Pension and Postretirement Benefit Plans (Tables)

9 Months Ended

Sep. 30, 2020

Retirement Benefits [Abstract]

Company's Share of Net Periodic Cost Components

for the three and nine months ended September 30, 2020 and 2019:

($ millions)	Pension				Postretirement				Pension				Postretirement
	Three months ended September 30								Nine months ended September 30
	2020		2019		2020		2019		2020		2019		2020		2019
Service cost	$	1.2	$	0.8	$	—	$	—	$	3.6	$	2.8	$	—	$	—
Interest cost	2.4		2.9		0.2		0.2		7.2		8.6		0.4		0.5
Expected return on plan assets	(4.4)		(4.2)		—		—		(13.1)		(12.5)		—		—
Amortization of:
Negative prior service cost	—		—		(1.4)		(1.4)		—		—		(4.1)		(4.1)
Net actuarial loss	2.3		1.5		0.1		0.1		6.9		4.5		0.2		0.2
Net periodic cost (benefit)	$	1.5	$	1.0	$	(1.1)	$	(1.1)	$	4.6	$	3.4	$	(3.5)	$	(3.4)