Basis of Presentation Restricted Cash (Details) - USD ($) $ in Millions		Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents		$ 97.1	$ 0.0	$ 0.0
Restricted cash(1)	[1]	29.2		26.1
Total cash, cash equivalents and restricted cash shown in the Condensed Consolidated Statements of Cash Flows		$ 126.3	$ 28.1	$ 26.1	$ 23.4

[1] As of June 30, 2019 and 2018, the restricted cash balance is cash held in an escrow account related to a title dispute between outside parties in the Williston Basin, and the restricted cash balance is recorded within "Other noncurrent assets" on the Condensed Consolidated Balance Sheets.