A 33-year-old lady present! with polydipsia and polyuria. Har symptoms started soon after a road traffic accident 6 months ago. The blood pressure is 120/80 mm Hg with no postural drop. The daily urinary output Is 6-8 liters. Investigation showed, Na 130 mEq/L, KJ.5 mEq/L, urea 15mg/dL, sugar-65 mg/dL. The plasma osmolality is 268 mosmol/L and urine osmolatity 45 mosmol/L. The most likely diagnosis is-