Benefit Plans (Tables)

3 Months Ended

Mar. 31, 2018

Retirement Benefits [Abstract]

Company's pension and postretirement benefit plans

Components of net periodic cost for pension and postretirement benefit plans
	Three months ended March 31,
($ in millions)	2018			2017
Pension benefits
Service cost	$	28		$	29
Interest cost	60			66
Expected return on plan assets	(105		)	(102		)
Amortization of:
Prior service credit	(14		)	(14		)
Net actuarial loss	44			47
Settlement loss	7			8
Net periodic pension cost	$	20		$	34
Postretirement benefits
Service cost	$	2		$	2
Interest cost	3			4
Amortization of:
Prior service credit	(5		)	(6		)
Net actuarial gain	(6		)	(6		)
Net periodic postretirement credit	$	(6	)	$	(6	)