[["in millions except per share amounts", "year ended december 2017", "year ended december 2016", "year ended december 2015"], ["common share repurchases", "29.0", "36.6", "22.1"], ["average cost per share", "$ 231.87", "$ 165.88", "$ 189.41"], ["total cost of common share repurchases", "$ 6721", "$ 6069", "$ 4195"]]