Investment Securities (Investments Classified by Contractual Maturity Date) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Available-for-sale securities
Due in one year or less, amortized cost	$ 114,853	$ 82,206	$ 147,550
Due in one to five years, amortized cost	137,427	168,855	103,903
Due in five to ten years, amortized cost	114,428	121,129	117,799
Due after ten years, amortized cost	168,539	128,339	6,719
Without single maturity date, amortized cost	1,646,323	1,684,965	1,641,432
Amortized Cost	2,181,570	2,185,494	2,017,403
Due in one year or less, fair value	115,231	82,153	147,271
Due in one to five years, fair value	139,958	169,307	103,919
Due in five to ten years, fair value	109,246	115,206	114,640
Due after ten years, fair value	172,296	129,580	6,764
Without single maturity date, fair value	1,649,423	1,629,835	1,568,193
Fair Value	2,186,154	2,126,081	1,940,787
Held-to-maturity securities
Held-to-maturity securities, Due in one year or less, amortized cost	7,573	10,009	5,667
Held-to-maturity securities, Due in one to five years, amortized cost	27,768	29,436	32,392
Held-to-maturity securities, Due in five to ten years, amortized cost	321,474	295,897	214,991
Held-to-maturity securities, Due after ten years, amortized cost	834,819	732,097	637,784
Amortized Cost	1,191,634	1,067,439	890,834
Held-to-maturity securities, Due in one year or less, fair value	7,566	9,979	5,658
Held-to-maturity securities, Due in one to five years, fair value	27,952	28,995	31,863
Held-to-maturity securities, Due in five to ten years, fair value	325,009	290,206	208,835
Held-to-maturity securities, Due after ten years, fair value	837,951	706,916	605,486
Fair Value	$ 1,198,478	$ 1,036,096	$ 851,842