Derivative Instruments - Credit Risk and Credit-Risk-Related Contingent Feature (Details) $ in Thousands	Jun. 30, 2018 USD ($) institution	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Derivative asset \| $	$ 15,410	$ 48,186
Derivative liability \| $	$ 7,716	$ 13,848
Moody's, Aa2 Rating
Derivative [Line Items]
Number of financial institutions company does transactions with	1
Moody's, Aa3 Rating
Derivative [Line Items]
Number of financial institutions company does transactions with	1
Moody's, A1 Rating
Derivative [Line Items]
Number of financial institutions company does transactions with	2
Moody's, A2 Rating
Derivative [Line Items]
Number of financial institutions company does transactions with	1
Moody's, A3 Rating
Derivative [Line Items]
Number of financial institutions company does transactions with	1

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Name:	us-gaap_DerivativeLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant