Basis of Presentation Restricted Cash (Details) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents		$ 9.5	$ 166.3	$ 92.4
Restricted cash(1)	[1]	31.0		29.7
Total cash, cash equivalents and restricted cash shown in the statements of cash flows		$ 40.5	$ 196.4	$ 122.1	$ 28.1

[1] As of September 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.