A 33-year-old lady presents with polydipsia and polyuria. Her 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 litres. Investigation showed, Na 130 mEq/L, K.3.5 mEq/L, urea 15mgAIL, sugar-65 mg/dL. The plasma osmolality is 268 mosmoUL and urine osmolality 45 mosmol/L. The most likely diagnosis is -