Morgan Stanley said Thursday it will buy discount brokerage E-Trade Financial Corp. in a stock deal worth about $13 billion, the biggest acquisition by a Wall Street bank since the 2008-2009 financial crisis.

Part of a broader consolidation in the discount brokerage sector, the move will add breadth to Morgan Stanley's wealth management unit.

Chief Executive Officer James Gorman has been trying to build the business out to insulate the bank from periods weak for trading and investment banking.