	$ 37.75
Restricted Stock [Member] | Director [Member] | Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage			33.30%
Restricted Stock [Member] | Director [Member] | Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage			33.30%
Restricted Stock [Member] | Director [Member] | Share-based Payment Arrangement, Tranche Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage			33.30%
Restricted Stock [Member] | Employees [Member]
Share-based Payment Arrangement, Expense							$ 0	$ 16,000	$ 70,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period							0	0	0
Restricted Stock Units (RSUs) [Member] | Employees [Member]
Share-based Payment Arrangement, Expense							$ 427,000	$ 351,000	$ 331,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period							13,270	14,780	10,995
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value							$ 44.45	$ 38.99	$ 33.28
Stock Performance Awards [Member] | Executive Officer [Member]
Share-based Payment Arrangement, Expense							$ 3,597,000	$ 2,645,000	$ 2,007,000
Stock Performance Awards [Member] | Executive Officer [Member] | Performance Period 2017 To 2019 [Member]
Adjusted Return, Targeted Percentage of Equity							9.50%
Payout as a Percent of Target							120.19%
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%
The 1999 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date										15.00%
Share-based Compensation Arrangements by Share-based Payment Award, Percentage of Market Price at Eligible Employees Granted Options to Purchase Shares						85.00%	100.00%				100.00%	15.00%
Share-based Payment Arrangement, Expense						$ 103,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized						1,400,000	1,400,000
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] | Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period							13,270
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value							$ 44.45
The 2014 Stock Incentive Plan [Member] | Restricted Stock [Member] | Employees [Member] | Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	100.00%
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				15,600
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value				$ 49.6225
The 2014 Stock Incentive Plan [Member] | Restricted Stock [Member] | Executive Officer [Member] | Share-based Payment Arrangement, 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 [Member] | Key Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period		600
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value		$ 52.165
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]
Period Specified for Average Adjusted Return							3 years
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized				55,600
Number of Trading Days				20 days
Number of Shares Authorized for Actual Payment				83,400
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]
Percentage of Target Amount as Actual Payment				150.00%
The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member] | Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized				27,800

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. 08, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Nonvested, Beginning of Year (in shares)		43,225	46,800	46,334
Nonvested, Beginning of Year, weighted average grant-date fair value (in dollars per share)		$ 37.53	$ 32.65	$ 29.71
Shares/Units Granted (in shares)	15,700	15,700	18,200	17,600
Granted, weighted average grant-date fair value (in dollars per share)	$ 49.73	$ 49.73	$ 43.40	$ 37.75
Vested (in shares)		18,320	21,775	17,134
Vested, weighted average grant-date fair value (in dollars per share)		$ 36.02	$ 31.94	$ 29.93
Forfeited (in shares)		0	0	0
Nonvested, End of Year (in shares)		40,605	43,225	46,800
Nonvested, End of Year, weighted average grant-date fair value (in dollars per share)		$ 42.93	$ 37.53	$ 32.65
Compensation Expense Recognized		$ 776,000	$ 661,000	$ 658,000
Fair Value of Shares Vested in Year		$ 660,000	$ 696,000	$ 513,000

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Nonvested, Beginning of Year (in shares)	0	2,895	7,180
Nonvested, Beginning of Year, weighted average grant-date fair value (in dollars per share)	$ 0	$ 29.41	$ 29.72
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	0	0	0
Vested (in shares)	0	2,895	4,285
Vested, weighted average grant-date fair value (in dollars per share)	$ 0	$ 29.41	$ 29.94
Forfeited (in shares)	0	0	0
Nonvested, End of Year (in shares)	0	0	2,895
Nonvested, End of Year, weighted average grant-date fair value (in dollars per share)	$ 0	$ 0	$ 29.41
Share-based Payment Arrangement, Expense	$ 0	$ 16,000	$ 70,000
Fair Value of Awards Vested	$ 0	$ 85,000	$ 128,000

Note 6 - Share-based Payments_4

Note 6 - Share-based Payments - Status of Dividends Equivalent Restricted Stock Unit Awards Granted to Executive and Key Employees (Details) - Executive Officers and Key Employees [Member] - Dividends Equivalent Restricted Stock Unit [Member] - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Nonvested, Beginning of Year (in shares)	45,300	47,750	41,825
Nonvested, Beginning of Year, weighted average grant-date fair value (in dollars per share)	$ 35.70	$ 32.71	$ 30.23
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	16,200	15,200	15,900
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 49.72	$ 41.325	$ 37.65
Vested (in shares)	15,850	17,650	9,975
Vested, weighted average grant-date fair value (in dollars per share)	$ 34.46	$ 32.462	$ 30.16
Forfeited (in shares)	0	0	0
Nonvested, End of Year (in shares)	45,650	45,300	47,750
Nonvested, End of Year, weighted average grant-date fair value (in dollars per share)	$ 41.10	$ 35.70	$ 32.71
Share-based Payment Arrangement, Expense	$ 1,055,000	$ 769,000	$ 576,000
Fair Value of Awards Vested	$ 546,000	$ 573,000	$ 301,000

Note 6 - Share-based Payments_5

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Nonvested, Beginning of Year (in shares)	49,470	46,440	47,370
Nonvested, Beginning of Year, weighted average grant-date fair value (in dollars per share)	$ 31.03	$ 27.07	$ 25.19
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	13,270	14,780	10,995
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 44.45	$ 38.99	$ 33.28
Vested (in shares)	13,250	8,925	11,550
Vested, weighted average grant-date fair value (in dollars per share)	$ 27.62	$ 25.23	$ 25.30
Forfeited (in shares)	1,900	2,825	375
Forfeited, weighted-average grant-date fair value (in dollars per share)	$ 34.57	$ 25.86	$ 26.92
Nonvested, End of Year (in shares)	47,590	49,470	46,440
Nonvested, End of Year, weighted average grant-date fair value (in dollars per share)	$ 35.58	$ 31.03	$ 27.07
Compensation Expense Recognized	$ 427,000	$ 351,000	$ 331,000
Fair Value of Shares Vested in Year	$ 366,000	$ 225,000	$ 292,000

Note 6 - Share-based Payments_6

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Compensation Expense Recognized	$ 3,597,000	$ 2,645,000	$ 2,007,000
Earned Shares (in shares)	297,943
Performance Period 2019 To 2021 [Member]
Maximum Shares Subject to Award (in shares)	83,400
Target Shares (in shares)	55,600
Compensation Expense Recognized	$ 2,168,000
Performance Period 2018 To 2020 [Member]
Maximum Shares Subject to Award (in shares)	81,000
Target Shares (in shares)	54,000
Compensation Expense Recognized	$ 897,000	1,121,000
Performance Period 2017 To 2019 [Member]
Maximum Shares Subject to Award (in shares)	89,250
Target Shares (in shares)	59,500
Compensation Expense Recognized	$ 524,000	729,000	854,000
Earned Shares (in shares)	69,997
Performance Period 2016 To 2018 [Member]
Maximum Shares Subject to Award (in shares)	122,250
Target Shares (in shares)	81,500
Compensation Expense Recognized	$ 8,000	772,000	580,000
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	$ 0	$ 23,000	$ 573,000
Earned Shares (in shares)	114,648

Note 7 - Retained Earnings an_2

Note 7 - Retained Earnings and Dividend Restriction (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Capitalization, Long-term Debt and Equity, Total	$ 1,471,063,000	$ 1,318,865,000
Otter Tail Power Company [Member]
Equity to Total Capitalization Ratio	51.20%
Net Assets Restricted from Distribution	$ 519,000,000
Capitalization, Long-term Debt and Equity, Total	$ 1,331,302,000
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Minimum [Member]
Public Utilities, Requested Equity Capital Structure, Percentage	46.00%
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Maximum [Member]
Public Utilities, Requested Equity Capital Structure, Percentage	56.20%

Note 8 - Leases (Details Textua

Note 8 - Leases (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Jan. 01, 2019
Operating Lease, Liability, Total	$ 20,369,000		$ 22,329,000
Operating Lease, Weighted Average Remaining Lease Term			6 years
Operating Lease, Weighted Average Discount Rate, Percent			5.30%
Operating Lease, Expense	6,273,000	$ 6,237,000
Operating Lease, Right-of-Use Asset	$ 0		$ 21,851,000
Accounting Standards Update 2016-02 [Member]
Operating Lease, Liability, Total				$ 20,000,000
Operating Lease, Right-of-Use Asset				$ 20,000,000

Note 8 - Leases - Lease Assets

Note 8 - Leases - Lease Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Gross	$ 25,919
Accumulated Amortization	(4,068)
Net of Accumulated Amortization	21,851	$ 0
Current Operating Lease Liabilities	4,136	0
Long-Term Operating Lease Liabilities	18,193	0
Total Lease Liabilities	22,329	$ 20,369
Electric [Member]
Gross	4,137
Accumulated Amortization	(1,166)
Net of Accumulated Amortization	2,971
Current Operating Lease Liabilities	1,116
Long-Term Operating Lease Liabilities	2,176
Total Lease Liabilities	3,292
Manufacturing [Member]
Gross	20,347
Accumulated Amortization	(2,375)
Net of Accumulated Amortization	17,972
Current Operating Lease Liabilities	2,609
Long-Term Operating Lease Liabilities	15,470
Total Lease Liabilities	18,079
Plastics [Member]
Gross	666
Accumulated Amortization	(395)
Net of Accumulated Amortization	271
Current Operating Lease Liabilities	256
Long-Term Operating Lease Liabilities	15
Total Lease Liabilities	271
Corporate Segment [Member]
Gross	769
Accumulated Amortization	(132)
Net of Accumulated Amortization	637
Current Operating Lease Liabilities	155
Long-Term Operating Lease Liabilities	532
Total Lease Liabilities	$ 687

Note 8 - Leases - Lease Obligat

Note 8 - Leases - Lease Obligations (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
2020	$ 5,212
2021	4,945
2022	3,914
2023	3,571
2024	3,087
Beyond 2024	5,587
Total Minimum Obligations	26,316
Interest Component of Obligations	(3,987)
Present Value of Minimum Obligations, December 31, 2019	22,329	$ 20,369
OTP [Member]
2020	1,243
2021	1,228
2022	326
2023	268
2024	217
Beyond 2024	283
Total Minimum Obligations	3,565
Interest Component of Obligations	(273)
Present Value of Minimum Obligations, December 31, 2019	3,292	3,609
Nonelectric Companies [Member]
2020	3,969
2021	3,717
2022	3,588
2023	3,303
2024	2,870
Beyond 2024	5,304
Total Minimum Obligations	22,751
Interest Component of Obligations	(3,714)
Present Value of Minimum Obligations, December 31, 2019	$ 19,037	$ 16,760

Note 8 - Leases - Operating Lea

Note 8 - Leases - Operating Lease Obligation (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Operating Lease Obligations, beginning balance	$ 20,369
Non-cash Acquisition of Right-of-Use Assets	6,318
Lease Adjustments/Modifications	(116)
Lease Obligation Payments	(5,371)
Interest Component of Lease Obligation Payment	1,129
Operating Lease Obligations, ending balance	22,329
OTP [Member]
Operating Lease Obligations, beginning balance	3,609
Non-cash Acquisition of Right-of-Use Assets	758
Lease Adjustments/Modifications	71
Lease Obligation Payments	(1,316)
Interest Component of Lease Obligation Payment	170
Operating Lease Obligations, ending balance	3,292
Nonelectric Companies [Member]
Operating Lease Obligations, beginning balance	16,760
Non-cash Acquisition of Right-of-Use Assets	5,560
Lease Adjustments/Modifications	(187)
Lease Obligation Payments	(4,055)
Interest Component of Lease Obligation Payment	959
Operating Lease Obligations, ending balance	$ 19,037

Note 8 - Leases - Allocation of

Note 8 - Leases - Allocation of Lease Costs (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Operating lease cost	$ 5,371
Variable lease cost	1,068
Total lease cost	6,439
Variable lease cost	1,068
Fixed Assets [Member]
Operating lease cost	29
Variable lease cost	0
Total lease cost	29
Variable lease cost	0
Production Fuel [Member]
Operating lease cost	952
Variable lease cost	0
Total lease cost	952
Variable lease cost	0
Cost of Sales [Member]
Operating lease cost	3,848
Variable lease cost	1,068
Total lease cost	4,916
Variable lease cost	1,068
Electric Operating and Maintenance Expenses [Member]
Operating lease cost	334
Variable lease cost	0
Total lease cost	334
Variable lease cost	0
Other Nonelectric Expenses [member]
Operating lease cost	208
Variable lease cost	0
Total lease cost	208
Variable lease cost	$ 0

Note 9 - Commitments and Cont_3

Note 9 - Commitments and Contingencies (Details Textual) - USD ($)	Oct. 01, 2019	Aug. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Land Easement Non-lease Payments Expense			$ 617,000	$ 605,000	$ 593,000
Otter Tail Power Company [Member]
Estimated Maximum Potential Asset Impairment Charge		$ 1,100,000
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member]
Estimated Liability of Refund Obligation			$ 3,000,000
Otter Tail Power Company [Member] | Construction Programs [Member]
Contract Expiration Year			2021
Long-term Purchase Commitment, Amount			$ 317,000,000
Other Commitment, Total			$ 283,000	$ 252,000	$ 280,000
Otter Tail Power Company [Member] | Coal Purchase Commitments 2 [Member]
Contract Expiration Year			2040
Otter Tail Power Company [Member] | Coal Purchase Commitments 3 [Member]
Contract Expiration Year			2020
Otter Tail Power Company [Member] | OTP Land Easements [Member]
Long-term Purchase Commitment, Amount			$ 10,200,000
T.O Plastics [Member] | Supply Agreement [Member]
Long-term Purchase Commitment, Amount	$ 1,900,000
Long-term Purchase Commitment, Period	6 years

Note 9 - Commitments and Cont_4

Note 9 - Commitments and Contingencies - Amounts of Commitments (Details) $ in Thousands	Dec. 31, 2019USD ($)
Otter Tail Power Company [Member]
2020, Land Easement Payments	$ 630
2021, Land Easement Payments	642
2022, Land Easement Payments	655
2023, Land Easement Payments	668
2024, Land Easement Payments	682
Beyond 2024, Land Easement Payments	6,931
Total, Land Easement Payments	10,208
Construction Program and Other Commitments [Member] | Subsidiaries [Member]
2020, Commitments Other than Operating Leases	269,774
2021, Commitments Other than Operating Leases	47,230
2022, Commitments Other than Operating Leases	22
2023, Commitments Other than Operating Leases	11
2024, Commitments Other than Operating Leases	6
Beyond 2024, Commitments Other than Operating Leases	0
Total, Commitments Other than Operating Leases	317,043
Capacity and Energy Requirements [Member] | Otter Tail Power Company [Member]
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
2024, Commitments Other than Operating Leases	11,801
Beyond 2024, Commitments Other than Operating Leases	131,913
Total, Commitments Other than Operating Leases	205,200
Coal Purchase Commitments [Member] | Otter Tail Power Company [Member]
2020, Commitments Other than Operating Leases	22,644
2021, Commitments Other than Operating Leases	22,935
2022, Commitments Other than Operating Leases	22,793
2023, Commitments Other than Operating Leases	23,955
2024, Commitments Other than Operating Leases	24,369
Beyond 2024, Commitments Other than Operating Leases	479,123
Total, Commitments Other than Operating Leases	$ 595,819

Note 10 - Short-term and Long_3

Note 10 - Short-term and Long-term Borrowings (Details Textual) - USD ($)	Oct. 10, 2019	Dec. 31, 2019	Dec. 31, 2018	Oct. 02, 2019	Sep. 12, 2019	Feb. 07, 2018	Sep. 23, 2016	Aug. 14, 2013	Oct. 29, 2012	Dec. 01, 2011
Short-term Debt, Maximum Amount Outstanding During Period		$ 109,200,000
Short-term Debt, Average Outstanding Amount		$ 36,200,000
Short-term Debt, Weighted Average Interest Rate, at Point in Time		3.20%
Line of Credit Facility, Maximum Borrowing Capacity		$ 340,000,000
Otter Tail Corporation Credit Agreement [Member]
Short-term Debt, Maximum Amount Outstanding During Period		38,900,000
Short-term Debt, Average Outstanding Amount		$ 21,900,000
Short-term Debt, Weighted Average Interest Rate, at Point in Time		3.80%	3.80%
Line of Credit Facility, Maximum Borrowing Capacity		$ 170,000,000
Otter Tail Corporation Credit Agreement [Member] | Maximum [Member]
Debt to Total Capitalization Ratio		60.00%
Otter Tail Corporation Credit Agreement [Member] | Minimum [Member]
Interest and Dividend Coverage Ratio		1.50
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
Reduced Line of Credit Facility, Borrowing Capacity by Outstanding Letters of Credit		$ 40,000,000
Otter Tail Corporation 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									290,000,000
OTP Credit Agreement [Member]
Short-term Debt, Average Outstanding Amount		$ 14,300,000
Short-term Debt, Weighted Average Interest Rate, at Point in Time		3.60%	3.00%
Line of Credit Facility, Maximum Borrowing Capacity		$ 170,000,000		$ 73,200,000
Line of Credit Facility, Maximum Amount Outstanding During Period, by Outstanding Letters of Credit		$ 50,000,000
OTP Credit Agreement [Member] | Maximum [Member]
Debt to Total Capitalization Ratio		60.00%
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
OTP Credit Agreement [Member] | Revolving Credit Facility [Member] | Otter Tail Power Company [Member]
Repayments of Debt	$ 69,900,000
The 2019 Note Purchase Agreement [Member] | Otter Tail Power Company [Member]
Proceeds from Issuance of Debt	$ 100,000,000
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount	10.00%
Debt Instrument, Redemption Price, Percentage	100.00%
The 2019 Note Purchase Agreement [Member] | Otter Tail Power Company [Member] | Senior Unsecured Note [Member[
Debt Instrument, Face Amount					$ 175,000,000
Series 2019 A Senior Unsecured Note Due October 10, 2029 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount					$ 10,000,000
Debt Instrument, Interest Rate, Stated Percentage					3.07%
Series 2019 B Senior Unsecured Note Due October 10, 2039 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount					$ 26,000,000
Debt Instrument, Interest Rate, Stated Percentage					3.52%
Series 2019 C Senior Unsecured Note Due October 10, 2049 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount					$ 64,000,000
Debt Instrument, Interest Rate, Stated Percentage					3.82%
Series 2020 A Senior Unsecured Notes Due February 25, 2030 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount					$ 10,000,000
Debt Instrument, Interest Rate, Stated Percentage					3.22%
Series 2020 B Senior Unsecured Notes Due August 20, 2030 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount					$ 40,000,000
Debt Instrument, Interest Rate, Stated Percentage					3.22%
Series 2020 C Senior Unsecured Notes Due February 25, 2040 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount					$ 10,000,000
Debt Instrument, Interest Rate, Stated Percentage					3.62%
Series 2020 D Senior Unsecured Notes Due February 25, 2050 [Member] | Otter Tail Power Company [Member]
Debt Instrument, Face Amount					$ 15,000,000
Debt Instrument, Interest Rate, Stated Percentage					3.92%
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%
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		60.00%
The 2011 and 2007 Note Purchase Agreements [Member] | Minimum [Member]
Interest and Dividend Coverage Ratio		1.50
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%

Note 10 - Short-term and Long_4

Note 10 - Short-term and Long-term Borrowings - Status of Lines of Credit (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Oct. 02, 2019	Dec. 31, 2018
Line of Credit Facility, Maximum Borrowing Capacity	$ 340,000
In Use	6,000
Restricted due to Outstanding Letters of Credit	15,476
Available	318,524		$ 281,101
Otter Tail Corporation Credit Agreement [Member]
Line of Credit Facility, Maximum Borrowing Capacity	170,000
In Use	6,000
Restricted due to Outstanding Letters of Credit	0
Available	164,000		120,785
OTP Credit Agreement [Member]
Line of Credit Facility, Maximum Borrowing Capacity	170,000	$ 73,200
In Use	0
Restricted due to Outstanding Letters of Credit	15,476
Available	$ 154,524		$ 160,316

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, 2019	Dec. 31, 2018
Short-Term Debt	$ 6,000	$ 18,599
Long-Term Debt	692,351	592,523
Less: Current Maturities net of Unamortized Debt Issuance Costs	183	172
Unamortized Long-Term Debt Issuance Costs	(2,587)	(2,349)
Total Long-Term Debt net of Unamortized Debt Issuance Costs	689,581	590,002
Total Short-Term and Long-Term Debt (with current maturities)	695,764	608,773
Unamortized Long-Term Debt Issuance Costs	2,587	2,349
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 3.07%, Series A, Due October 10, 2029 [Member]
Long-Term Debt	10,000
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 3.52%, Series 2019B, Due October 10, 2039 [Member]
Long-Term Debt	26,000
Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member]
Long-Term Debt	100,000	100,000
Partnership in Assisting Community Expansion (PACE) Note, 2.54%, due March 18, 2021 [Member]
Long-Term Debt	351	523
Senior Unsecured Notes 3.82%, Sericies C, Due October 10, 2049 [Member]
Long-Term Debt	64,000
Otter Tail Power Company [Member]
Short-Term Debt		9,384
Long-Term Debt	612,000	512,000
Unamortized Long-Term Debt Issuance Costs	(2,231)	(1,942)
Total Long-Term Debt net of Unamortized Debt Issuance Costs	609,769	510,058
Total Short-Term and Long-Term Debt (with current maturities)	609,769	519,442
Unamortized Long-Term Debt Issuance Costs	2,231	1,942
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 3.07%, Series A, Due October 10, 2029 [Member]
Long-Term Debt	10,000
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 3.52%, Series 2019B, Due October 10, 2039 [Member]
Long-Term Debt	26,000	0
Otter Tail Power Company [Member] | Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member]
Long-Term Debt	100,000	100,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 3.82%, Sericies C, Due October 10, 2049 [Member]
Long-Term Debt	64,000	0
Parent Company [Member]
Short-Term Debt	6,000	9,215
Long-Term Debt	80,351	80,523
Less: Current Maturities net of Unamortized Debt Issuance Costs	183	172
Unamortized Long-Term Debt Issuance Costs	(356)	(407)
Total Long-Term Debt net of Unamortized Debt Issuance Costs	79,812	79,944
Total Short-Term and Long-Term Debt (with current maturities)	85,995	89,331
Unamortized Long-Term Debt Issuance Costs	356	407
Parent Company [Member] | The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member]
Long-Term Debt	80,000	80,000
Parent Company [Member] | Partnership in Assisting Community Expansion (PACE) Note, 2.54%, due March 18, 2021 [Member]
Long-Term Debt	$ 351	$ 523

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, 2019	Dec. 31, 2018	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 3.07%, Series A, Due October 10, 2029 [Member]
Debt Instrument, Interest Rate, Stated Percentage	3.07%
Long-Term Debt, Due Date	Oct. 10, 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 3.52%, Series 2019B, Due October 10, 2039 [Member]
Debt Instrument, Interest Rate, Stated Percentage	3.52%
Long-Term Debt, Due Date	Oct. 10, 2039
Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member]
Debt Instrument, Interest Rate, Stated Percentage	4.07%	4.07%
Long-Term Debt, Due Date	Feb. 7, 2048	Feb. 7, 2048
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
Senior Unsecured Notes 3.82%, Sericies C, Due October 10, 2049 [Member]
Debt Instrument, Interest Rate, Stated Percentage	3.82%
Long-Term Debt, Due Date	Oct. 10, 2049

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, 2019USD ($)
Aggregate amounts of debt maturities in 2020	$ 183
Aggregate amounts of debt maturities in 2021	140,168
Aggregate amounts of debt maturities in 2022	30,000
Aggregate amounts of debt maturities in 2023	0
Aggregate amounts of debt maturities in 2024	$ 0

Note 11 - Pension Plan and Ot_3

Note 11 - Pension Plan and Other Postretirement Benefits (Details Textual) - USD ($)	1 Months Ended		12 Months Ended
	Jan. 31, 2020	Jul. 30, 2015	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year			$ 3,300,000
Defined Benefit Plan, Expected Future Prescription Drug Subsidy Receipt, Next Twelve Months			100,000
Defined Contribution Plan, Cost			5,265,000	$ 4,532,000	$ 4,211,000
Employee Stock Ownership Plan (ESOP), Cash Contributions to ESOP			$ 374,000	398,000	$ 612,000
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			6.88%
Defined Benefit Plan, Plan Assets at Fair Value, Valuation Inputs, Period Over Which Investment Gains or Losses are Recognized			5 years
Defined Benefit Plan, Plan Assets, Contributions by Employer			$ 22,500,000	20,000,000
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	$ 11,200,000
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer			$ 2,757,000	3,183,000
Health Insurance Benefits, Requisite Age			55 years
Health Insurance Benefits, Requisite Service Period			10 years
Health Insurance Benefits, Reduction in Prescription Reimbursement, Percent			27.00%
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment			$ (20,864,000)	$ 0
Health Insurance Benefits, Reduction in Benefits, Period for Recognition			4 years 3 months 18 days

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan [Member]
Service Cost		$ 5,491	$ 6,459	$ 5,629
Interest Cost		14,412	13,452	14,139
Expected Return on Assets		(21,297)	(21,199)	(19,229)
From Regulatory Asset		5	16	120
From Other Comprehensive Income	[1]	9	0	3
From Regulatory Asset		4,642	7,135	5,090
From Other Comprehensive Income	[1]	114	183	125
Net Periodic Pension Cost	[2]	3,376	6,046	5,877
From Other Comprehensive Income	[1]	114	183	125
Amortization of Prior Service Cost
From Regulatory Asset		5	16	120
From Other Comprehensive Income	[1]	9	0	3
Amortization of Net Actuarial Loss
From Regulatory Asset		(4,642)	(7,135)	(5,090)
From Other Comprehensive Income	[1]	114	183	125
Net Periodic Pension Cost	[2]	3,376	6,046	5,877
Pension Plan [Member] | Costs Included in OTP Capital Expenditures [Member]
Net Periodic Pension Cost		1,365	1,542	1,094
Amortization of Net Actuarial Loss
Net Periodic Pension Cost		1,365	1,542	1,094
Pension Plan [Member] | Costs Included in Electric Operation and Maintenance Expenses [Member]
Net Periodic Pension Cost		3,994	4,756	4,400
Amortization of Net Actuarial Loss
Net Periodic Pension Cost		3,994	4,756	4,400
Pension Plan [Member] | Costs Included in Other Nonelectric Expenses [Member]
Net Periodic Pension Cost		132	161	135
Amortization of Net Actuarial Loss
Net Periodic Pension Cost		132	161	135
Pension Plan [Member] | Nonservice Costs Included in Nonservice Cost Components of Postretirement Benefits [Member]
Net Periodic Pension Cost		(1,589)	(314)	200
Amortization of Net Actuarial Loss
Net Periodic Pension Cost		(1,589)	(314)	200
Pension Plan [Member] | Nonservice Costs Capitalized as Regulatory Assets [Member]
Net Periodic Pension Cost		(526)	(99)	48
Amortization of Net Actuarial Loss
Net Periodic Pension Cost		(526)	(99)	48
Executive Survivor and Supplemental Retirement Plan [Member]
Service Cost		418	408	290
Interest Cost		1,735	1,589	1,686
From Regulatory Asset		5	20	16
From Other Comprehensive Income	[3]	17	34	38
From Regulatory Asset		124	206	285
From Other Comprehensive Income	[3]	348	722	440
Net Periodic Pension Cost	[4]	2,647	2,979	2,755
From Other Comprehensive Income	[3]	348	722	440
Amortization of Prior Service Cost
From Regulatory Asset		5	20	16
From Other Comprehensive Income	[3]	17	34	38
Amortization of Net Actuarial Loss
From Regulatory Asset		(124)	(206)	(285)
From Other Comprehensive Income	[3]	348	722	440
Net Periodic Pension Cost	[4]	2,647	2,979	2,755
Executive Survivor and Supplemental Retirement Plan [Member] | Costs Included in Electric Operation and Maintenance Expenses [Member]
Net Periodic Pension Cost		104	99	94
Amortization of Net Actuarial Loss
Net Periodic Pension Cost		104	99	94
Executive Survivor and Supplemental Retirement Plan [Member] | Costs Included in Other Nonelectric Expenses [Member]
Net Periodic Pension Cost		314	309	196
Amortization of Net Actuarial Loss
Net Periodic Pension Cost		314	309	196
Executive Survivor and Supplemental Retirement Plan [Member] | Nonservice Costs Included in Nonservice Cost Components of Postretirement Benefits [Member]
Net Periodic Pension Cost		2,229	2,571	2,465
Amortization of Net Actuarial Loss
Net Periodic Pension Cost		2,229	2,571	2,465
Other Postretirement Benefits Plan [Member]
Service Cost		1,286	1,526	1,425
Interest Cost		3,083	2,583	2,712
From Regulatory Asset		0	0	(4)
From Other Comprehensive Income	[1]	0	0	4
From Regulatory Asset		(1,571)	(1,648)	(936)
From Other Comprehensive Income	[1]	38	42	19
Net Periodic Pension Cost	[5]	5,978	5,799	5,092
From Other Comprehensive Income	[1]	38	42	19
Amortization of Prior Service Cost
From Regulatory Asset		0	0	(4)
From Other Comprehensive Income	[1]	0	0	4
Amortization of Net Actuarial Loss
From Regulatory Asset		1,571	1,648	936
From Other Comprehensive Income	[1]	38	42	19
Net Periodic Pension Cost	[5]	5,978	5,799	5,092
Effect of Medicare Part D Subsidy		(179)	(470)	(561)
Other Postretirement Benefits Plan [Member] | Costs Included in OTP Capital Expenditures [Member]
Net Periodic Pension Cost		320	364	277
Amortization of Net Actuarial Loss
Net Periodic Pension Cost		320	364	277
Other Postretirement Benefits Plan [Member] | Costs Included in Electric Operation and Maintenance Expenses [Member]
Net Periodic Pension Cost		935	1,124	1,114
Amortization of Net Actuarial Loss
Net Periodic Pension Cost		935	1,124	1,114
Other Postretirement Benefits Plan [Member] | Costs Included in Other Nonelectric Expenses [Member]
Net Periodic Pension Cost		31	38	34
Amortization of Net Actuarial Loss
Net Periodic Pension Cost		31	38	34
Other Postretirement Benefits Plan [Member] | Nonservice Costs Included in Nonservice Cost Components of Postretirement Benefits [Member]
Net Periodic Pension Cost		3,525	3,253	2,955
Amortization of Net Actuarial Loss
Net Periodic Pension Cost		3,525	3,253	2,955
Other Postretirement Benefits Plan [Member] | Nonservice Costs Capitalized as Regulatory Assets [Member]
Net Periodic Pension Cost		1,167	1,020	712
Amortization of Net Actuarial Loss
Net Periodic Pension Cost		$ 1,167	$ 1,020	$ 712
[1]	Corporate cost included in nonservice cost components of postretirement benefits.
[2]	Allocation of Costs: 2019 2018 2017 Service costs included in OTP Capital Expenditures $ 1,365 $ 1,542 $ 1,094 Service costs included in electric operation and maintenance expenses 3,994 4,756 4,400 Service costs included in other nonelectric expenses 132 161 135 Nonservice costs capitalized (526 ) (99 ) 48 Nonservice costs included in nonservice cost components of postretirement benefits (1,589 ) (314 ) 200
[3]	Amortization of prior service costs and net actuarial losses from other comprehensive income are included in nonservice cost components of postretirement benefits.
[4]	Allocation of Costs: 2019 2018 2017 Service costs included in electric operation and maintenance expenses $ 104 $ 99 $ 94 Service costs included in other nonelectric expenses 314 309 196 Nonservice costs included in nonservice cost components of postretirement benefits 2,229 2,571 2,465
[5]	Allocation of Cost: 2019 2018 2017 Service Costs included in OTP capital expenditures $ 320 $ 364 $ 277 Service costs included in electric operation and maintenance expenses 935 1,124 1,114 Service costs included in other nonelectric expenses 31 38 34 Nonservice costs capitalized 1,167 1,020 712 Nonservice costs included in nonservice cost components of postretirement benefits 3,525 3,253 2,955

Note 11 - Pension Plan and Ot_5

Note 11 - Pension Plan and Other Postretirement Benefits - Weighted-average Assumptions Used to Determine Net Periodic Benefit Cost (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan [Member]
Discount Rate	4.50%	3.90%	4.60%
Long-Term Rate of Return on Plan Assets	7.25%	7.50%	7.50%
Rate of Increase in Future Compensation Level			3.00%
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%	4.60%
Rate of Increase in Future Compensation Level	3.40%	2.92%	3.00%
Other Postretirement Benefits Plan [Member]
Discount Rate	4.44%	3.81%	4.46%

Note 11 - Pension Plan and Ot_6

Note 11 - Pension Plan and Other Postretirement Benefits - Amounts Recognized in Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan [Member]
Regulatory Assets:
Unrecognized Prior Service Cost	$ 0	$ 5
Unrecognized Actuarial Loss	120,592	104,891
Total Regulatory Assets	120,592	104,896
Projected Benefit Obligation Liability – Net Amount Recognized	(384,785)	(328,442)	$ (352,718)
Unrecognized Prior Service Credit	0	5
Accumulated Other Comprehensive Loss:
Unrecognized Prior Service Cost	0	9
Unrecognized Actuarial (Gain) Loss	(82)	137
Total Accumulated Other Comprehensive Loss	(82)	146
Noncurrent Liability	55,004	58,659
Unrecognized Prior Service Cost	0	9
Executive Survivor and Supplemental Retirement Plan [Member]
Regulatory Assets:
Unrecognized Prior Service Cost	0	20
Unrecognized Actuarial Loss	2,170	1,768
Total Regulatory Assets	2,170	1,788
Projected Benefit Obligation Liability – Net Amount Recognized	(43,966)	(39,699)	(42,308)
Unrecognized Prior Service Credit	0	20
Accumulated Other Comprehensive Loss:
Unrecognized Prior Service Cost	1	64
Unrecognized Actuarial (Gain) Loss	9,170	6,455
Total Accumulated Other Comprehensive Loss	9,171	6,519
Unrecognized Prior Service Cost	1	64
Other Postretirement Benefits Plan [Member]
Regulatory Assets:
Unrecognized Prior Service Cost	(20,363)	0
Total Regulatory Assets	14,959	18,094
Projected Benefit Obligation Liability – Net Amount Recognized	(71,437)	(71,561)	$ (69,774)
Unrecognized Prior Service Credit	(20,363)	0
Unrecognized Net Actuarial Loss (Gain)	35,322	18,094
Accumulated Other Comprehensive Loss:
Unrecognized Prior Service Cost	(501)	0
Unrecognized Actuarial (Gain) Loss	184	(107)
Total Accumulated Other Comprehensive Loss	(317)	(107)
Unrecognized Prior Service Cost	$ (501)	$ 0

Note 11 - Pension Plan and Ot_7

Note 11 - Pension Plan and Other Postretirement Benefits - Funded Status (Details) - Pension Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Benefit Obligation	$ (346,723)	$ (297,972)
Projected Benefit Obligation Liability – Net Amount Recognized	(384,785)	(328,442)	$ (352,718)
Fair Value of Plan Assets	329,781	269,783	$ 285,319
Funded Status	$ (55,004)	$ (58,659)

Note 11 - Pension Plan and Ot_8

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan [Member]
Reconciliation of Fair Value of Plan Assets:
Fair Value of Plan Assets, beginning balance		$ 269,783	$ 285,319
Actual Return on Plan Assets		52,640	(21,334)
Discretionary Company Contributions		22,500	20,000
Benefit Payments		(15,142)	(14,202)
Fair Value of Plan Assets, ending balance		$ 329,781	$ 269,783	$ 285,319
Estimated Asset Return		19.30%	(7.30%)
Reconciliation of Projected Benefit Obligation:
Projected Benefit Obligation, beginning balance		$ 328,442	$ 352,718
Service Cost		5,491	6,459	5,629
Interest Cost		14,412	13,452	14,139
Benefit Payments		(15,142)	(14,202)
Actuarial Loss (Gain)		51,582	(29,985)
Projected Benefit Obligation, ending balance		384,785	328,442	352,718
Reconciliation of Accrued Postretirement Cost:
Expense	[1]	(3,376)	(6,046)	(5,877)
Discretionary Company Contributions		22,500	20,000
Executive Survivor and Supplemental Retirement Plan [Member]
Reconciliation of Fair Value of Plan Assets:
Fair Value of Plan Assets, beginning balance		0	0
Actual Return on Plan Assets		0	0
Discretionary Company Contributions		1,475	1,505
Benefit Payments		(1,475)	(1,505)
Fair Value of Plan Assets, ending balance		0	0	0
Reconciliation of Projected Benefit Obligation:
Projected Benefit Obligation, beginning balance		39,699	42,308
Service Cost		418	408	290
Interest Cost		1,735	1,589	1,686
Benefit Payments		(1,475)	(1,505)
Actuarial Loss (Gain)		5,260	(3,101)
Projected Benefit Obligation, ending balance		43,966	39,699	42,308
Curtailments		(1,671)	0
Reconciliation of Accrued Postretirement Cost:
Expense	[2]	(2,647)	(2,979)	(2,755)
Discretionary Company Contributions		1,475	1,505
Other Postretirement Benefits Plan [Member]
Reconciliation of Fair Value of Plan Assets:
Fair Value of Plan Assets, beginning balance		0	0
Actual Return on Plan Assets		0	0
Discretionary Company Contributions		2,757	3,183
Benefit Payments		(7,164)	(6,684)
Fair Value of Plan Assets, ending balance		0	0	0
Participant Premium Payments		4,407	3,501
Reconciliation of Projected Benefit Obligation:
Projected Benefit Obligation, beginning balance		71,561	69,774
Service Cost		1,286	1,526	1,425
Interest Cost		3,083	2,583	2,712
Benefit Payments		(7,164)	(6,684)
Actuarial Loss (Gain)		19,128	861
Projected Benefit Obligation, ending balance		71,437	71,561	69,774
Participant Premium Payments		4,407	3,501
Plan Amendments		(20,864)	0
Reconciliation of Accrued Postretirement Cost:
Accrued Postretirement Cost at January 1		(53,574)	(50,958)
Expense	[3]	(5,978)	(5,799)	(5,092)
Discretionary Company Contributions		2,757	3,183
Accrued Postretirement Cost at December 31		$ (56,795)	$ (53,574)	$ (50,958)
[1]	Allocation of Costs: 2019 2018 2017 Service costs included in OTP Capital Expenditures $ 1,365 $ 1,542 $ 1,094 Service costs included in electric operation and maintenance expenses 3,994 4,756 4,400 Service costs included in other nonelectric expenses 132 161 135 Nonservice costs capitalized (526 ) (99 ) 48 Nonservice costs included in nonservice cost components of postretirement benefits (1,589 ) (314 ) 200
[2]	Allocation of Costs: 2019 2018 2017 Service costs included in electric operation and maintenance expenses $ 104 $ 99 $ 94 Service costs included in other nonelectric expenses 314 309 196 Nonservice costs included in nonservice cost components of postretirement benefits 2,229 2,571 2,465
[3]	Allocation of Cost: 2019 2018 2017 Service Costs included in OTP capital expenditures $ 320 $ 364 $ 277 Service costs included in electric operation and maintenance expenses 935 1,124 1,114 Service costs included in other nonelectric expenses 31 38 34 Nonservice costs capitalized 1,167 1,020 712 Nonservice costs included in nonservice cost components of postretirement benefits 3,525 3,253 2,955

Note 11 - Pension Plan and Ot_9

Note 11 - Pension Plan and Other Postretirement Benefits - Weighted-average Assumptions Used to Determine Benefit Obligations (Details)	Dec. 31, 2019	Dec. 31, 2018
Pension Plan [Member]
Discount Rate	3.47%	4.50%
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	3.36%	4.46%
Rate of Increase in Future Compensation Level	3.50%	3.40%
Other Postretirement Benefits Plan [Member]
Discount Rate	3.43%	4.44%

Note 11 - Pension Plan and O_10

Note 11 - Pension Plan and Other Postretirement Benefits - Measurement Dates (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Pension Plan [Member]
Net Periodic Pension Cost	Jan. 1, 2019	Jan. 1, 2018
End of Year Benefit Obligations	January 1, 2019 projected to December 31, 2019	January 1, 2018 projected to December 31, 2018
Market Value of Assets	Dec. 31, 2019	Dec. 31, 2018
Other Postretirement Benefits Plan [Member]
Net Periodic Postretirement Benefit Cost	Jan. 1, 2019	Jan. 1, 2018
End of Year Benefit Obligations	January 1, 2019 projected to December 31, 2019	January 1, 2018 projected to December 31, 2018

Note 11 - Pension Plan and O_11

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, 2019USD ($)
2020	$ 15,908
2021	16,477
2022	17,116
2023	17,768
2024	18,374
Years 2025-2029	98,994
Executive Survivor and Supplemental Retirement Plan [Member]
2020	1,571
2021	1,681
2022	2,279
2023	2,680
2024	2,627
Years 2025-2029	13,976
Other Postretirement Benefits Plan [Member]
2020	3,323
2021	3,470
2022	3,653
2023	3,720
2024	3,778
Years 2025-2029	$ 19,852

Note 11 - Pension Plan and O_12

Note 11 - Pension Plan and Other Postretirement Benefits - Asset Allocation Ranges By Asset Class and PBO Funded Status (Details) - Pension Plan [Member]	Dec. 31, 2019
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 or the SEI Energy Debt Collective Fund.

Note 11 - Pension Plan and O_13

Note 11 - Pension Plan and Other Postretirement Benefits - Pension Plan Asset Allocations by Asset Category (Details) - Pension Plan [Member]	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Actual Plan Asset Allocations	100.00%	100.00%
Mixed Equities Fund [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	20.40%
Large Capitalization Equity Securities [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	11.30%	17.50%
International Equity Securities [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	9.30%	17.00%
Emerging Markets Equity Funds [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	4.20%	3.40%
Small and Mid Capitalization Equity Securities [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	4.10%	6.70%
SEI Dynamic Asset Allocation Fund [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	3.10%	4.00%
Equity Securities [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	52.40%	48.60%
Fixed-income Securities and Cash [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	44.70%	47.10%
Other - SEI Energy Debt Collective Fund [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	2.90%	4.30%

Note 11 - Pension Plan and O_14

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value of Plan Assets	$ 329,781	$ 269,783	$ 285,319
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	9,540	11,476
Fair Value, Inputs, Level 1 [Member]
Fair Value of Plan Assets	$ 320,241	$ 258,307

Note 11 - Pension Plan and O_15

Note 11 - Pension Plan and Other Postretirement Benefits - Pension Fund Assets Measured at Fair Value (Details) - Pension Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value of Plan Assets	$ 329,781	$ 269,783	$ 285,319
Fair Value, Inputs, Level 1 [Member]
Fair Value of Plan Assets	320,241	258,307
Fair Value, Inputs, Level 1 [Member] | Mixed Equities Fund [Member]
Fair Value of Plan Assets	67,184	0
Fair Value, Inputs, Level 1 [Member] | Large Capitalization Equity Securities [Member]
Fair Value of Plan Assets	37,357	47,198
Fair Value, Inputs, Level 1 [Member] | International Equity Securities [Member]
Fair Value of Plan Assets	30,653	45,912
Fair Value, Inputs, Level 1 [Member] | Small and Mid Capitalization Equity Securities [Member]
Fair Value of Plan Assets	13,447	17,971
Fair Value, Inputs, Level 1 [Member] | SEI Dynamic Asset Allocation Fund [Member]
Fair Value of Plan Assets	10,168	10,929
Fair Value, Inputs, Level 1 [Member] | Emerging Markets Equity Funds [Member]
Fair Value of Plan Assets	13,792	9,197
Fair Value, Inputs, Level 1 [Member] | Fixed Income Securities [Member]
Fair Value of Plan Assets	147,639	127,098
Fair Value, Inputs, Level 1 [Member] | Cash Management Money Market Fund [Member]
Fair Value of Plan Assets	$ 1	$ 2

Note 11 - Pension Plan and O_16

Note 11 - Pension Plan and Other Postretirement Benefits - Healthcare Cost-trend Rates (Details) - Other Postretirement Benefits Plan [Member]	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Healthcare Cost-Trend Rate Assumed for Next Year	6.72%	7.00%
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

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, 2019	Dec. 31, 2018
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, 2019	Dec. 31, 2018
Reported Value Measurement [Member]
Cash and Cash Equivalents	$ 21,199	$ 861
Short-Term Debt	(6,000)	(18,599)
Long-Term Debt including Current Maturities	(689,764)	(590,174)
Estimate of Fair Value Measurement [Member]
Cash and Cash Equivalents	21,199	861
Short-Term Debt	(6,000)	(18,599)
Long-Term Debt including Current Maturities	$ (742,279)	$ (601,513)

Note 13 - Property, Plant and_3

Note 13 - Property, Plant and Equipment (Details Textual)	12 Months Ended
	Dec. 31, 2019
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, 2019	Dec. 31, 2018
Total Gross Plant	$ 2,645,478	$ 2,429,467
Less Accumulated Depreciation and Amortization	891,684	848,369
Net Plant	1,753,794	1,581,098
Electric Plant [Member]
Total Gross Plant	2,390,468	2,189,811
Less Accumulated Depreciation and Amortization	731,110	699,642
Net Plant	1,659,358	1,490,169
Nonelectric Operations Plant [Member]
Total Gross Plant	255,010	239,656
Less Accumulated Depreciation and Amortization	160,574	148,727
Net Plant	94,436	90,929
Electric Generation Equipment [Member] | Electric Plant [Member]
Total Gross Plant	915,996	905,224
Transmission Plant [Member] | Electric Plant [Member]
Total Gross Plant	647,474	512,832
Distribution Plant [Member] | Electric Plant [Member]
Total Gross Plant	526,146	502,261
General Plant [Member] | Electric Plant [Member]
Total Gross Plant	123,268	99,404
Electric Plant in Service [Member] | Electric Plant [Member]
Total Gross Plant	2,212,884	2,019,721
Construction in Progress [Member] | Electric Plant [Member]
Total Gross Plant	177,584	170,090
Construction in Progress [Member] | Nonelectric Plant [Member]
Total Gross Plant	7,654	11,536
Equipment [Member] | Nonelectric Plant [Member]
Total Gross Plant	187,904	170,634
Buildings and Leasehold Improvements [Member] | Nonelectric Plant [Member]
Total Gross Plant	53,412	53,011
Land [Member] | Nonelectric Plant [Member]
Total Gross Plant	6,040	4,475
Nonelectric Operations Plant [Member] | Nonelectric Plant [Member]
Total Gross Plant	$ 247,356	$ 228,120

Note 13 - Property, Plant and_5

Note 13 - Property, Plant and Equipment - Estimated Service Lives for Properties (Details)	12 Months Ended
	Dec. 31, 2019
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)	51 years
Minimum [Member] | Electric Plant [Member] | Distribution Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	15 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)	75 years
Maximum [Member] | Electric Plant [Member] | Distribution Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	70 years
Maximum [Member] | Electric Plant [Member] | General Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	57 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	35.00%
Increase (Decrease) in Percentage of Production Tax Credits		(53.00%)
Wind Tax Credits, Amortization Period		25 years
Period for Unrecognized Tax Benefits Not Expected Change	12 months
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	$ 0
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 0	$ 0	$ 1,756
Domestic Tax Authority [Member]
Open Tax Year	2016 2017 2018 2019
State and Local Jurisdiction [Member] | Minnesota Department of Revenue [Member]
Open Tax Year	2015 2016 2017 2018 2019
State and Local Jurisdiction [Member] | North Dakota Department of Revenue [Member]
Open Tax Year	2016 2017 2018 2019

Note 14 - Income Taxes - Effect

Note 14 - Income Taxes - Effective Income Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Tax Computed at Federal Statutory Rate	$ 21,901	$ 20,356	$ 34,893
Increases (Decreases) in Tax from:
State Income Taxes Net of Federal Income Tax Expense	3,561	5,210	4,368
Differences Reversing in Excess of Federal Rates	(3,357)	(3,432)	551
Permanent Differences, R&D Tax Credits, Unitary Tax and Other Adjustments	(1,315)	(1,864)	(1,873)
North Dakota Wind Tax Credit Amortization – Net of Federal Taxes	(1,033)	(1,033)	(850)
Corporate-owned Life Insurance	(749)	(3)	(845)
Excess Tax deduction – Equity Method Stock Awards	(744)	(708)	(751)
Allowance for Funds Used During Construction – Equity	(501)	(431)	(322)
Employee Stock Ownership Plan Dividend Deduction	(281)	(298)	(509)
Investment Tax Credit Amortization	(41)	(98)	(164)
Federal Production Tax Credits (PTCs)	0	(3,111)	(7,527)
Section 199 Domestic Production Activities Deduction	0	0	(1,471)
Effect of TCJA Tax Rate Reduction on Value of Net Deferred Tax Assets	0	0	1,756
Income Tax Expense	$ 17,441	$ 14,588	$ 27,256
Overall Effective Federal and State Income Tax Rate	16.70%	15.00%	27.30%
Current Federal Income Taxes	$ 5,156	$ 4,960	$ 4,434
Current State Income Taxes	1,333	1,395	1,128
Deferred Federal Income Taxes	8,859	8,065	25,648
Deferred State Income Taxes	3,167	4,410	4,587
Federal PTCs	0	(3,111)	(7,527)
North Dakota Wind Tax Credit Amortization – Net of Federal Taxes	(1,033)	(1,033)	(850)
Investment Tax Credit Amortization	(41)	(98)	(164)
Total	17,441	14,588	27,256
Total Income Before Income Taxes	$ 104,288	$ 96,933	$ 99,695

Note 14 - Income Taxes - Deferr

Note 14 - Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Assets
Benefit Liabilities	$ 36,246	$ 33,967
Retirement Benefits Liabilities	36,206	32,664
Regulatory Tax Liability	35,700	33,228
North Dakota Wind Tax Credits	31,611	32,570
Cost of Removal	25,604	21,787
Federal PTCs	20,017	32,101
Differences Related to Property	6,979	6,842
Lease Liability	5,733	0
Vacation Accrual	1,884	1,919
Net Operating Loss Carryforward	1,860	2,489
Investment Tax Credits	408	449
Other	344	3,218
Valuation Allowance	(800)	(600)
Total Deferred Tax Assets	201,792	200,634
Deferred Tax Liabilities
Differences Related to Property	(268,495)	(261,396)
Retirement Benefits Regulatory Asset	(36,206)	(32,664)
Excess Tax over Book Pension	(17,556)	(15,145)
Right of Use Asset	(5,705)	0
North Dakota Wind Tax Credits	(3,126)	(4,386)
Impact of State Net Operating Losses on Federal Taxes	(385)	(523)
Other	(2,260)	(7,496)
Total Deferred Tax Liabilities	(333,733)	(321,610)
Deferred Income Taxes	$ (131,941)	$ (120,976)

Note 14 - Income Taxes - Expira

Note 14 - Income Taxes - Expiration of Tax Net Operating Losses and Tax Credits Available (Details) $ in Thousands	Dec. 31, 2019USD ($)
Domestic Tax Authority [Member]
Tax Credits	$ 23,002
Domestic Tax Authority [Member] | Tax Years 2022 to 2032 [Member]
Tax Credits	0
Domestic Tax Authority [Member] | Tax Years 2033 to 2038 [Member]
Tax Credits	22,220
Domestic Tax Authority [Member] | Tax Years 2039 to 2043 [Member]
Tax Credits	782
State and Local Jurisdiction [Member]
Tax Credits	32,177
State Net Operating Losses	1,860
State and Local Jurisdiction [Member] | Tax Years 2022 to 2032 [Member]
Tax Credits	0
State Net Operating Losses	1,833
State and Local Jurisdiction [Member] | Tax Years 2033 to 2038 [Member]
Tax Credits	2,643
State Net Operating Losses	27
State and Local Jurisdiction [Member] | Tax Years 2039 to 2043 [Member]
Tax Credits	29,534
State Net Operating Losses	$ 0

Note 14 - Income Taxs - Unrecog

Note 14 - Income Taxs - Unrecognized Tax Benefit Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Balance, beginning	$ 1,282	$ 684	$ 891
Increases Related to Tax Positions for Prior Years	37	6	28
Decreases Related to Tax Positions for Prior Years	0	0	(172)
Increases Related to Tax Positions for Current Year	339	778	143
Uncertain Positions Resolved During Year	(170)	(186)	(206)
Balance, ending	$ 1,488	$ 1,282	$ 684

Note 15 - Asset Retirement Ob_3

Note 15 - Asset Retirement Obligations (AROs) (Details Textual) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Asset Retirement Obligation, Liabilities Incurred	$ 0	$ 0
Otter Tail Power Company [Member]
Asset Retirement Obligation, Liabilities Incurred	$ 0
North Dakota 1 [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, 2019	Dec. 31, 2018
Asset Retirement Obligations
Beginning Balance	$ 9,117	$ 8,719
Asset Retirement Obligation, Liabilities Incurred	0	0
Adjustments Due to Revisions in Cash Flow Estimates	3,099	0
Accrued Accretion	440	398
Settlements	0	0
Ending Balance	12,656	9,117
Asset Retirement Costs Capitalized
Beginning Balance	2,983	2,983
New Obligations Recognized	0	0
Adjustments Due to Revisions in Cash Flow Estimates	3,099	0
Settlements	0	0
Ending Balance	6,082	2,983
Accumulated Depreciation – Asset Retirement Costs Capitalized
Beginning Balance	1,034	915
New Obligations Recognized	0	0
Adjustments Due to Revisions in Cash Flow Estimates	0	0
Depreciation Expense	163	119
Settlements	0	0
Ending Balance	1,197	1,034
Original Capitalized Asset Retirement Cost – Retired	0	0
Accumulated Depreciation	0	0
Asset Retirement Obligation	0	0
Settlement Cost	0	0
Gain on Settlement – Deferred Under Regulatory Accounting	$ 0	$ 0

Note 16 - Subsequent Events (De

Note 16 - Subsequent Events (Details Textual) - The 2014 Stock Incentive Plan [Member] - USD ($) $ in Millions	Feb. 12, 2020	Feb. 13, 2019	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized			1,900,000
Executive Officer [Member] | Stock Performance Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		55,600
Period Specified for Average Adjusted Return			3 years
Number of Trading Days		20 days
Number of Shares Authorized for Actual Payment		83,400
Executive Officer [Member] | Stock Performance Awards [Member] | Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		27,800
Subsequent Event [Member] | Stock Performance Awards [Member]
Period Specified for Average Adjusted Return	3 years
Number of Trading Days	20 days
Subsequent Event [Member] | Executive Officer [Member] | Stock Performance Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Grant Date Fair Value	$ 3.3
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	55,000
Number of Shares Authorized for Actual Payment	82,500
Subsequent Event [Member] | Executive Officer [Member] | Stock Performance Awards [Member] | Minimum [Member]
Actual Payment, Percentage of Target Amount	0.00%
Subsequent Event [Member] | Executive Officer [Member] | Stock Performance Awards [Member] | Maximum [Member]
Actual Payment, Percentage of Target Amount	150.00%
Subsequent Event [Member] | Executive Officer [Member] | Stock Performance Awards [Member] | Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	27,500
Subsequent Event [Member] | Executive Officer [Member] | Stock Performance Awards [Member] | Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	27,500

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. 12, 2020shares
The 2014 Stock Incentive Plan [Member] | Restricted Stock Units (RSUs) [Member]
Shares/Units Granted (in shares)	15,300
The 2014 Stock Incentive Plan Under Executive Agreement [Member] | Performance Shares [Member]
Shares/Units Granted (in shares)	47,600
Vesting date	Dec. 31, 2022
The 2014 Stock Incentive Plan Under Legacy Agreement [Member] | Performance Shares [Member]
Shares/Units Granted (in shares)	7,400
Vesting date	Dec. 31, 2022

Schedule 1 - Condensed Financ_3

Schedule 1 - Condensed Financial Information of Registrant - Condensed Balance Sheets, Parent Company (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Cash and Cash Equivalents	$ 21,199	$ 861	$ 16,216	$ 0
Accounts Receivable	77,947	75,144
Accounts Receivable from Subsidiaries	2,144	1,931
Interest Receivable from Subsidiaries	117	117
Notes Receivable from Subsidiaries	0	1,167
Income Taxes Receivable	1,487	2,439
Other	5,042	6,114
Total Current Assets	254,860	241,420
Investments in Subsidiaries	9,894	8,961
Notes Receivable from Subsidiaries	79,251	79,422
Operating Lease, Right-of-Use Asset	21,851	0
Other Assets	40,196	35,759
Identifiable Assets	2,273,595	2,052,517	2,004,278
Liabilities and Equity
Short-Term Debt	6,000	18,599
Less: Current Maturities--net of Unamortized Debt Issuance Costs	183	172
Accounts Payable to Subsidiaries	7	7
Notes Payable to Subsidiaries	89,611	60,626
Current Operating Lease Liabilities	4,136	0
Other	10,912	12,149
Total Current Liabilities	189,741	170,093
Long Term Operating Lease Liabilities	18,193	0
Other Noncurrent Liabilities	30,833	24,326
Capitalization
Total Long-Term Debt net of Unamortized Debt Issuance Costs	689,581	590,002
Common Shareholder Equity	781,482	728,863	696,892	670,104
Capitalization, Long-term Debt and Equity, Total	1,471,063	1,318,865
Total Liabilities and Equity	2,273,595	2,052,517
Parent Company [Member]
Assets
Cash and Cash Equivalents	4,959	0	$ 16,371	$ 6,218
Accounts Receivable	0	0
Accounts Receivable from Subsidiaries	2,144	1,931
Interest Receivable from Subsidiaries	117	117
Notes Receivable from Subsidiaries	0	1,167
Income Taxes Receivable	1,487	0
Other	1,050	3,482
Total Current Assets	9,757	6,697
Investments in Subsidiaries	860,646	787,869
Notes Receivable from Subsidiaries	79,251	79,422
Deferred Income Taxes	25,505	21,100
Operating Lease, Right-of-Use Asset	637	0
Other Assets	35,503	31,547
Identifiable Assets	1,011,299	926,635
Liabilities and Equity
Short-Term Debt	6,000	9,215
Less: Current Maturities--net of Unamortized Debt Issuance Costs	183	172
Accounts Payable to Subsidiaries	7	7
Notes Payable to Subsidiaries	89,611	60,626
Current Operating Lease Liabilities	156	0
Other	9,473	9,994
Total Current Liabilities	105,430	80,014
Long Term Operating Lease Liabilities	533	0
Other Noncurrent Liabilities	44,042	37,814
Capitalization
Total Long-Term Debt net of Unamortized Debt Issuance Costs	79,812	79,944
Common Shareholder Equity	781,482	728,863
Capitalization, Long-term Debt and Equity, Total	861,294	808,807
Total Liabilities and Equity	$ 1,011,299	$ 926,635

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Loss
Operating Expenses	$ 784,623	$ 787,058	$ 717,063
Operating Loss	134,880	129,389	132,287
Other Income (Expense)
Interest Charges	(31,411)	(30,408)	(29,604)
Nonservice Cost Components of Postretirement Benefits	4,293	5,509	5,620
Other Income	5,112	3,461	2,632
Income Tax (Benefit) Expense	17,441	14,588	27,256
Net Income	86,847	82,345	72,439
Parent Company [Member]
Operating Loss
Revenue from Contracts with Customers	0	0	0
Operating Expenses	10,529	9,916	7,138
Operating Loss	(10,529)	(9,916)	(7,138)
Other Income (Expense)
Equity Income in Earnings of Subsidiaries	93,731	91,446	82,715
Interest Charges	(4,863)	(4,043)	(4,270)
Interest Charges to Subsidiaries	(306)	(387)	(244)
Interest Income from Subsidiaries	3,063	2,839	2,848
Nonservice Cost Components of Postretirement Benefits	1,297	1,422	1,215
Other Income	1,566	550	1,054
Total Other Income	91,894	88,983	80,888
Income Before Income Taxes	81,365	79,067	73,750
Income Tax (Benefit) Expense	(5,482)	(3,278)	1,311
Net Income	$ 86,847	$ 82,345	$ 72,439

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Flows from Operating Activities
Net Cash Provided by Operating Activities	$ 185,037	$ 143,448	$ 173,577
Cash Flows from Investing Activities
Investment in Subsidiaries	(10,626)	(4,372)	(4,168)
Net Cash (Used in) Provided by Investing Activities	(209,472)	(107,419)	(132,590)
Cash Flows from Financing Activities
Change in Checks Written in Excess of Cash	(2,814)	(345)	2,434
Net Short-Term (Repayments) Borrowings	(12,599)	(93,772)	69,488
Proceeds from Issuance of Common Stock	20,338	0	4,349
Common Stock Issuance Expenses	(577)	(108)	0
Payments for Retirement of Capital Stock	(2,730)	(3,011)	(1,799)
Short-Term and Long-Term Debt Issuance Expenses	(950)	(761)	(380)
Payments for Retirement of Long-Term Debt	(172)	(189)	(48,231)
Dividends Paid	(55,723)	(53,198)	(50,632)
Net Cash Used in Financing Activities	44,773	(51,384)	(24,771)
Net Change in Cash and Cash Equivalents	20,338	(15,355)	16,216
Cash and Cash Equivalents at Beginning of Period	861	16,216	0
Cash and Cash Equivalents at End of Period	21,199	861	16,216
Parent Company [Member]
Cash Flows from Operating Activities
Net Cash Provided by Operating Activities	52,263	56,947	50,205
Cash Flows from Investing Activities
Investment in Subsidiaries	(34,990)	(24,764)	0
Debt Repaid by Subsidiaries	1,338	774	151
Cash Used in Investing Activities	(257)	(623)	(121)
Net Cash (Used in) Provided by Investing Activities	(33,909)	(24,613)	30
Cash Flows from Financing Activities
Change in Checks Written in Excess of Cash	(31)	31	0
Net Short-Term (Repayments) Borrowings	(3,215)	9,215	0
Borrowings from (Repayments to) Subsidiaries	28,985	(1,281)	23,389
Proceeds from Issuance of Common Stock	20,338	0	4,349
Common Stock Issuance Expenses	(577)	(108)	0
Payments for Retirement of Capital Stock	(2,730)	(3,011)	(1,799)
Short-Term and Long-Term Debt Issuance Expenses	(270)	(164)	(158)
Payments for Retirement of Long-Term Debt	(172)	(189)	(15,231)
Dividends Paid	(55,723)	(53,198)	(50,632)
Net Cash Used in Financing Activities	(13,395)	(48,705)	(40,082)
Net Change in Cash and Cash Equivalents	4,959	(16,371)	10,153
Cash and Cash Equivalents at Beginning of Period	0	16,371	6,218
Cash and Cash Equivalents at End of Period	$ 4,959	$ 0	$ 16,371

Schedule 1 - Condensed Financ_6

Schedule 1 - Condensed Financial Information of Registrant - Related Party Transactions (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable	$ 2,144	$ 1,931
Interest Receivable	117	117
Current Notes Receivable	0	1,167
Long-Term Notes Receivable	79,251	79,422
Accounts Payable	7	7
Current Notes Payable	89,611	60,626
Otter Tail Corporation [Member] | Otter Tail Power Company [Member]
Accounts Receivable	2,056	1,877
Interest Receivable	0	0
Current Notes Receivable	0	0
Long-Term Notes Receivable	0	0
Accounts Payable	7	7
Current Notes Payable	0	0
Otter Tail Corporation [Member] | Northern Pipe Products, Inc. [Member]
Accounts Receivable	0	0
Interest Receivable	8	8
Current Notes Receivable	0	0
Long-Term Notes Receivable	5,351	5,522
Accounts Payable	0	0
Current Notes Payable	3,056	5,623
Otter Tail Corporation [Member] | Vinyltech Corporation [Member]
Accounts Receivable	4	4
Interest Receivable	17	17
Current Notes Receivable	0	0
Long-Term Notes Receivable	11,500	11,500
Accounts Payable	0	0
Current Notes Payable	15,099	15,305
Otter Tail Corporation [Member] | BTD Manufacturing, Inc. [Member]
Accounts Receivable	0	0
Interest Receivable	77	77
Current Notes Receivable	0	415
Long-Term Notes Receivable	52,000	52,000
Accounts Payable	0	0
Current Notes Payable	18,474	0
Otter Tail Corporation [Member] | T. O. Plastics, Inc. [Member]
Accounts Receivable	0	0
Interest Receivable	15	15
Current Notes Receivable	0	0
Long-Term Notes Receivable	10,400	10,400
Accounts Payable	0	0
Current Notes Payable	3,099	14,308
Otter Tail Corporation [Member] | Varistar Corporation [Member]
Accounts Receivable	0	0
Interest Receivable	0	0
Current Notes Receivable	0	752
Long-Term Notes Receivable	0	0
Accounts Payable	0	0
Current Notes Payable	49,883	25,390
Otter Tail Corporation [Member] | Otter Tail Assurance Limited [Member]
Accounts Receivable	84	50
Interest Receivable	0	0
Current Notes Receivable	0	0
Long-Term Notes Receivable	0	0
Accounts Payable	0	0
Current Notes Payable	$ 0	$ 0

Schedule 1 - Condensed Financ_7

Schedule 1 - Condensed Financial Information of Registrant - Dividends (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Dividends Paid to Parent by Subsidiaries	$ 55,660	$ 53,134	$ 50,571