Stockholders' Equity (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stockholders' Equity Note [Abstract]
Authorized share repurchase	$ 1,000.0
Stock repurchase program, value	787.6
Value of shares withheld related to tax withholding for share-based compensation	$ 19.6	$ 15.9	$ 4.7

Amount of cash outflow to satisfy an employee's income tax withholding obligation as part of a net-share settlement of a share-based award.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 25
-Paragraph 19A
-URI http://asc.fasb.org/extlink&oid=107668666&loc=SL79513924-113897

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