CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income (loss)		$ 7.3	$ (3.3)	$ (382.3)	$ 119.0
Postretirement medical plan change(1)	[1]	0.0	0.0	0.0	1.6
Fair value of plan assets adjustment(2)	[2]	0.0	0.0	0.3	0.0
Pension and other postretirement plans adjustments:
Amortization of prior service costs(3)	[3]	0.2	0.1	0.4	0.4
Amortization of actuarial losses(4)	[4]	0.1	0.1	0.5	0.2
Other comprehensive income		0.3	0.2	1.2	2.2
Comprehensive income (loss)		7.6	(3.1)	(381.1)	121.2
Postretirement medical plan changes, income tax expense		0.0	0.0	0.0	(1.0)
Amortization of prior service cost, income tax expense		0.0	(0.1)	(0.1)	(0.3)
Amortization of net actuarial loss, income tax expense		$ 0.1	$ 0.0	$ (0.1)	$ (0.1)

[1] Presented net of income tax expense of $1.0 million during the nine months ended September 30, 2017.

[2] Adjustment recorded during the nine months ended September 30, 2018, related to a change in the fair value of plan assets as of December 31, 2017.

[3] Presented net of income tax expense of $0.1 million during the nine months ended September 30, 2018. Presented net of income tax expense of $0.1 million and $0.3 million during the three and nine months ended September 30, 2017, respectively.

[4] Presented net of income tax benefit of $0.1 million and expense of $0.1 million during the three and nine months ended September 30, 2018, respectively. Presented net of income tax expense of $0.1 million during the nine months ended September 30, 2017.