A 33 years 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 liters. Investigations showed Na 130 mEq/l, blood glucose 65 mg/dL, plasma osmolality 268 mosmol/l and urine osmolality 45 mosmol/l. The most likely diagnosis is: