Components of net periodic benefit cost (Tables)

9 Months Ended

Sep. 30, 2018

Components of net periodic benefit cost [Abstract]

Schedule of net periodic benefit cost

	Pension Benefits
	Three months ended September 30,						Nine months ended September 30,
	2018			2017			2018			2017
Service cost	$	1.1		$	1.1		$	3.2		$	3.3
Interest cost	3.1			3.3			9.2			10.0
Expected return on plan assets	(5.3		)	(4.8		)	(15.8		)	(14.6		)
Amortization of prior service costs	0.0			0.0			0.1			0.1
Amortization of net loss	1.3			1.2			3.9			3.9
Net periodic benefit cost	$	0.2		$	0.8		$	0.6		$	2.7

	Other Postretirement Benefits ("OPEB")
	Three months ended September 30,						Nine months ended September 30,
	2018			2017			2018			2017
Service cost	$	0.1		$	0.2		$	0.2		$	0.7
Interest cost	1.0			1.4			0.4			5.7
Amortization of prior service cost	(1.8		)	(0.7		)	(5.1		)	(1.7		)
Amortization of net loss	1.0			1.0			5.4			0.9
Net periodic benefit cost	$	0.3		$	1.9		$	0.9		$	5.6