Preheat the oven to 400 degrees F. In a mixing bowl, toss the sweet potatoes, parsnips and celery root, with the oil. Season with salt and pepper. Place on a baking sheet and roast for 20 to 25 minutes, or until golden brown. Put the Idaho potatoes in a large saucepan and cover with salted water. Bring to a boil, then reduce heat to medium and cook until tender, about 12 to 15 minutes. Drain, then return the potatoes to the pot. Add the roasted vegetables and butter and stir and mash until the butter melts completely. Add the milk and stir to mix. Season salt and pepper. Serve hot.