Common Stock - Narrative (Details) $ in Millions	Sep. 30, 2017 USD ($)
Equity, Class of Treasury Stock [Line Items]
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value	$ 1,150

Current fair value of each cash settlement alternative for each freestanding forward contract.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 40
-Section 50
-Paragraph 5
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6445032&loc=d3e90205-114008

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