Wells Fargo, the largest bank in the US by market value, has been rewarded for keeping its business simple. Now it is slowly creeping into more complex - and riskier - territory.

While many rivals retreat from risk amid regulatory pressures, the bank is expanding into a range of businesses a step removed from its long history as a Main Street lender. Among the initiatives: ramping up its investment-banking unit, bolstering trading assets and building up some overseas offices.