Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Measurements, Recurring | Parent Company [Member] | Level 3 | Electric Portfolio
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	$ 4,522
Derivative Liabilities	3,160
Fair Value, Measurements, Recurring | Parent Company [Member] | Level 3 | Natural Gas Portfolio
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	3,485
Derivative Liabilities	1,812
Fair Value, Measurements, Recurring | Subsidiaries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	49,019	$ 24,405
Derivative Liabilities	57,756	86,094
Fair Value, Measurements, Recurring | Subsidiaries [Member] | Electric Portfolio
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	33,287	13,391
Derivative Liabilities	27,284	49,050
Fair Value, Measurements, Recurring | Subsidiaries [Member] | Natural Gas Portfolio
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	15,732	11,014
Derivative Liabilities	30,472	37,044
Fair Value, Measurements, Recurring | Subsidiaries [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	41,012	16,839
Derivative Liabilities	52,784	81,549
Fair Value, Measurements, Recurring | Subsidiaries [Member] | Level 2 | Electric Portfolio
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	28,765	9,866
Derivative Liabilities	24,124	46,623
Fair Value, Measurements, Recurring | Subsidiaries [Member] | Level 2 | Natural Gas Portfolio
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	12,247	6,973
Derivative Liabilities	28,660	34,926
Fair Value, Measurements, Recurring | Subsidiaries [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	8,007	7,566
Derivative Liabilities	4,972	4,545
Fair Value, Measurements, Recurring | Subsidiaries [Member] | Level 3 | Electric Portfolio
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets		3,525
Derivative Liabilities		2,427
Fair Value, Measurements, Recurring | Subsidiaries [Member] | Level 3 | Natural Gas Portfolio
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets		4,041
Derivative Liabilities		2,118
Carrying Amount | Parent Company [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable and other	$ 49,500	$ 48,500