What was the change in the Balance as of 31 December from 2018 to 2019?
{"Partners and commercial managements": {"2019": 1.9, "2018": "-"}, "Derivative financial instruments": {"2019": 0.5, "2018": 3.7}, "Tax receivables": {"2019": 1.5, "2018": 1.2}, "Other": {"2019": 2.3, "2018": 2.6}, "Balance as of 31 December": {"2019": 6.2, "2018": 7.5}}
balance_2019 = df["Balance as of 31 December"]["2019"]
balance_2018 = df["Balance as of 31 December"]["2018"]
answer = balance_2019 - balance_2018