Investment Securities (Investments Classified by Contractual Maturity Date) (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Available-for-sale securities
Due in one year or less, amortized cost	$ 338,702	$ 183,996	$ 201,572
Due in one to five years, amortized cost	76,843	62,679	141,319
Due in five to ten years, amortized cost	108,688	186,683	200,118
Due after ten years, amortized cost	186,718	292,308	139,007
Mortgage-backed, without single maturity date, amortized cost	2,134,459	2,361,158	1,565,629
Amortized Cost	2,845,410	3,086,824	2,247,645
Due in one year or less, fair value	338,954	185,035	201,866
Due in one to five years, fair value	78,843	64,064	143,804
Due in five to ten years, fair value	105,968	184,666	198,250
Due after ten years, fair value	193,339	292,924	143,317
Mortgage-backed, without single maturity date, fair value	2,229,355	2,379,525	1,582,822
Fair Value	2,946,459	3,106,214	2,270,059
Held-to-maturity securities
Held-to-maturity securities, Due in one year or less, amortized cost	6,911	6,061	7,748
Held-to-maturity securities, Due in one to five years, amortized cost	21,392	28,697	28,138
Held-to-maturity securities, Due in five to ten years, amortized cost	144,468	213,104	231,021
Held-to-maturity securities, Due after ten years, amortized cost	387,559	886,538	828,895
Amortized Cost	560,330	1,134,400	1,095,802
Less: Allowance for credit losses	(63)	0	0
Held-to-maturity securities, net of allowance for credit losses	560,267	1,134,400	1,095,802
Held-to-maturity securities, Due in one year or less, fair value	6,942	6,074	7,749
Held-to-maturity securities, Due in one to five years, fair value	21,999	28,986	28,335
Held-to-maturity securities, Due in five to ten years, fair value	151,175	216,957	235,138
Held-to-maturity securities, Due after ten years, fair value	395,831	886,379	833,681
Fair Value	$ 575,947	$ 1,138,396	$ 1,104,903