Question: What U.S. state forms the western boundary of the state where area code 406 is located?

full_context: Document 1: Area codes 208 and 986: Area codes 208 and 986 are the North American telephone area codes for all of Idaho. 208 is the main area code, and is one of the 86 original area codes created in 1947. It was Idaho's sole area code until 2017, when 986 was added as an overlay for the entire state.
Document 2: Montana: Montana is one of the nine Mountain States, located in the north of the region known as the Western United States. It borders North Dakota and South Dakota to the east. Wyoming is to the south, Idaho is to the west and southwest, and three Canadian provinces, British Columbia, Alberta, and Saskatchewan, are to the north.
Document 3: North Dakota: North Dakota is in the U.S. region known as the Great Plains. The state shares the Red River of the North with Minnesota to the east. South Dakota is to the south, Montana is to the west, and the Canadian provinces of Saskatchewan and Manitoba are to the north. North Dakota is situated near the middle of North America with a stone marker in Rugby, North Dakota marking the ``Geographic Center of the North American Continent ''. With an area of 70,762 square miles (183,273 km), North Dakota is the 19th largest state.
Document 4: Area code 406: Coordinates: 47 ° 00 ′ 01 ''N 109 ° 45 ′ 04'' W ﻿ / ﻿ 47.00028 ° N 109.75111 ° W ﻿ / 47.00028; - 109.75111 ﻿ (State of Montana) Area code 406 is the telephone area code covering the entire state of Montana. It has been Montana's area code since area codes were created in 1947.
Document 5: Montana: The topography of the state is roughly defined by the Continental Divide, which splits much of the state into distinct eastern and western regions. Most of Montana's 100 or more named mountain ranges are concentrated in the western half of the state, most of which is geologically and geographically part of the Northern Rocky Mountains. The Absaroka and Beartooth ranges in the south-central part of the state are technically part of the Central Rocky Mountains. The Rocky Mountain Front is a significant feature in the north-central portion of the state, and there are a number of isolated island ranges that interrupt the prairie landscape common in the central and eastern parts of the state. About 60 percent of the state is prairie, part of the northern Great Plains.
Document 6: Montana: Montana has a larger Native American population numerically and percentage-wise than most U.S. states. Although the state ranked 45th in population (according to the 2010 U.S. Census), it ranked 19th in total native people population. Native people constituted 6.5 percent of the state's total population, the sixth highest percentage of all 50 states. Montana has three counties in which Native Americans are a majority: Big Horn, Glacier, and Roosevelt. Other counties with large Native American populations include Blaine, Cascade, Hill, Missoula, and Yellowstone counties. The state's Native American population grew by 27.9 percent between 1980 and 1990 (at a time when Montana's entire population rose just 1.6 percent), and by 18.5 percent between 2000 and 2010. As of 2009, almost two-thirds of Native Americans in the state live in urban areas. Of Montana's 20 largest cities, Polson (15.7 percent), Havre (13.0 percent), Great Falls (5.0 percent), Billings (4.4 percent), and Anaconda (3.1 percent) had the greatest percentage of Native American residents in 2010. Billings (4,619), Great Falls (2,942), Missoula (1,838), Havre (1,210), and Polson (706) have the most Native Americans living there. The state's seven reservations include more than twelve distinct Native American ethnolinguistic groups.
Document 7: Montana: Montana has 56 counties with the United States Census Bureau stating Montana's contains 364 "places", broken down into 129 incorporated places and 235 census-designated places. Incorporated places consist of 52 cities, 75 towns, and two consolidated city-counties. Montana has one city, Billings, with a population over 100,000; and two cities with populations over 50,000, Missoula and Great Falls. These three communities are considered the centers of Montana's three Metropolitan Statistical Areas.
Document 8: Area code 607: North American area code 607 is a state of New York telephone area code servicing parts of its Southern Tier (which borders Pennsylvania). It was split from parts of area codes 315 and 716 in 1954. Initially, in the late 1940s, area codes with a middle digit of ``0 ''(x0x - xxx - xxxx) were assigned only to jurisdictions covering an entire state. Along with 507 in Minnesota and 606 in Kentucky, 607 was among the first such area codes to be added to a jurisdiction.
Document 9: Geography of the United States: The Great Plains come to an abrupt end at the Rocky Mountains. The Rocky Mountains form a large portion of the Western U.S., entering from Canada and stretching nearly to Mexico. The Rocky Mountain region is the highest region of the United States by average elevation. The Rocky Mountains generally contain fairly mild slopes and wider peaks compared to some of the other great mountain ranges, with a few exceptions (such as the Teton Mountains in Wyoming and the Sawatch Range in Colorado). The highest peaks of the Rockies are found in Colorado, the tallest peak being Mount Elbert at 14,440 ft (4,400 m). The Rocky Mountains contain some of the most spectacular, and well known scenery in the world. In addition, instead of being one generally continuous and solid mountain range, it is broken up into a number of smaller, intermittent mountain ranges, forming a large series of basins and valleys.
Document 10: Montana: The northern section of the Divide, where the mountains give way rapidly to prairie, is part of the Rocky Mountain Front. The front is most pronounced in the Lewis Range, located primarily in Glacier National Park. Due to the configuration of mountain ranges in Glacier National Park, the Northern Divide (which begins in Alaska's Seward Peninsula) crosses this region and turns east in Montana at Triple Divide Peak. It causes the Waterton River, Belly, and Saint Mary rivers to flow north into Alberta, Canada. There they join the Saskatchewan River, which ultimately empties into Hudson Bay.
Document 11: Montana: East of the divide, several roughly parallel ranges cover the southern part of the state, including the Gravelly Range, the Madison Range, Gallatin Range, Absaroka Mountains and the Beartooth Mountains. The Beartooth Plateau is the largest continuous land mass over 10,000 feet (3,000 m) high in the continental United States. It contains the highest point in the state, Granite Peak, 12,799 feet (3,901 m) high. North of these ranges are the Big Belt Mountains, Bridger Mountains, Tobacco Roots, and several island ranges, including the Crazy Mountains and Little Belt Mountains.
Document 12: Montana: Vegetation of the state includes lodgepole pine, ponderosa pine; Douglas fir, larch, spruce; aspen, birch, red cedar, hemlock, ash, alder; rocky mountain maple and cottonwood trees. Forests cover approximately 25 percent of the state. Flowers native to Montana include asters, bitterroots, daisies, lupins, poppies, primroses, columbine, lilies, orchids, and dryads. Several species of sagebrush and cactus and many species of grasses are common. Many species of mushrooms and lichens are also found in the state.
Document 13: Montana: With a total area of 147,040 square miles (380,800 km2), Montana is slightly larger than Japan. It is the fourth largest state in the United States after Alaska, Texas, and California; the largest landlocked U.S. state; and the 56th largest national state/province subdivision in the world. To the north, Montana shares a 545-mile (877 km) border with three Canadian provinces: British Columbia, Alberta, and Saskatchewan, the only state to do so. It borders North Dakota and South Dakota to the east, Wyoming to the south and Idaho to the west and southwest.
Document 14: Montana: East of the divide the Missouri River, which is formed by the confluence of the Jefferson, Madison and Gallatin rivers near Three Forks, flows due north through the west-central part of the state to Great Falls. From this point, it then flows generally east through fairly flat agricultural land and the Missouri Breaks to Fort Peck reservoir. The stretch of river between Fort Benton and the Fred Robinson Bridge at the western boundary of Fort Peck Reservoir was designated a National Wild and Scenic River in 1976. The Missouri enters North Dakota near Fort Union, having drained more than half the land area of Montana (82,000 square miles (210,000 km2)). Nearly one-third of the Missouri River in Montana lies behind 10 dams: Toston, Canyon Ferry, Hauser, Holter, Black Eagle, Rainbow, Cochrane, Ryan, Morony, and Fort Peck.
Document 15: Area codes 304 and 681: North American telephone area code 304 was established October 1947 as one of the original area codes, and serves all of the U.S. state of West Virginia. It was overlaid with area code 681, effective March 28, 2009.
ground_truth: Idaho
sub_queries: where is area code 406 located in usa Answer: Montana
what us state forms the western boundary of Montana Answer: Idaho