Employee Benefits, Projected Benefit Obligation, Fair Value of Assets, Funded Status and the Amounts Not Yet Recognized As Components of Net Periodic Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of period	$ 125.1	$ 140.7
Service cost	3.0	3.3	$ 3.5
Interest cost	3.9	4.5	4.4
Plan participants’ contributions	0.0	0.0
Actuarial (gain) loss	5.9	(4.6)
Benefits paid	(0.5)	(0.4)
Settlement	(15.2)	(18.4)
Benefit obligation at end of period	122.2	125.1	140.7
Change in Plan Assets [Roll Forward]
Fair value of plan assets at beginning of period	100.3	118.9
Actual return on plan assets	12.5	(3.6)
Benefits paid	(0.5)	(0.4)
Settlement	(15.2)	(18.4)
Company contributions	4.6	3.8
Plan participants’ contributions	0.0	0.0
Fair value of plan assets at end of period	101.7	100.3	118.9
Funded status	(20.5)	(24.8)
Items Not Recognized As Components Of Net Periodic Cost [Abstract]
Net actuarial loss	20.6	25.8
PBOP
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of period	35.6	41.4
Service cost	0.1	0.1	0.1
Interest cost	1.4	1.5	1.6
Plan participants’ contributions	1.1	1.0
Actuarial (gain) loss	1.9	(4.0)
Benefits paid	(3.6)	(4.4)
Settlement	0.0	0.0
Benefit obligation at end of period	36.5	35.6	41.4
Change in Plan Assets [Roll Forward]
Fair value of plan assets at beginning of period	85.0	88.2
Actual return on plan assets	8.2	0.0
Benefits paid	(3.6)	(4.4)
Settlement	0.0	0.0
Company contributions	0.1	0.2
Plan participants’ contributions	1.1	1.0
Fair value of plan assets at end of period	90.8	85.0	$ 88.2
Funded status	54.3	49.4
Items Not Recognized As Components Of Net Periodic Cost [Abstract]
Net actuarial loss	$ 1.1	$ 4.4