Accumulated Other Comprehensive Income: Accumulated Other Comprehensive (Loss) Income (Roll-Forward) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)
Unrealized Net Gains on Interest Rate Swaps
Accumulated other comprehensive loss [Roll Forward]
Beginning Balance	$ 17.6
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0.0
Amounts reclassified from other accumulated comprehensive income (a)	(4.9)
Ending Balance	12.7
Pension and Postretirement Plans
Accumulated other comprehensive loss [Roll Forward]
Beginning Balance	5.0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(0.7)
Amounts reclassified from other accumulated comprehensive income (a)	(0.5)
Ending Balance	3.8
Total
Accumulated other comprehensive loss [Roll Forward]
Beginning Balance	22.6
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(0.7)
Amounts reclassified from other accumulated comprehensive income (a)	(5.4)
Ending Balance	$ 16.5