Subsequent Events (Details) - Subsequent Event $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)
Minimum
Subsequent Event [Line Items]
2017 BTC net benefit	$ 210
Maximum
Subsequent Event [Line Items]
2017 BTC net benefit	$ 220

Monetary effect of a change in the tax status effective after year-end but before the financial statements for the reporting period are issued or available to be issue by the entity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32590-109319

Name:	us-gaap_SubsequentEventEffectOfChangeInTaxStatus
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration