Restructuring Charges (Summary of Restructuring Charges) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	$ 2.0	$ 18.0	$ 5.0	$ 18.0
E&P
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	2.0	16.0	5.0	16.0
Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring liability	0.4		0.4		$ 5.0
Severance (including payroll taxes)	1.0	17.0	3.0	17.0
Office consolidation	1.0	0.0	2.0	0.0
Professional fees	0.0	1.0	0.0	1.0
Total restructuring charges	2.0	18.0	5.0	18.0
Workforce Reduction \| E&P
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	$ 2.0	$ 16.0	$ 5.0	$ 2.0

Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Includes, but is not limited to, one-time termination benefits, termination of an operating lease or other contract, consolidating or closing facilities, and relocating employees, and termination benefits associated with an ongoing benefit arrangement. Excludes expenses associated with special or contractual termination benefits, a discontinued operation or an asset retirement obligation.

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Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

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Carrying amount (including both current and noncurrent portions of the accrual) as of the balance sheet date pertaining to a specified type of cost associated with exit from or disposal of business activities or restructuring pursuant to a duly authorized plan.

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Amount of expenses for special or contractual termination benefits provided to current employees involuntarily terminated under a benefit arrangement associated exit or disposal activities pursuant to an authorized plan. Excludes expenses related to one-time termination benefits, a discontinued operation or an asset retirement obligation.

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