Investment Securities (Investments Classified by Contractual Maturity Date) (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Available-for-sale securities
Due in one year or less, amortized cost	$ 120,316	$ 183,996	$ 114,853
Due in one to five years, amortized cost	74,427	62,679	137,427
Due in five to ten years, amortized cost	173,429	186,683	114,428
Due after ten years, amortized cost	234,743	292,308	168,539
Mortgage-backed, without single maturity date, amortized cost	2,476,384	2,361,158	1,646,323
Amortized Cost	3,079,299	3,086,824	2,181,570
Due in one year or less, fair value	121,023	185,035	115,231
Due in one to five years, fair value	76,284	64,064	139,958
Due in five to ten years, fair value	169,249	184,666	109,246
Due after ten years, fair value	241,248	292,924	172,296
Mortgage-backed, without single maturity date, fair value	2,587,157	2,379,525	1,649,423
Fair Value	3,194,961	3,106,214	2,186,154
Held-to-maturity securities
Held-to-maturity securities, Due in one year or less, amortized cost	6,988	6,061	7,573
Held-to-maturity securities, Due in one to five years, amortized cost	21,818	28,697	27,768
Held-to-maturity securities, Due in five to ten years, amortized cost	146,937	213,104	321,474
Held-to-maturity securities, Due after ten years, amortized cost	552,787	886,538	834,819
Amortized Cost	728,530	1,134,400	1,191,634
Less: Allowance for credit losses	(65)	0	0
Held-to-maturity securities, net of allowance for credit losses	728,465	1,134,400	1,191,634
Held-to-maturity securities, Due in one year or less, fair value	7,028	6,074	7,566
Held-to-maturity securities, Due in one to five years, fair value	22,362	28,986	27,952
Held-to-maturity securities, Due in five to ten years, fair value	153,664	216,957	325,009
Held-to-maturity securities, Due after ten years, fair value	561,232	886,379	837,951
Fair Value	$ 744,286	$ 1,138,396	$ 1,198,478