Damage from two destructive Northern California wildfires that killed six and sent thousands fleeing their homes topped $1 billion in insured losses, according to a preliminary estimate by the state’s insurance department. The two fires started days apart in September, burning more than 200 square miles of remote, mountainous territory north and northeast of San Francisco. The preliminary figure announced Monday includes $700 million from a fire centered largely in Lake County that killed four and destroyed nearly 2,000 structures, including some 1,300 homes.