A 33 year old lady presents with polydipsia and polyuria. Her symptoms staed soon after a road traffic accident 6 months ago. The blood pressure is 120/80mm Hg with no postural drop. The daily Na 130mEq/L, K 3.5 mEq/L, urea 15 mg/dL, sugar 65 mg/dL. The plasma osmolality is 268 mosmol/L and urine osmolality 45 mosmol/L. The most likely diagnosis is: