Supplemental Cash Flow Information - Supplemental Cash Flow Information and Non-cash Investing and Financing Activities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Supplemental cash flow information
Cash paid for interest	$ 7,832	$ 7,469
Establishment of operating lease right-of-use assets and liabilities	363
Non-cash investing and financing activities
Change in accrued additions to electric plant	(242)	(2,902)
Allowance for funds used during construction - debt	988	820
Redemption of operating partnership units for common stock	116	3,106
Dividends and distributions payable	$ 15,182	$ 15,176	$ 15,176