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_CurrencyAxis=currency_JPY
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, ¥ in Thousands, NOK in Thousands, $ in Thousands	Sep. 30, 2017 USD ($) instrument	Sep. 30, 2017 JPY (¥) instrument	Sep. 30, 2017 NOK instrument	Sep. 30, 2017 EUR (€) instrument
Derivative Instrument Detail
Fair value, foreign currency derivatives	$ 18,572
Derivatives Designated as Hedging Instruments \| Cash Flow Hedging \| Foreign currency forward contracts \| Euro
Derivative Instrument Detail
Number of Instruments \| instrument	47	47	47	47
Notional Amount \| €				€ 95,676
Fair value, foreign currency derivatives	$ 17,022
Derivatives Designated as Hedging Instruments \| Cash Flow Hedging \| Foreign currency collars \| Euro
Derivative Instrument Detail
Number of Instruments \| instrument	2	2	2	2
Notional Amount \| €				€ 15,100
Fair value, foreign currency derivatives	$ (1,071)
Derivatives Designated as Hedging Instruments \| Cash Flow Hedging \| Foreign currency collars \| NOK
Derivative Instrument Detail
Number of Instruments \| instrument	3	3	3	3
Notional Amount \| NOK			NOK 2,000
Fair value, foreign currency derivatives	$ (21)
Derivatives Designated as Hedging Instruments \| Designated as Net Investment Hedging Instruments \| Foreign currency forward contracts \| NOK
Derivative Instrument Detail
Number of Instruments \| instrument	2	2	2	2
Notional Amount \| NOK			NOK 4,329
Fair value, foreign currency derivatives	$ 42
Derivatives Designated as Hedging Instruments \| Designated as Net Investment Hedging Instruments \| Foreign currency forward contracts \| JPY
Derivative Instrument Detail
Number of Instruments \| instrument	2	2	2	2
Notional Amount \| ¥		¥ 660,156
Fair value, foreign currency derivatives	$ 2,552
Derivatives Designated as Hedging Instruments \| Designated as Net Investment Hedging Instruments \| Foreign currency collars \| NOK
Derivative Instrument Detail
Number of Instruments \| instrument	1	1	1	1
Notional Amount \| NOK			NOK 2,500
Fair value, foreign currency derivatives	$ (23)
Not Designated as Hedging Instrument \| Foreign currency forward contracts \| NOK
Derivative Instrument Detail
Number of Instruments \| instrument	10	10	10	10
Notional Amount \| NOK			NOK 7,240
Fair value, foreign currency derivatives	$ 71