Investment Securities (Investments Classified by Contractual Maturity Date) (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 128,184	$ 300,833	$ 150,907
Due in one to five years, Amortized Cost	174,968	97,019	282,443
Due in five to ten years, Amortized Cost	118,229	33,947	38,339
Due after ten years, Amortized Cost	46,860	15,249	8,188
Amortized Cost	2,260,750	1,833,069	1,691,632
Due in one year or less, Fair Value	127,844	299,285	150,241
Due in one to five years, Fair Value	174,877	97,326	282,121
Due in five to ten years, Fair Value	113,875	35,029	39,458
Due after ten years, Fair Value	46,011	15,499	8,283
Available-for-sale securities	2,164,985	1,803,666	1,665,903
Held-to-maturity securities, Due in one year or less, Amortized Cost	5,521	170	170
Held-to-maturity securities, Due in one to five years, Amortized Cost	31,782	38,392	36,914
Held-to-maturity securities, Due in five to ten years, Amortized Cost	255,146	205,227	193,387
Held-to-maturity securities, Due after ten years, Amortized Cost	673,989	582,660	588,869
Amortized Cost	966,438	826,449	819,340
Held-to-maturity securities, Due in one year or less, Fair Value	5,511	171	171
Held-to-maturity securities, Due in one to five years, Fair Value	31,160	38,012	36,734
Held-to-maturity securities, Due in five to ten years, Fair Value	243,608	203,680	192,581
Held-to-maturity securities, Due after ten years, Fair Value	631,318	570,653	577,550
Held-to-maturity securities, Fair Value	911,597	812,516	807,036
Mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,792,509	1,351,787	1,178,589
Available-for-sale securities	1,702,378	1,319,725	1,149,448
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	34,234	33,166
Available-for-sale securities	$ 0	$ 36,802	$ 36,352