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Employee Benefit Plans - Summary of Components of Net Periodic Benefit Cost and Other Amounts Recognized in Other Comprehensive Loss (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Benefits      
Components of Net Periodic Benefit Cost      
Service cost $ (0.8) $ (2.3) $ (1.8)
Interest cost (6.5) (8.0) (7.4)
Expected return on plan assets 6.8 7.3 8.2
Amortization of net loss (2.5) (4.1) (4.2)
Amortization of prior service cost 0.0 0.6 0.5
Curtailment gain (loss) 0.0 1.3 0.0
Settlement gain (loss) 0.0 0.0 0.0
Net periodic benefit cost (3.0) (5.2) (4.7)
Net gain (loss) (3.8) 5.2 (6.5)
Amortization of net loss 2.5 4.1 4.2
Prior service cost (0.1) 0.0 0.0
Amortization of prior service credit 0.0 (0.6) (0.5)
Curtailment gain recognition of prior service credit 0.0 (1.3) 0.0
Recognition of settlement loss 0.0 0.0 0.0
Total recognized in Other comprehensive income (loss) (1.4) 7.4 (2.8)
Total recognized in net periodic benefit cost and Other comprehensive income (loss) (4.4) 2.2 (7.5)
Other Post-retirement Benefits      
Components of Net Periodic Benefit Cost      
Service cost (0.1) (0.1) (0.1)
Interest cost (0.3) (0.4) (0.4)
Expected return on plan assets 0.0 0.0 0.0
Amortization of net loss 0.1 0.1 (0.3)
Amortization of prior service cost 0.0 0.0 0.0
Curtailment gain (loss) 0.0 0.0 0.0
Settlement gain (loss) 0.0 0.0 0.0
Net periodic benefit cost (0.3) (0.4) (0.8)
Net gain (loss) (3.7) 0.3 1.4
Amortization of net loss (0.1) (0.1) 0.3
Prior service cost 0.0 0.0 0.0
Amortization of prior service credit 0.0 0.0 0.0
Curtailment gain recognition of prior service credit 0.0 0.0 0.0
Recognition of settlement loss 0.0 0.0 0.0
Total recognized in Other comprehensive income (loss) (3.8) 0.2 1.7
Total recognized in net periodic benefit cost and Other comprehensive income (loss) (4.1) (0.2) 0.9
Other Post-retirement Benefits | Non-qualified Plan Benefits      
Components of Net Periodic Benefit Cost      
Service cost 0.0 (0.1) (0.1)
Interest cost (0.1) (0.1) (0.1)
Expected return on plan assets 0.0 0.0 0.0
Amortization of net loss (0.1) 0.0 (0.1)
Amortization of prior service cost 0.0 0.1 0.2
Curtailment gain (loss) 0.0 0.1 0.6
Settlement gain (loss) 0.0 0.0 (0.1)
Net periodic benefit cost (0.2) 0.0 0.4
Net gain (loss) (0.2) (0.2) 0.2
Amortization of net loss 0.1 0.0 0.1
Prior service cost 0.0 0.0 0.0
Amortization of prior service credit 0.0 (0.1) (0.2)
Curtailment gain recognition of prior service credit 0.0 (0.1) (0.6)
Recognition of settlement loss 0.0 0.0 0.1
Total recognized in Other comprehensive income (loss) (0.1) (0.4) (0.4)
Total recognized in net periodic benefit cost and Other comprehensive income (loss) $ (0.3) $ (0.4) $ 0.0