Significant business acquisitions - Estimated fair values of assets acquired and liabilities assumed (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 19, 2013 NV Energy [Member]
Business Acquisition [Line Items]
Property, plant and equipment				$ 9,518
Goodwill	58,669	57,011	54,523	2,363
Other assets, including cash and cash equivalents of $304 million				2,505
Assets acquired				14,386
Accounts payable, accruals and other liabilities				3,456
Notes payable and other borrowings				5,334
Liabilities assumed				8,790
Net assets acquired				$ 5,596