Employee Benefits - Amounts Not Yet Recognized in Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension Benefits
Accumulated other comprehensive loss (pre-tax)
Net actuarial loss (gain)	$ 10.4	$ 10.6
Prior service credits	0.0	0.0
Total	10.4	10.6
Net regulatory assets (liabilities)
Net actuarial loss (gain)	1,101.2	1,067.7
Prior service costs (credits)	1.1	2.7
Total	1,102.3	1,070.4
OPEB Benefits
Accumulated other comprehensive loss (pre-tax)
Net actuarial loss (gain)	(1.4)	(1.6)
Prior service credits	(0.1)	(0.1)
Total	(1.5)	(1.7)
Net regulatory assets (liabilities)
Net actuarial loss (gain)	(288.7)	(266.6)
Prior service costs (credits)	(78.6)	(88.6)
Total	$ (367.3)	$ (355.2)