You should get a separate bike insurance, if you don't have any kind of household contents insurance coverage, which covers your bike against theft. If you have a very expensive bike a separate insurance might also make sense to protect it against damages from e.g. accidents or vandalism.
The more expensive your bike and the more add-ons, the more expensive the insurance. For a €1000 bike insurances start at roughly €4 a month.
The coverage depends on the plan you choose. Protection against theft is usually included. You can also protect your bike against damages from accidents, necessary repairs, vandalism, flaws in materials and damage to others which you unintentionally cause yourself.