XML 41 R27.htm IDEA: XBRL DOCUMENT v3.8.0.1
Schedule of Other Operating Revenues and Operating Expenses (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Component of Operating Other Cost and Expense [Abstract]    
Usource Contract Revenue $ 1.6  
Less: Revenue Sharing Payments 0.3 $ 0.3
Total Other Operating Revenues 1.3  
Operation and Maintenance Expense $ 17.3 16.0
ASU 2014-09    
Component of Operating Other Cost and Expense [Abstract]    
Usource Contract Revenue   1.7
Less: Revenue Sharing Payments   0.3
Total Other Operating Revenues   1.4
Operation and Maintenance Expense   $ 15.7