Basis of Presentation Restricted Cash (Details) - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents		$ 89.9	$ 0.0	$ 0.0
Restricted cash(1)	[1]	28.6		24.5
Total cash, cash equivalents and restricted cash shown in the Condensed Consolidated Statements of Cash Flows		$ 118.5	$ 28.1	$ 24.5	$ 23.4

[1] As of March 31, 2019 and 2018, the restricted cash balance related to cash deposited into an escrow account for 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.