Employee Benefits - Amounts Not Yet Recognized in Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Pension Benefits
Pre-tax accumulated other comprehensive income (loss)
Net actuarial loss (gain)	$ 7.5	$ 10.4
Prior service credits	0.0	0.0
Total	7.5	10.4
Net regulatory assets (liabilities)
Net actuarial loss (gain)	798.6	1,101.2
Prior service costs (credits)	(0.5)	1.1
Total	798.1	1,102.3
OPEB Benefits
Pre-tax accumulated other comprehensive income (loss)
Net actuarial loss (gain)	(1.4)	(1.4)
Prior service credits	(0.1)	(0.1)
Total	(1.5)	(1.5)
Net regulatory assets (liabilities)
Net actuarial loss (gain)	(300.1)	(288.7)
Prior service costs (credits)	(60.3)	(78.6)
Total	$ (360.4)	$ (367.3)