Supplemental Consolidated Balance Sheet Information (Narrative) (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018
Supplemental Consolidated Balance Sheet Information [Line Items]
Sign-on and retention bonuses issued	$ 6.6	$ 3.9
Purchases of property and equipment	8.5
Performance-based long-term incentive compensation liabilities	$ 0.8
Restricted Stock Units [Member]
Supplemental Consolidated Balance Sheet Information [Line Items]
Vesting period	3 years
Computer Equipment and Software (Member)
Supplemental Consolidated Balance Sheet Information [Line Items]
Retired fully-depreciated assets	$ 17.8
Maximum [Member]
Supplemental Consolidated Balance Sheet Information [Line Items]
Employee retention and signing bonuses, term, years	6 years

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