Pension and Postretirement Benefits (Tables)

6 Months Ended

Jun. 30, 2021

Compensation And Retirement Disclosure [Abstract]

Schedule of Components of Net Periodic Benefit Cost (Credit)

The estimated components of the recorded net periodic benefit cost (credit) for pension and postretirement benefits are as follows:

	Pension						Postretirement Benefits
	Three Months Ended June 30,
	2021			2020			2021			2020
	(Dollars in Millions)
Service cost	$	10.6		$	10.4		$	—		$	—
Interest cost		8.3			9.8			0.1			0.1
Expected return on assets		(16.3	)		(14.8	)		—			—
Amortization of:
Prior service cost (credit)		0.3			0.2			(0.2	)		(0.2	)
Actuarial loss		2.8			3.2			—			—
Net periodic benefit cost (credit)	$	5.7		$	8.8		$	(0.1	)	$	(0.1	)

	Pension						Postretirement Benefits
	Six Months Ended June 30,
	2021			2020			2021			2020
	(Dollars in Millions)
Service cost	$	23.0		$	19.6		$	—		$	—
Interest cost		17.7			18.5			0.2			0.2
Expected return on assets		(35.1	)		(28.6	)		—			—
Amortization of:
Prior service cost (credit)		0.4			0.3			(0.4	)		(0.4	)
Actuarial loss (gain)		6.2			6.7			(0.1	)		(0.1	)
Net periodic benefit cost (credit)	$	12.2		$	16.5		$	(0.3	)	$	(0.3	)