type 1 diabetes (diagnosed clinically) for 12 months or longer at the time of screening (visit 1)

currently treated with a basal-bolus insulin regimen for at least 12 months prior to screening (visit 1)

currently treated with a basal insulin analogue (any regimen of insulin detemir or insulin glargine) for at least 4 months prior to screening (visit 1)

hba1c 7.0-9.5% (53-80 mmol/mol) (both inclusive) as assessed by central laboratory

body mass index (bmi) below or equal to 35.0 kg/m^2

use of any anti-diabetic drug other than insulin within the last 3 months prior to screening (visit 1)

recurrent severe hypoglycaemia (more than one severe hypoglycaemic event during the last 12 months) or hypoglycaemic unawareness as judged by the investigator, or hospitalisation for diabetic ketoacidosis during the previous 6 months prior to screening (visit 1)

cardiovascular disease, within the last 6 months prior to screening (visit 1), defined as stroke, decompensated heart failure new york heart association (nyha) class iii or iv, myocardial infarction, unstable angina pectoris, coronary arterial bypass graft or angioplasty