Schedule of Other Operating Revenues and Operating Expenses (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Component of Operating Other Cost and Expense [Abstract]
Revenue	$ 83.4	$ 79.4	$ 227.9	$ 203.7
Less: Revenue Sharing Payments	0.2	0.3	0.5	0.5
Total Other Operating Revenues	1.1		2.4
Operation and Maintenance Expense	17.8	16.5	35.1	32.5
Usource Contract Revenues [Member]
Component of Operating Other Cost and Expense [Abstract]
Revenue	$ 1.3		$ 2.9
ASU 2014-09
Component of Operating Other Cost and Expense [Abstract]
Less: Revenue Sharing Payments		0.3		0.5
Total Other Operating Revenues		1.1		2.6
Operation and Maintenance Expense		16.2		32.0
ASU 2014-09 | Usource Contract Revenues [Member]
Component of Operating Other Cost and Expense [Abstract]
Revenue		$ 1.4		$ 3.1