What was the percentage change in the balance at December 31 between 2018 and 2019?
{"Balance at January 1": {"2019": 107.7, "2018": 116.7, "2017": 107.6}, "Additions based on tax positions related to the current year": {"2019": 33.3, "2018": 8.1, "2017": 7.6}, "Additions and reductions for tax positions of prior years": {"2019": 37.5, "2018": 0.3, "2017": "\u2014"}, "Foreign currency": {"2019": -1.6, "2018": -8.1, "2017": 1.9}, "Reduction as a result of the lapse of statute of limitations": {"2019": -1.3, "2018": -2.6, "2017": -0.4}, "Reduction as a result of effective settlements": {"2019": "\u2014", "2018": -6.7, "2017": "\u2014"}, "Balance at December 31": {"2019": 175.6, "2018": 107.7, "2017": 116.7}}
balance_2019 = df["Balance at December 31"]["2019"]
balance_2018 = df["Balance at January 1"]["2019"]
answer = (balance_2019 - balance_2018) / balance_2018 * 100.0