Investment Securities (Investments Classified by Contractual Maturity Date) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 147,550	$ 300,833	$ 125,706
Due in one to five years, Amortized Cost	103,903	97,019	289,688
Due in five to ten years, Amortized Cost	117,799	33,947	38,213
Due after ten years, Amortized Cost	6,719	15,249	7,309
Amortized Cost	2,017,403	1,833,069	1,676,137
Due in one year or less, Fair Value	147,271	299,285	125,170
Due in one to five years, Fair Value	103,919	97,326	289,243
Due in five to ten years, Fair Value	114,640	35,029	39,463
Due after ten years, Fair Value	6,764	15,499	7,433
Available-for-sale securities	1,940,787	1,803,666	1,649,636
Held-to-maturity securities, Due in one year or less, Amortized Cost	5,667	170	0
Held-to-maturity securities, Due in one to five years, Amortized Cost	32,392	38,392	32,925
Held-to-maturity securities, Due in five to ten years, Amortized Cost	214,991	205,227	172,398
Held-to-maturity securities, Due after ten years, Amortized Cost	637,784	582,660	588,053
Amortized Cost	890,834	826,449	793,376
Held-to-maturity securities, Due in one year or less, Fair Value	5,658	171	0
Held-to-maturity securities, Due in one to five years, Fair Value	31,863	38,012	32,776
Held-to-maturity securities, Due in five to ten years, Fair Value	208,835	203,680	172,800
Held-to-maturity securities, Due after ten years, Fair Value	605,486	570,653	581,913
Held-to-maturity securities, Fair Value	851,842	812,516	787,489
Mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,641,432	1,351,787	1,182,579
Available-for-sale securities	1,568,193	1,319,725	1,153,290
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	34,234	32,642
Available-for-sale securities	$ 0	$ 36,802	$ 35,037