Fair Value (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Level 1 [Member]
Total Master Trust Assets, Fair Value	$ 0.2	$ 0.3
Total Derivative Assets	0.0	0.0
Total Assets	0.2	0.3
Total Liabilities	0.0	0.0
Level 1 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Level 1 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.2	0.3
Total Derivative Assets	0.0	0.0
Total Assets	0.2	0.3
Total Liabilities	0.0	0.0
Level 1 [Member] | Subsidiaries [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Level 2 [Member]
Total Master Trust Assets, Fair Value	8.0	8.5
Total Derivative Assets	0.0	0.1
Total Assets	8.0	8.6
Total Liabilities	1,482.0	1,386.5
Level 2 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.1
Level 2 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	8.0	8.5
Total Derivative Assets	0.0	0.1
Total Assets	8.0	8.6
Total Liabilities	597.8	583.0
Level 2 [Member] | Subsidiaries [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.1
Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Total Derivative Assets	0.0	0.0
Total Assets	0.0	0.0
Total Liabilities	17.4	17.5
Level 3 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Level 3 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Total Derivative Assets	0.0	0.0
Total Assets	0.0	0.0
Total Liabilities	17.4	17.5
Level 3 [Member] | Subsidiaries [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Fair Value [Member]
Total Master Trust Assets, Fair Value	8.2	8.8
Total Derivative Assets	0.0	0.1
Total Assets	8.2	8.9
Total Liabilities	1,499.4	1,404.0
Fair Value [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.1
Fair Value [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	8.2	8.8
Total Derivative Assets	0.0	0.1
Total Assets	8.2	8.9
Total Liabilities	615.2	600.5
Fair Value [Member] | Subsidiaries [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.1
Money Market Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.2	0.3
Money Market Funds [Member] | Level 1 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.2	0.3
Money Market Funds [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Money Market Funds [Member] | Level 2 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Money Market Funds [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Money Market Funds [Member] | Level 3 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Money Market Funds [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.2	0.3
Money Market Funds [Member] | Fair Value [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.2	0.3
Equity Securities [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Equity Securities [Member] | Level 1 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Equity Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	3.9	4.2
Equity Securities [Member] | Level 2 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	3.9	4.2
Equity Securities [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Equity Securities [Member] | Level 3 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Equity Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	3.9	4.2
Equity Securities [Member] | Fair Value [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	3.9	4.2
Debt Securities [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] | Level 1 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	4.1	4.1
Debt Securities [Member] | Level 2 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	4.1	4.1
Debt Securities [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] | Level 3 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	4.1	4.1
Debt Securities [Member] | Fair Value [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	4.1	4.1
Hedge Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Hedge Funds [Member] | Level 1 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Hedge Funds [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Hedge Funds [Member] | Level 2 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Hedge Funds [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Hedge Funds [Member] | Level 3 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Hedge Funds [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Hedge Funds [Member] | Fair Value [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Tangible Assets [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Tangible Assets [Member] | Level 1 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Tangible Assets [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Tangible Assets [Member] | Level 2 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Tangible Assets [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Tangible Assets [Member] | Level 3 [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Tangible Assets [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Tangible Assets [Member] | Fair Value [Member] | Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Debt [Member] | Level 1 [Member]
Debt	0.0	0.0
Debt [Member] | Level 1 [Member] | Subsidiaries [Member]
Debt	0.0	0.0
Debt [Member] | Level 2 [Member]
Debt	1,482.0	1,386.5
Debt [Member] | Level 2 [Member] | Subsidiaries [Member]
Debt	597.8	583.0
Debt [Member] | Level 3 [Member]
Debt	17.4	17.5
Debt [Member] | Level 3 [Member] | Subsidiaries [Member]
Debt	17.4	17.5
Debt [Member] | Fair Value [Member]
Debt	1,499.4	1,404.0
Debt [Member] | Fair Value [Member] | Subsidiaries [Member]
Debt	$ 615.2	$ 600.5