Minecraft wiki entry for fill, associated text: fill <from: x y z> <to: x y z> <tileName: Block> [tileData: int] [oldBlockHandling: FillMode]
fill <from: x y z> <to: x y z> <tileName: Block> [blockStates: block states] [oldBlockHandling: FillMode]
The fill command also has an optional alternate syntax for replace mode:
fill <from: x y z> <to: x y z> <tileName: Block> <tileData: int> replace [replaceTileName: Block] [replaceDataValue: int]
fill <from: x y z> <to: x y z> <tileName: Block> <blockStates: block states> replace [replaceTileName: Block] [replaceDataValue: int]