Income Taxes: TCJA (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit)		$ 103.0
Revenue Refunded To Customers As A Result Of The TCJA Tax Benefits	$ 7.6
Deferred Income Tax Charges
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit)		$ 97.0
Tax Cuts and Jobs Act of 2017, Provisional Income Tax Expense (Benefit)	100.0
Revenue Subject to Refund
Revenue Refunded To Customers As A Result Of The TCJA Tax Benefits	$ 0.9