Recent Accounting Pronouncements (Schedule of New Accounting Pronouncements and Changes in Accounting Princicples) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017		Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance	$ 365.2	$ 391.6	$ 767.7	$ 803.2
Total Operating Expenses	888.6	866.7	2,238.8	2,049.9
Operating Income	118.4	124.0	519.0	539.4
Other, net	12.8	4.2	44.1	6.5
Total Other Deductions	(88.4)	(195.0)	(150.2)	(277.9)
Income before Income Taxes	30.0	(71.0)	368.8	261.5
Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance					$ 1,612.3		$ 1,453.7
Total Operating Expenses					3,964.0		3,634.3
Operating Income					910.6		858.2
Other, net					(2.8)		1.5
Total Other Deductions					(467.5)		(348.0)
Income before Income Taxes					443.1		510.2
As Adjusted
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance					1,601.7		1,445.8
Total Operating Expenses					3,953.4		3,626.4
Operating Income					921.2		866.1
Other, net					(13.4)		(6.4)
Total Other Deductions					(478.1)		(355.9)
Income before Income Taxes					443.1		510.2
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance					(10.6)	[1]	(7.9)	[2]
Total Operating Expenses					(10.6)	[1]	(7.9)	[2]
Operating Income					10.6	[1]	7.9	[2]
Other, net					(10.6)	[1]	(7.9)	[2]
Total Other Deductions					(10.6)	[1]	(7.9)	[2]
Income before Income Taxes					0.0	[1]	0.0	[2]
Gas Distribution Operations
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Income	39.1	43.7	360.8	382.5
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Operating Results					(4.4)		4.3
Electric Operations
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Income	82.4	85.6	165.5	163.2
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Operating Results					(2.6)		(9.8)
Corporate and Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Income	$ (3.1)	$ (5.3)	$ (7.3)	$ (6.3)
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Operating Results					$ (3.6)		$ (2.4)

[1] The effect of this change is attributable to our business segments: Gas Distribution Operations, Electric Operations, and Corporate and Other in the amounts of $(4.4) million, $(2.6) million, and $(3.6) million, respectively.

[2] The effect of this change is attributable to our business segments: Gas Distribution Operations, Electric Operations, and Corporate and Other in the amounts of $4.3 million, $(9.8) million, and $(2.4) million, respectively.