CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income (loss)		$ 45.4	$ (197.0)	$ 122.3	$ (1,060.8)
Postretirement medical plan change	[1]	0.0	0.0	1.6	0.0
Pension and other postretirement plans adjustments:
Amortization of prior service costs	[2]	0.2	0.3	0.3	0.4
Amortization of net actuarial losses	[3]	(0.1)	0.1	0.1	0.2
Other comprehensive income		0.1	0.4	2.0	0.6
Comprehensive income (loss)		45.5	(196.6)	124.3	(1,060.2)
Postretirement medical plan changes, income tax expense		0.0	0.0	(1.0)	0.0
Amortization of prior service cost, income tax expense		(0.1)	(0.2)	(0.2)	(0.3)
Amortization of net actuarial loss, income tax expense		$ 0.0	$ (0.1)	$ (0.1)	$ (0.1)

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

[2] Presented net of income tax expense of $0.1 million and $0.2 million during the three and six months ended June 30, 2017, respectively. Presented net of income tax expense of $0.2 million and $0.3 million during the three and six months ended June 30, 2016, respectively.

[3] Presented net of income tax expense of $0.1 million during the six months ended June 30, 2017. Presented net of income tax expense of $0.1 million during the three and six months ended June 30, 2016, respectively.