Benefit Plans (Tables)

6 Months Ended

Jun. 30, 2018

Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]

Schedule of Components of Net Periodic Benefit Cost

The components of net periodic benefit cost for the three months endedJune 30 were as follows:

	Pension benefits						Other retirement benefits						Total
($ in millions)	2018			2017			2018			2017			2018			2017
Service cost	$	2.7		$	2.6		$	—		$	—		$	2.7		$	2.6
Interest cost	2.4			2.4			—			0.1			2.4			2.5
Expected return on assets	(3.9		)	(3.3		)	—			—			(3.9		)	(3.3		)
Amortization of prior service credit	(0.5		)	(0.4		)	(0.1		)	(0.2		)	(0.6		)	(0.6		)
Recognized actuarial losses (gains)	1.0			1.2			(0.6		)	(0.4		)	0.4			0.8
Net periodic benefit cost	$	1.7		$	2.5		$	(0.7	)	$	(0.5	)	$	1.0		$	2.0

	Pension benefits				Other retirement benefits						Total
($ in millions)	2018		2017		2018			2017			2018		2017
U.S. plans	$	1.2	$	1.8	$	(0.7	)	$	(0.5	)	$	0.5	$	1.3
International plans	0.5		0.7		—			—			0.5		0.7
Net periodic benefit cost	$	1.7	$	2.5	$	(0.7	)	$	(0.5	)	$	1.0	$	2.0

The components of net periodic benefit cost for the six months ended June 30 were as follows ($ in millions):

	Pension benefits						Other retirement benefits						Total
	2018			2017			2018			2017			2018			2017
Service cost	$	5.5		$	5.3		$	—		$	—		$	5.5		$	5.3
Interest cost	4.7			4.9			0.1			0.2			4.8			5.1
Expected return on assets	(7.8		)	(6.7		)	—			—			(7.8		)	(6.7		)
Amortization of prior service credit	(0.8		)	(0.7		)	(0.3		)	(0.4		)	(1.1		)	(1.1		)
Recognized actuarial losses (gains)	1.9			2.4			(1.1		)	(1.1		)	0.8			1.3
Net periodic benefit cost	$	3.5		$	5.2		$	(1.3	)	$	(1.3	)	$	2.2		$	3.9

	Pension benefits				Other retirement benefits						Total
	2018		2017		2018			2017			2018		2017
U.S. plans	$	2.5	$	3.8	$	(1.3	)	$	(1.3	)	$	1.2	$	2.5
International plans	1.0		1.4		—			—			1.0		1.4
Net periodic benefit cost	$	3.5	$	5.2	$	(1.3	)	$	(1.3	)	$	2.2	$	3.9