Schedule of Interest Rate Swap (Details) - Interest Rate Swap (3-months LIBOR) [Member] - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
2010 Senior Notes [Member]
Derivative [Line Items]
Hedged Item	2010 Senior Notes due 2020
Nature of Swap	Pay Floating/Receive Fixed
Notional amount	$ 500.0	$ 500.0
Floating Interest Rate	3-month LIBOR
2012 Senior Notes [Member]
Derivative [Line Items]
Hedged Item	2012 Senior Notes due 2022
Nature of Swap	Pay Floating/Receive Fixed
Notional amount	$ 80.0	80.0
Floating Interest Rate	3-month LIBOR
2014 Senior Notes (5-Year) [Member]
Derivative [Line Items]
Hedged Item	2014 Senior Notes due 2019
Nature of Swap	Pay Floating/Receive Fixed
Notional amount	$ 450.0	$ 450.0
Floating Interest Rate	3-month LIBOR

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

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- Definition

Description of the hedged item related to the derivative. Includes the type of instrument hedged, contract amount, term, and so forth.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579240-113959

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- Definition

Description of the relevant terms of the derivative. Includes the type of instrument, risk being hedged, notional amount, counterparty, inception date, maturity date, relevant interest rates, strike price, cap price, and floor price.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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