Supplemental Consolidated Balance Sheet Information (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Supplemental Consolidated Balance Sheet Information [Line Items]
Sign-on and retention bonuses issued	$ 3.0	$ 2.5
Purchases of property and equipment	3.0
Performance-based long-term incentive compensation liabilities	$ 2.3
Restricted Stock Units [Member]
Supplemental Consolidated Balance Sheet Information [Line Items]
Vesting period	3 years
Maximum [Member]
Supplemental Consolidated Balance Sheet Information [Line Items]
Employee retention and signing bonuses, term, years	3 years

Employee retention and signing bonuses issued.

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- Definition

Supplemental consolidated balance sheet information.

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Amount of liability, recognized in statement of financial position, for defined benefit other postretirement plan, classified as noncurrent. Excludes pension plan.

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The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.

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