Investment Securities (Investments Classified by Contractual Maturity Date) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 180,899	$ 300,833	$ 142,323
Due in one to five years, Amortized Cost	90,073	97,019	320,945
Due in five to ten years, Amortized Cost	116,909	33,947	39,520
Due after ten years, Amortized Cost	8,730	15,249	9,826
Amortized Cost	1,962,868	1,833,069	1,847,780
Due in one year or less, Fair Value	180,333	299,285	142,001
Due in one to five years, Fair Value	89,953	97,326	320,278
Due in five to ten years, Fair Value	116,517	35,029	40,519
Due after ten years, Fair Value	8,732	15,499	10,174
Available-for-sale securities	1,895,688	1,803,666	1,803,733
Held-to-maturity securities, Due in one year or less, Amortized Cost	3,786	170	0
Held-to-maturity securities, Due in one to five years, Amortized Cost	34,495	38,392	32,771
Held-to-maturity securities, Due in five to ten years, Amortized Cost	210,705	205,227	70,533
Held-to-maturity securities, Due after ten years, Amortized Cost	643,951	582,660	564,460
Amortized Cost	892,937	826,449	667,764
Held-to-maturity securities, Due in one year or less, Fair Value	3,775	171	0
Held-to-maturity securities, Due in one to five years, Fair Value	33,994	38,012	32,525
Held-to-maturity securities, Due in five to ten years, Fair Value	205,823	203,680	69,348
Held-to-maturity securities, Due after ten years, Fair Value	618,935	570,653	546,022
Held-to-maturity securities, Fair Value	862,527	812,516	647,895
Mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,566,257	1,351,787	1,302,532
Available-for-sale securities	1,500,153	1,319,725	1,256,044
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	34,234	32,634
Available-for-sale securities	$ 0	$ 36,802	$ 34,717