Basis of Presentation Restricted Cash (Details) - USD ($) $ in Millions		Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents		$ 3.4	$ 166.3	$ 97.1
Restricted cash(1)	[1]	30.8		29.2
Total cash, cash equivalents and restricted cash shown in the statements of cash flows		$ 34.2	$ 196.4	$ 126.3	$ 28.1

[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 balance sheets.