Employee Benefits, Master Trust Pension and PBOP Fair Value Hierarchy (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Amount committed to future capital calls in exchange for an ownership interest	$ 2.0
PBOP
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	$ 93.0	$ 96.2	$ 90.8
Allocation to fixed income securities	100.00%	100.00%
PBOP | Short-term investments
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	$ 4.3	$ 5.7
PBOP | Fixed income mutual funds
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	19.2	19.5
PBOP | Asset-backed securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	6.9	14.4
PBOP | Corporate Bond Securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	31.0	23.7
PBOP | Tax Exempt Securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	31.6	32.9
Retirement Plans
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	$ 103.5	$ 102.7	$ 101.7
Master Trust percentage of pension plan assets	43.20%	43.90%
Fair Value, Measurements, Recurring | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	$ 184.5	$ 176.3
Investments, Excluding Net Asset Value, Fair Value Disclosure	184.5	176.3
Alternative Investment	54.9	57.5
Investments, Fair Value Disclosure	239.4	233.8
Fair Value, Measurements, Recurring | Pension Plan | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	184.5	176.3
Fair Value, Measurements, Recurring | Pension Plan | Fair Value, Inputs, Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | Pension Plan | Fair Value, Inputs, Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | Pension Plan | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	69.1	59.9
Fair Value, Measurements, Recurring | Pension Plan | Equity securities | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	69.1	59.9
Fair Value, Measurements, Recurring | Pension Plan | Equity securities | Fair Value, Inputs, Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | Pension Plan | Equity securities | Fair Value, Inputs, Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | Pension Plan | Short-term investments
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	1.9	3.9
Fair Value, Measurements, Recurring | Pension Plan | Short-term investments | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	1.9	3.9
Fair Value, Measurements, Recurring | Pension Plan | Short-term investments | Fair Value, Inputs, Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | Pension Plan | Short-term investments | Fair Value, Inputs, Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | Pension Plan | Other assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	2.2
Fair Value, Measurements, Recurring | Pension Plan | Other assets | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	2.2
Fair Value, Measurements, Recurring | Pension Plan | Fixed income mutual funds
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	111.3	112.5
Fair Value, Measurements, Recurring | Pension Plan | Fixed income mutual funds | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	111.3	112.5
Fair Value, Measurements, Recurring | Pension Plan | Fixed income mutual funds | Fair Value, Inputs, Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | Pension Plan | Fixed income mutual funds | Fair Value, Inputs, Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | PBOP | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	23.5	25.2
Fair Value, Measurements, Recurring | PBOP | Fair Value, Inputs, Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	69.5	71.0
Fair Value, Measurements, Recurring | PBOP | Fair Value, Inputs, Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | PBOP | Short-term investments | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	4.3	5.7
Fair Value, Measurements, Recurring | PBOP | Short-term investments | Fair Value, Inputs, Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | PBOP | Short-term investments | Fair Value, Inputs, Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | PBOP | Fixed income mutual funds | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	19.2	19.5
Fair Value, Measurements, Recurring | PBOP | Fixed income mutual funds | Fair Value, Inputs, Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | PBOP | Fixed income mutual funds | Fair Value, Inputs, Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | PBOP | Asset-backed securities | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | PBOP | Asset-backed securities | Fair Value, Inputs, Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	6.9	14.4
Fair Value, Measurements, Recurring | PBOP | Asset-backed securities | Fair Value, Inputs, Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | PBOP | Corporate Bond Securities | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | PBOP | Corporate Bond Securities | Fair Value, Inputs, Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	31.0	23.7
Fair Value, Measurements, Recurring | PBOP | Corporate Bond Securities | Fair Value, Inputs, Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | PBOP | Tax Exempt Securities | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Measurements, Recurring | PBOP | Tax Exempt Securities | Fair Value, Inputs, Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	31.6	32.9
Fair Value, Measurements, Recurring | PBOP | Tax Exempt Securities | Fair Value, Inputs, Level 3
Defined Contribution Plan Disclosure [Line Items]
Fair value of plan assets	$ 0.0	$ 0.0
Minimum | Pension Plan | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations Range (up to)	40.00%
Minimum | Pension Plan | Alternative Investments
Defined Contribution Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations Range (up to)	40.00%
Maximum | Pension Plan | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations Range (up to)	60.00%
Maximum | Pension Plan | Alternative Investments
Defined Contribution Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations Range (up to)	60.00%