Employee Benefits - Amounts Not Yet Recognized in Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Accumulated other comprehensive loss (pre-tax)
Net actuarial loss (gain)	$ 10.6	$ 14.5
Prior service credits	0.0	0.0
Total	10.6	14.5
Net regulatory assets (liabilities)
Net actuarial loss (gain)	1,067.7	1,184.1
Prior service costs (credits)	2.7	4.9
Total	1,070.4	1,189.0
Estimated amounts that will be amortized into net periodic benefit cost next year
Net actuarial loss (gain)	(97.1)
Prior service costs (credits)	1.6
Total 2020 – estimated amortization	98.7
OPEB Benefits
Accumulated other comprehensive loss (pre-tax)
Net actuarial loss (gain)	(1.6)	(1.6)
Prior service credits	(0.1)	(0.1)
Total	(1.7)	(1.7)
Net regulatory assets (liabilities)
Net actuarial loss (gain)	(266.6)	(133.0)
Prior service costs (credits)	(88.6)	(100.0)
Total	(355.2)	$ (233.0)
Estimated amounts that will be amortized into net periodic benefit cost next year
Net actuarial loss (gain)	21.5
Prior service costs (credits)	(15.0)
Total 2020 – estimated amortization	$ (36.5)