Fair Value Measurements and Derivative Instruments - Credit Features (Details)	3 Months Ended
Mar. 31, 2018 derivative
Derivative Instruments
Number of derivatives matured	4
Moody's, Baa3 Rating
Derivative Instruments
Credit ratings for senior debt	Baa3
Standard & Poor's, BBB- Rating \| Minimum
Derivative Instruments
Credit ratings for senior debt	BBB-

Description of the credit rating indicative of likelihood of repayment of the debt instrument given by a credit rating agency (for example, Standard and Poor's, Moody's, Fitch).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration