SECURITIES - Amortized Cost and Fair Value of Securities, by Contractual Maturities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Securities [Line Items]
Available for sale, due in one year or less, amortized cost	$ 121,270
Available for sale, due from one to five years, amortized cost	258,824
Available for sale, due from five to ten years, amortized cost	57,811
Available for sale, due after ten years, amortized cost	111,546
Available for sale, with contractual maturities, amortized cost	549,451
Total securities available for sale, amortized cost	3,402,446
Available for sale, due in one year or less, fair value	120,729
Available for sale, due from one to five years, fair value	252,577
Available for sale, due from five to ten years, fair value	57,603
Available for sale, due after ten years, fair value	110,909
Available for sale, with contractual maturities, fair value	541,818
Total securities available for sale, fair value	3,298,894
Held to maturity, due in one year or less, amortized cost	40,450
Held to maturity, due from one to five years, amortized cost	202,164
Held to maturity, due from five to ten years, amortized cost	109,606
Held to maturity, due after ten years, amortized cost	901,097
Securities held to maturity, with contractual maturities, amortized cost	1,253,317
Held to maturity, with contractual maturities, amortized cost	3,206,345	$ 3,242,268
Held to maturity, due in one year or less, fair value	40,308
Held to maturity, due from one to five years, fair value	195,882
Held to maturity, due from five to ten years, fair value	105,281
Held to maturity, due after ten years, fair value	818,633
Securities held to maturity, with contractual maturities, fair value	1,160,104
Held to maturity, with contractual maturities, fair value	3,032,947	3,218,379
Agency Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, amortized cost	1,540,049	1,615,168
Total securities available for sale, fair value	1,483,820	1,598,874
Held to maturity, with contractual maturities, amortized cost	1,078,270	1,219,802
Held to maturity, with contractual maturities, fair value	1,037,526	1,214,219
Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, amortized cost	1,083,989	813,034
Total securities available for sale, fair value	1,046,481	794,957
Held to maturity, with contractual maturities, amortized cost	797,940	777,146
Held to maturity, with contractual maturities, fair value	760,362	757,083
Commercial Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, amortized cost	228,957
Total securities available for sale, fair value	226,775
Held to maturity, with contractual maturities, amortized cost	76,818	80,786
Held to maturity, with contractual maturities, fair value	$ 74,955	$ 80,625