Significant business acquisitions - Estimated fair values of assets acquired and liabilities assumed (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 19, 2013 NV Energy [Member]
Business Acquisition [Line Items]
Property, plant and equipment				$ 9,550
Goodwill	58,264	57,011	54,523	2,362
Other assets, including cash of $304 million				2,481
Assets acquired				14,393
Accounts payable, accruals and other liabilities				3,455
Notes payable and other borrowings				5,342
Liabilities assumed				8,797
Net assets acquired				$ 5,596