Employee Benefits Schedule of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Plan and SERP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 0.2	$ 0.2	$ 0.6	$ 0.6
Interest cost		1.2	1.2	3.4	3.6
Expected return on plan assets		(1.4)	(1.3)	(4.3)	(4.0)
Amortization of prior service costs(1)	[1]	0.2	0.3	0.6	0.9
Amortization of actuarial losses(1)	[1]	0.0	0.1	0.6	0.3
Curtailment loss(2)	[2]	0.3	0.7	0.3	0.7
Periodic expense		0.5	(1.2)	(1.2)	(2.1)
Medical Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost		0.0	0.0	0.1	0.1
Amortization of prior service costs(1)	[1]	(0.1)	(0.1)	(0.2)	(0.2)
Periodic expense		$ 0.1	$ 0.1	$ 0.1	$ 0.1

[1] Amortization of prior service costs and actuarial losses out of accumulated other comprehensive income are recognized on the Condensed Consolidated Statements of Operations within "Interest and other income (expense)".

[2] A curtailment is recognized when there is a significant reduction in, or an elimination of, defined benefit accruals for current employees' future services. These expenses relate to the Uinta Basin Divestiture for the three and nine months ended September 30, 2018 and the Pinedale Divestiture for the three and nine months ended September 30, 2017. The Uinta Basin Divestiture curtailment is recognized on the Condensed Consolidated Statements of Operations within "Interest and other income (expense)" for the three and nine months ended September 30, 2018. The Pinedale Divestiture curtailment is recognized on the Condensed Consolidated Statements of Operations within "Net gain (loss) from asset sales, inclusive of severance costs" for the three and nine months ended September 30, 2017.