Supplemental Cash Flow Information - Supplemental Cash Flow Information and Non-cash Investing and Financing Activities (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Supplemental cash flow information
Cash paid for interest	$ 28,317	$ 26,004
Non-cash investing and financing activities
Change in accrued additions to electric plant	(2,943)	8,901
Allowance for funds used during construction - debt	1,971	2,261
Redemption of operating partnership units for common stock	3,133	492
Dividends and distributions payable	$ 15,176	$ 15,169	$ 15,169

Change in accrued additions to electric plant.

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- Definition

Non cash or part non cash allowance for funds used during construction.

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Noncash redemption of operating partnership units for common stock.

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Carrying value as of the balance sheet date of dividends declared but unpaid on equity securities issued by the entity and outstanding. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Amount of cash paid for interest, excluding capitalized interest, classified as operating activity. Includes, but is not limited to, payment to settle zero-coupon bond for accreted interest of debt discount and debt instrument with insignificant coupon interest rate in relation to effective interest rate of borrowing attributable to accreted interest of debt discount.

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