Derivative Instruments - Interest Rate Swaps Future Maturities (Details) $ in Millions	Mar. 31, 2018 USD ($)
Derivative [Line Items]
Notional balance of outstanding contracts	$ 4,832
Year 2018
Derivative [Line Items]
Notional balance of outstanding contracts	544
Year 2019
Derivative [Line Items]
Notional balance of outstanding contracts	1,344
Year 2020
Derivative [Line Items]
Notional balance of outstanding contracts	1,072
Year 2021
Derivative [Line Items]
Notional balance of outstanding contracts	1,072
Year 2022
Derivative [Line Items]
Notional balance of outstanding contracts	$ 800

Nominal or face amount used to calculate payments on the derivative liability.

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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant