Shareholders' Equity And Earnings Per Share (Narrative) (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended					3 Months Ended		9 Months Ended
	Jul. 31, 2021	Apr. 30, 2021	Jan. 31, 2021	May 31, 2020	Jun. 30, 2015	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Temporary Equity [Line Items]
Cash dividends declared per common share (usd per share)	$ 0.31	$ 0.31	$ 0.31			$ 0.31	$ 0.28	$ 0.93	$ 0.84
Common stock dividends per share declared annualized (usd per share)	$ 1.24	$ 1.24	$ 1.24
Series D Preferred Stock
Temporary Equity [Line Items]
Sale of stock (in shares)					5,000,000
Preferred stock, liquidation value per share (usd per share)					$ 25	25	25	25	25	$ 25
Sale of stock, proceeds					$ 125.0
Preferred stock, dividend rate, percentage					6.50%
Series E Preferred Stock
Temporary Equity [Line Items]
Sale of stock (in shares)				11,500
Preferred stock, liquidation value per share (usd per share)				$ 25,000		$ 25,000	$ 25,000	$ 25,000	$ 25,000	$ 25,000
Sale of stock, proceeds				$ 287.5
Preferred stock, dividend rate, percentage				6.875%
Depositary Shares
Temporary Equity [Line Items]
Sale of stock (in shares)				11,500,000
London Interbank Offered Rate (LIBOR) | Series D Preferred Stock
Temporary Equity [Line Items]
Preferred stock, dividend rate, percentage, variable spread					4.06%