Employee Benefits - Activity of Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at prior year end	$ 874.9	$ 998.3
Fair value of plan assets at year end	900.0	874.9
Fair Value, Measurements, Recurring
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at prior year end	874.9
Fair value of plan assets at year end	900.0	874.9
Fair Value, Measurements, Recurring \| Private Equities
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at prior year end	43.4
Fair value of plan assets at year end	36.4	43.4
Fair Value, Measurements, Recurring \| Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Purchases	8.1	0.6
Sales	(17.5)	(0.3)
Realized gains	1.9
Unrealized gain (loss)	8.0	(8.6)
Fair value of plan assets at prior year end	44.3	52.6
Fair value of plan assets at year end	44.8	44.3
Fair Value, Measurements, Recurring \| Level 3 \| Private Equities
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets beginning of year	43.4	46.6
Sales	(16.7)	(0.3)
Realized gains	1.9
Unrealized gain (loss)	7.8	(2.9)
Fair value of plan assets at year end	36.4	43.4
Fair value of plan assets at prior year end	43.4
Fair value of plan assets at year end	36.4	43.4
Fair Value, Measurements, Recurring \| Level 3 \| Fixed Income
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets beginning of year	0.9	6.0
Purchases	8.1	0.6
Sales	(0.8)
Unrealized gain (loss)	0.2	(5.7)
Fair value of plan assets at year end	$ 8.4	$ 0.9

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets sold.

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- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held.

+ References

+ Details

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

+ References

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-Publisher FASB
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- Definition

Amount of investment within defined benefit plan asset category.

+ References

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- Definition

Amount, measured using unobservable input, of decrease in plan asset of defined benefit plan from sale.

+ References

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- Definition

Amount, measured using unobservable input, of increase in plan asset of defined benefit plan from purchase.

+ References

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