Income Taxes Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Non-current
Future tax benefits	$ 430.4	$ 382.8
Employee benefits and compensation	222.0	229.9
Deferred revenues	207.2	219.9
Property-related	54.5	59.5
Other	230.6	177.1
Total deferred tax assets	1,144.7	1,069.2
Valuation allowance	(15.0)	(17.1)
Net deferred tax assets	1,129.7	1,052.1
Non-current
Property-related	4,979.3	4,451.5
Investment in transmission affiliate	476.9	420.4
Employee benefits and compensation	401.6	428.9
Deferred transmission costs	93.1	76.7
Other	325.4	296.9
Total deferred tax liabilities	6,276.3	5,674.4
Deferred tax liability, net	$ 5,146.6	$ 4,622.3