Investment Securities (Investments Classified by Contractual Maturity Date) (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Available-for-sale securities
Due in one year or less, amortized cost	$ 45,459	$ 343,601	$ 338,702
Due in one to five years, amortized cost	92,457	67,901	76,843
Due in five to ten years, amortized cost	121,099	111,886	108,688
Due after ten years, amortized cost	67,608	95,672	186,718
Amortized Cost	2,347,419	2,960,081	2,845,410
Due in one year or less, fair value	45,549	343,846	338,954
Due in one to five years, fair value	94,134	70,334	78,843
Due in five to ten years, fair value	121,650	112,178	105,968
Due after ten years, fair value	71,660	101,441	193,339
Fair Value	2,373,478	3,055,839	2,946,459
Held-to-maturity securities
Held-to-maturity securities, due in one year or less, amortized cost	5,466	7,138	6,911
Held-to-maturity securities, due in one to five years, amortized cost	75,724	22,217	21,392
Held-to-maturity securities, due in five to ten years, amortized cost	150,908	150,621	144,468
Held-to-maturity securities, due after ten years, amortized cost	186,398	198,690	387,559
Amortized Cost	2,736,805	579,197	560,330
Less: Allowance for credit losses	(83)	(59)	(63)
Held-to-maturity securities, net of allowance for credit losses	2,736,722	579,138	560,267
Held-to-maturity securities, due in one year or less, fair value	5,493	7,186	6,942
Held-to-maturity securities, due in one to five years, fair value	76,173	23,068	21,999
Held-to-maturity securities, due in five to ten years, fair value	157,282	159,293	151,175
Held-to-maturity securities, due after ten years, fair value	186,401	203,689	395,831
Fair Value	2,699,480	593,767	575,947
Mortgage-backed
Available-for-sale securities
Mortgage-backed, without single maturity date, amortized cost	2,020,796	2,341,021	2,134,459
Mortgage-backed, without single maturity date, fair value	2,040,485	2,428,040	2,229,355
Held-to-maturity securities
Mortgage-backed, amortized cost	2,318,309	200,531	0
Mortgage-backed, fair value	$ 2,274,131	$ 200,531	$ 0