CONDENSED CONSOLIDATED STATEMENTS OF CAPITAL STOCK AND LONG-TERM DEBT (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Common Stock, No Par Value	$ 217,976	$ 217,451
Retained Earnings	130,872	128,757
TOTAL COMMON EQUITY	348,848	346,208
TOTAL PREFERRED STOCK	2,084	2,084
SUBTOTAL LONG-TERM DEBT	276,605	277,241
Add: Premium on Issuance of Long-term Debt	7,569	7,669
Less: Unamortized Debt Expense	(4,351)	(4,411)
Less: Current Portion of Long-term Debt	(7,159)	(7,255)
TOTAL LONG-TERM DEBT	272,664	273,244
Amortizing Secured Note, due December 20, 2021 [Member]
SUBTOTAL LONG-TERM DEBT	255	336
Amortizing Secured Note, due May 19, 2028 [Member]
SUBTOTAL LONG-TERM DEBT	3,010	3,115
Amortizing Secured Note, due August 25, 2030 [Member]
SUBTOTAL LONG-TERM DEBT	2,637	2,707
Amortizing Secured Note, due September 19, 2031 [Member]
SUBTOTAL LONG-TERM DEBT	2,917	2,987
State Revolving Trust Note, due December 31, 2022 [Member]
SUBTOTAL LONG-TERM DEBT	119	119
State Revolving Trust Note, due May 1, 2025 [Member]
SUBTOTAL LONG-TERM DEBT	1,170	1,170
State Revolving Trust Note, due March 1, 2026 [Member]
SUBTOTAL LONG-TERM DEBT	243	266
State Revolving Trust Note, due January 25, 2027 [Member]
SUBTOTAL LONG-TERM DEBT	286	307
State Revolving Trust Note, due December 1, 2026 [Member]
SUBTOTAL LONG-TERM DEBT	390	389
State Revolving Trust Bond 0.00%, due August 1, 2021 [Member]
SUBTOTAL LONG-TERM DEBT	10	11
State Revolving Trust Note, due July 1, 2028 [Member]
SUBTOTAL LONG-TERM DEBT	192	192
State Revolving Trust Note, due January 1, 2028 [Member]
SUBTOTAL LONG-TERM DEBT	62	62
State Revolving Trust Note, due August 1, 2031 [Member]
SUBTOTAL LONG-TERM DEBT	763	793
Amortizing Secured Note, due April 20, 2029 [Member]
SUBTOTAL LONG-TERM DEBT	2,819	2,907
Amortizing Secured Note 7.05%, due January 20, 2030 [Member]
SUBTOTAL LONG-TERM DEBT	2,208	2,271
Amortizing Secured Note 5.69%, due January 20, 2030 [Member]
SUBTOTAL LONG-TERM DEBT	4,530	4,658
Amortizing Secured Note, due April 20, 2040 [Member]
SUBTOTAL LONG-TERM DEBT	8,397	8,506
Amortizing Secured Note, due April 20, 2040 [Member]
SUBTOTAL LONG-TERM DEBT	3,116	3,156
State Revolving Trust Note, due July 1, 2031 [Member]
SUBTOTAL LONG-TERM DEBT	1,699	1,699
State Revolving Trust Note, due February 1, 2036 [Member]
SUBTOTAL LONG-TERM DEBT	934	961
State Revolving Trust Note, due November 1, 2038 [Member]
SUBTOTAL LONG-TERM DEBT	1,543	1,543
State Revolving Trust Note, due November 30, 2030 [Member]
SUBTOTAL LONG-TERM DEBT	883	883
Construction Loans [Member]
SUBTOTAL LONG-TERM DEBT	51,223	50,536
First Mortgage Bonds - Series BB [Member]
SUBTOTAL LONG-TERM DEBT	116	119
First Mortgage Bonds - Series CC [Member]
SUBTOTAL LONG-TERM DEBT	164	164
First Mortgage Bonds - Series EE [Member]
SUBTOTAL LONG-TERM DEBT	996	1,036
First Mortgage Bonds - Series FF [Member]
SUBTOTAL LONG-TERM DEBT	1,870	1,870
First Mortgage Bonds - Series GG [Member]
SUBTOTAL LONG-TERM DEBT	531	541
First Mortgage Bonds - Series HH [Member]
SUBTOTAL LONG-TERM DEBT	620	620
First Mortgage Bonds - Series II [Member]
SUBTOTAL LONG-TERM DEBT	326	338
First Mortgage Bonds - Series JJ [Member]
SUBTOTAL LONG-TERM DEBT	500	500
First Mortgage Bonds - Series KK [Member]
SUBTOTAL LONG-TERM DEBT	704	719
First Mortgage Bonds - Series LL [Member]
SUBTOTAL LONG-TERM DEBT	846	846
First Mortgage Bonds - Series MM [Member]
SUBTOTAL LONG-TERM DEBT	903	937
First Mortgage Bonds - Series NN [Member]
SUBTOTAL LONG-TERM DEBT	1,105	1,105
First Mortgage Bonds - Series OO [Member]
SUBTOTAL LONG-TERM DEBT	1,605	1,656
First Mortgage Bonds - Series PP [Member]
SUBTOTAL LONG-TERM DEBT	600	600
First Mortgage Bonds - Series QQ [Member]
SUBTOTAL LONG-TERM DEBT	9,915	9,915
First Mortgage Bonds - Series RR [Member]
SUBTOTAL LONG-TERM DEBT	22,500	22,500
First Mortgage Bonds - Series SS [Member]
SUBTOTAL LONG-TERM DEBT	23,000	23,000
First Mortgage Bonds - Series TT [Member]
SUBTOTAL LONG-TERM DEBT	1,756	1,806
First Mortgage Bonds - Series UU [Member]
SUBTOTAL LONG-TERM DEBT	705	705
First Mortgage Bonds - Series VV [Member]
SUBTOTAL LONG-TERM DEBT	1,813	1,861
First Mortgage Bonds - Series WW [Member]
SUBTOTAL LONG-TERM DEBT	715	715
First Mortgage Bonds - Series XX [Member]
SUBTOTAL LONG-TERM DEBT	10,121	10,247
First Mortgage Bonds - Series YY [Member]
SUBTOTAL LONG-TERM DEBT	3,710	3,710
0.00%, Series 2018A, due August 1, 2047 [Member]
SUBTOTAL LONG-TERM DEBT	6,167	6,246
3.00%-5.00%, Series 2018B, due August 1, 2047 [Member]
SUBTOTAL LONG-TERM DEBT	2,211	2,211
4.00%, Series 2019A, due August 1, 2059 [Member]
SUBTOTAL LONG-TERM DEBT	32,500	32,500
5.00%, Series 2019B, due August 1, 2059 [Member]
SUBTOTAL LONG-TERM DEBT	21,200	21,200
2.90%, Series 2020A, due November 18, 2050 [Member]
SUBTOTAL LONG-TERM DEBT	40,000	40,000
Convertible Preferred Stock $7.00 Series [Member]
TOTAL PREFERRED STOCK	1,005	1,005
Nonredeemable Preferred Stock $7.00 Series [Member]
TOTAL PREFERRED STOCK	79	79
Nonredeemable Preferred Stock $4.75 Series [Member]
TOTAL PREFERRED STOCK	$ 1,000	$ 1,000