CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income (loss)		$ (53.6)	$ 76.9
Postretirement medical plan change(1)	[1]	0.0	1.6
Fair value of plan assets adjustment(2)	[2]	0.3	0.0
Pension and other postretirement plans adjustments:
Amortization of prior service costs(3)	[3]	0.1	0.1
Amortization of actuarial losses(4)	[4]	0.2	0.2
Other comprehensive income		0.6	1.9
Comprehensive income (loss)		(53.0)	78.8
Postretirement medical plan changes, income tax expense		0.0	(1.0)
Amortization of prior service cost, income tax expense		0.0	(0.1)
Amortization of net actuarial loss, income tax expense		$ (0.1)	$ (0.1)

[1] Presented net of income tax expense of $1.0 million for the three months ended March 31, 2017.

[2] Adjustment recorded during the three months ended March 31, 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 for the three months ended March 31, 2017.

[4] Presented net of income tax expense of $0.1 million for the three months ended March 31, 2018 and 2017, respectively.