XML 114 R98.htm IDEA: XBRL DOCUMENT v3.8.0.1
Income Taxes - Summary of income tax expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
Current tax expense $ 111.8 $ 72.7 $ 15.1
Deferred income taxes, net 274.4 498.7 420.4
Investment tax credit, net (2.7) (4.9) (1.7)
Total income tax expense $ 383.5 $ 566.5 $ 433.8