Convertible Senior Notes - Capped call transactions (Narrative) (Details) - Call Option - Senior Notes - Convertible Senior Notes Due 2023 - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	9 Months Ended
Jun. 30, 2016	Sep. 30, 2018
Debt Instrument [Line Items]
Net proceeds used to pay the cost of the capped call transactions	$ 33.9
Cap price (usd per share)		$ 64.68

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No definition available.

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- Definition

The per share price of the Company's stock at which the contract holder of the freestanding contract has the right to purchase or sell the Company's stock at a future date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 40
-Section 50
-Paragraph 5
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6445032&loc=d3e90205-114008

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-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=109262807&loc=d3e22047-110879

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- Definition

The cash outflow for derivative instruments during the period, which are classified as investing activities, excluding those designated as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3213-108585

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