CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income (loss)		$ (336.0)	$ 45.4	$ (389.6)	$ 122.3
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]	0.1	0.2	0.2	0.3
Amortization of actuarial losses(4)	[4]	0.2	(0.1)	0.4	0.1
Other comprehensive income		0.3	0.1	0.9	2.0
Comprehensive income (loss)		(335.7)	45.5	(388.7)	124.3
Postretirement medical plan changes, income tax expense		0.0	0.0	0.0	(1.0)
Amortization of prior service cost, income tax expense		(0.1)	(0.1)	(0.1)	(0.2)
Amortization of net actuarial loss, income tax expense		$ (0.1)	$ 0.0	$ (0.2)	$ (0.1)

[1] Presented net of income tax expense of $1.0 million for the six months ended June 30, 2017.

[2] Adjustment recorded during the six months ended June 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 and $0.2 million for the three and six months ended June 30, 2017, respectively.

[4] Presented net of income tax expense of $0.1 million and $0.2 million for the three and six months ended June 30, 2018, respectively. Presented net of income tax expense of $0.1 million for the six months ended June 30, 2017.