Schedule of Other Operating Revenues and Operating Expenses (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Component of Operating Other Cost and Expense [Abstract]
Revenue	$ 87.1	$ 82.6	$ 315.0	$ 286.3
Less: Revenue Sharing Payments	0.3	0.3	0.8	0.8
Total Other Operating Revenues	1.1		3.5
Operation and Maintenance Expense	16.4	16.9	51.5	49.4
Usource Contract Revenues [Member]
Component of Operating Other Cost and Expense [Abstract]
Revenue	$ 1.4		$ 4.3
ASU 2014-09
Component of Operating Other Cost and Expense [Abstract]
Less: Revenue Sharing Payments		0.3		0.8
Total Other Operating Revenues		1.1		3.7
Operation and Maintenance Expense		16.6		48.6
ASU 2014-09 | Usource Contract Revenues [Member]
Component of Operating Other Cost and Expense [Abstract]
Revenue		$ 1.4		$ 4.5