AltaGas Ltd. (TSX:ALA) is in talks to combine with WGL Holdings, Inc. (NYSE:WGL) in a transaction that could value the parent of Washington, D.C.’s natural-gas utility at $5 billion to $6 billion, as increasing use of natural gas spurs merger activity. A deal could be announced this month, people familiar with the matter said assuming the talks don’t fall apart and another bidder doesn’t re-emerge.