Basis of Presentation (Tables)

6 Months Ended

Jun. 30, 2020

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Restrictions on Cash and Cash Equivalents [Table Text Block]

The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the balance sheets to the amounts shown in the statements of cash flows:

	June 30,
	2020		2019
	(in millions)
Cash and cash equivalents	$	3.4	$	97.1
Restricted cash(1)	30.8		29.2
Total cash, cash equivalents and restricted cash shown in the statements of cash flows	$	34.2	$	126.3

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(1) As of June 30, 2020 and 2019, the restricted cash balance is cash held in an escrow account related to a title dispute between outside parties in the Williston Basin. The restricted cash balance is recorded within "Other noncurrent assets" on the

Schedule of Deferred Compensation Mark To Market And Unrealized Gain Loss [Table Text Block]

	Three Months Ended June 30,						Six Months Ended June 30,
	2020		2019		Change		2020		2019		Change
	(in millions)
Deferred compensation mark-to-market adjustments	$	5.0	$	0.3	$	4.7	$	(3.2)	$	3.7	$	(6.9)
Unrealized (gain)/loss on marketable securities	(3.3)		0.8		$	(4.1)	—		(2.7)		$	2.7