Investment Securities (Investments Classified by Contractual Maturity Date) (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 115,227	$ 160,856	$ 164,374
Due in one to five years, Amortized Cost	141,364	166,550	186,199
Due in five to ten years, Amortized Cost	28,696	228,652	343,468
Due after ten years, Amortized Cost	10,539	13,753	662,015
Amortized Cost	1,637,399	1,731,292	2,226,376
Due in one year or less, Fair Value	115,487	160,756	164,429
Due in one to five years, Fair Value	141,368	166,468	186,592
Due in five to ten years, Fair Value	31,319	225,699	342,271
Due after ten years, Fair Value	10,029	14,182	651,940
Available-for-sale securities	1,650,096	1,716,388	2,214,281
Held-to-maturity securities, Due in one year or less, Amortized Cost	0	0	0
Held-to-maturity securities, Due in one to five years, Amortized Cost	25,927	19,208	0
Held-to-maturity securities, Due in five to ten years, Amortized Cost	64,835	96,454	0
Held-to-maturity securities, Due after ten years, Amortized Cost	842,005	769,164	0
Held-to-maturity securities, Due in one year or less, Fair Value	0	0	0
Held-to-maturity securities, Due in one to five years, Fair Value	26,023	19,156	0
Held-to-maturity securities, Due in five to ten years, Fair Value	65,842	96,091	0
Held-to-maturity securities, Due after ten years, Fair Value	850,801	762,864	0
Held-to-maturity securities, Amortized Cost	932,767	884,826	0
Held-to-maturity securities, Fair Value	942,666	878,111	0
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,293,005	1,110,101	820,964
Available-for-sale securities	1,301,111	1,092,597	815,045
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48,568	51,380	49,356
Available-for-sale securities	$ 50,782	$ 56,686	$ 54,004