Basis of Presentation Restricted Cash (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents		$ 0.0	$ 0.0	$ 782.6
Restricted cash(1)	[1]	27.2		59.2
Total cash, cash equivalents and restricted cash shown in the Condensed Consolidated Statements of Cash Flows		27.2	$ 23.4	841.8	$ 465.4
Long-term [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash(1)		$ 27.2		$ 59.2

[1] As of September 30, 2018, the restricted cash balance consisted of $27.2 million related to cash deposited into an escrow account for a title dispute between outside parties in the Williston Basin and is included within "Other noncurrent assets" on the Condensed Consolidated Balance Sheet. As of September 30, 2017, the restricted cash balance consisted of $59.2 million related to cash deposited into an escrow account related to the 2017 Permian Basin Acquisition (defined below) and cash deposited into an escrow account for a title dispute between outside parties in the Williston Basin and is included within "Other noncurrent assets" on the Condensed Consolidated Balance Sheet provided within the Quarterly Report on Form 10-Q.