Subsequent Event Subsequent Event (Detail) (Subsequent Event, Illinois Extension Pipeline Company LLC, USD $) In Millions, unless otherwise specified	1 Months Ended		17 Months Ended
Jul. 31, 2014	Jul. 01, 2014	Dec. 31, 2015 Scenario, Forecast
Subsequent Event [Line Items]
Equity method investments, ownership percentage		35.00%
Cash paid to acquire equity method investments	$ 69		$ 226

The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 13

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585

Name:	us-gaap_PaymentsToAcquireEquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration