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

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_DerivativeInstrumentDetailAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNumberOfInstrumentsHeld
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value as of the balance sheet date of all foreign currency derivatives designated as cash flow hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(b),(c)(1)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

+ Details

Name:	us-gaap_ForeignCurrencyCashFlowHedgeDerivativeAtFairValueNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CurrencyAxis=currency_EUR
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CurrencyAxis=currency_NOK
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_NetInvestmentHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Risk Management and Use of Derivative Financial Instruments - Foreign Currency Derivatives Details (Details) € in Thousands, kr in Thousands, $ in Thousands	Dec. 31, 2017 USD ($) instrument	Dec. 31, 2017 NOK (kr) instrument	Dec. 31, 2017 EUR (€) instrument
Derivative Instrument Detail
Fair value, foreign currency derivatives	$ 13,037
Designated as hedging \| Cash Flow Hedging \| Foreign currency forward contracts \| Euro
Derivative Instrument Detail
Derivative number of instruments \| instrument	43	43	43
Derivative notional amount \| €			€ 88,173
Fair value, foreign currency derivatives	$ 14,325
Designated as hedging \| Cash Flow Hedging \| Foreign currency collars \| Euro
Derivative Instrument Detail
Derivative number of instruments \| instrument	2	2	2
Derivative notional amount \| €			€ 15,100
Fair value, foreign currency derivatives	$ (1,395)
Designated as hedging \| Cash Flow Hedging \| Foreign currency collars \| NOK
Derivative Instrument Detail
Derivative number of instruments \| instrument	3	3	3
Derivative notional amount \| kr		kr 2,000
Fair value, foreign currency derivatives	$ (16)
Designated as hedging \| Net Investment Hedging \| Foreign currency forward contracts \| NOK
Derivative Instrument Detail
Derivative number of instruments \| instrument	2	2	2
Derivative notional amount \| kr		kr 4,329
Fair value, foreign currency derivatives	$ 57
Designated as hedging \| Net Investment Hedging \| Foreign currency collars \| NOK
Derivative Instrument Detail
Derivative number of instruments \| instrument	1	1	1
Derivative notional amount \| kr		kr 2,500
Fair value, foreign currency derivatives	$ (20)
Not designated \| Foreign currency forward contracts \| NOK
Derivative Instrument Detail
Derivative number of instruments \| instrument	9	9	9
Derivative notional amount \| kr		kr 6,521
Fair value, foreign currency derivatives	$ 86