Employee Retirement Plans (Plan Assets and Investment Strategy) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 303.3	$ 313.0	$ 443.5
Redemption Notice Period | JP Morgan Chase Bank Strategic Property Fund
Defined Benefit Plan Disclosure [Line Items]
Redemption notice period	30 days
Redemption Notice Period | Pyramis Long Corporate A or Better
Defined Benefit Plan Disclosure [Line Items]
Redemption notice period	15 days
Redemption Notice Period | Pyramis Long Duration
Defined Benefit Plan Disclosure [Line Items]
Redemption notice period	15 days
Redemption Notice Period | Pyramis 810 Corporate
Defined Benefit Plan Disclosure [Line Items]
Redemption notice period	15 days
Redemption Notice Period | Russell 3000 Index NL
Defined Benefit Plan Disclosure [Line Items]
Redemption notice period	1 day
Redemption Notice Period | NT Collective Short Term Investment Fund
Defined Benefit Plan Disclosure [Line Items]
Redemption notice period	1 day
Fair Value, Recurring | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0.5	0.6
Fair Value, Recurring | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	86.8	89.6
Fair Value, Recurring | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Recurring | NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	216.0	222.8
Fair Value, Recurring | NAV | JP Morgan Chase Bank Strategic Property Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9.7	11.7
Fair Value, Recurring | NAV | Pyramis Long Corporate A or Better
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24.0	24.0
Fair Value, Recurring | NAV | Pyramis Long Duration
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12.2	12.4
Fair Value, Recurring | NAV | Pyramis 810 Corporate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	123.5	124.7
Fair Value, Recurring | NAV | Russell 3000 Index NL
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	42.7	46.0
Fair Value, Recurring | NAV | NT Collective Short Term Investment Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3.9	4.0
Fair Value, Recurring | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.5	0.6
Fair Value, Recurring | Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.5	0.6
Fair Value, Recurring | Cash and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Recurring | Cash and cash equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Recurring | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	75.4	76.2
Fair Value, Recurring | Debt securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Recurring | Debt securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	75.4	76.2
Fair Value, Recurring | Debt securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Recurring | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11.4	13.4
Fair Value, Recurring | Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Recurring | Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11.4	13.4
Fair Value, Recurring | Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Recurring | Pension Plan Assets, Excluding Investments Measured At NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 87.3	$ 90.2