Employee Benefits - Amounts Not Yet Recognized in Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Pension Costs
Accumulated other comprehensive loss (pre-tax)
Net actuarial loss (gain)	$ 14.5	$ 10.0
Prior service credits	0.0	0.0
Total	14.5	10.0
Net regulatory assets
Net actuarial loss (gain)	1,184.1	1,136.8
Prior service costs (credits)	4.9	7.5
Total	1,189.0	1,144.3
Estimated amounts that will be amortized into net periodic benefit cost next year
Net actuarial loss (gain)	76.1
Prior service costs (credits)	2.2
Total 2019 – estimated amortization	78.3
OPEB Costs
Accumulated other comprehensive loss (pre-tax)
Net actuarial loss (gain)	(1.6)	(1.0)
Prior service credits	(0.1)	(0.1)
Total	(1.7)	(1.1)
Net regulatory assets
Net actuarial loss (gain)	(133.0)	(4.7)
Prior service costs (credits)	(100.0)	(111.8)
Total	(233.0)	$ (116.5)
Estimated amounts that will be amortized into net periodic benefit cost next year
Net actuarial loss (gain)	(5.7)
Prior service costs (credits)	(15.4)
Total 2019 – estimated amortization	$ (21.1)