Shareholders' Equity and Earnings Per Common Share - Common Shares Repurchased (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	13 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021
Shareholders' Equity and Earnings Per Common Share
Authorized value of shares for repurchase		$ 500.0
Repurchase and retired common shares	619,749	1,400,000	4,400,000
Repurchase and retired common shares, Value	$ 19.2	$ 41.2	$ 124.0
Average purchase price	$ 30.96	$ 29.01	$ 28.18

Equity impact of the value of stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital).

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