Debt Securities (Sales of Debt Securities) (Narrative) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investments Debt And Equity Securities [Abstract]
Realized gains on sales of debt securities	$ 7,100	$ 7,100	$ 25,300	$ 16,400
Realized losses on sales of debt securities	200	400	7,400	3,400
Proceeds from sales of debt securities	$ 189,100	$ 200,100	$ 869,057	$ 694,588