Stockholders' Equity (Narrative) (Detail) - USD ($) shares in Millions, $ in Millions	9 Months Ended	127 Months Ended
Sep. 30, 2021	Sep. 30, 2021	Aug. 31, 2021
Stock repurchase program, authorized amount			$ 300.0
Remaining authorized amount under stock repurchase program	$ 463.1	$ 463.1
Common stock repurchased, shares	1.4	2.6
Purchase of Company, value	$ 78.8	$ 136.9
Maximum
Stock repurchase program, authorized amount			$ 600.0

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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