Income Taxes - Tax Charges and Benefits Recorded Directly to Stockholders' Equity (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Tax Effect of Other Comprehensive Income	$ 168.1	$ 88.9	$ 25.7

Amount of tax expense (benefit) allocated to other comprehensive income (loss).

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