The average account balance grew 12 percent last year, according to Fidelity Investments. The nation's largest 401(k) administrator said Thursday that the year-end average was $77,300, up from $69,100 a year earlier. In the fourth quarter, the average balance rose 2 percent. 

Good news...The average balance among the 12 million 401(k) accounts that Fidelity administers is at the highest level since it began tracking the data in 2000.