Investment Securities - Amortized Cost and Fair Value of Investment Securities by Contractual Maturities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	$ 1,200
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	1,208
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	10,100
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	10,206
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	14,847
Held-to-maturity Securities, Debt Maturities, Rolling Year Six Through Ten, Fair Value	14,475
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	236,157
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	229,529
Held-to-maturity Securities	262,304	$ 255,050
Held-to-maturity Securities, Fair Value	255,418	256,314
Securities available for sale, Due in one year or less, Amortized Cost	55,801
Securities available for sale, After one year through five years, Amortized Cost	15,947
Securities available for sale, After five years through ten years, Amortized Cost	256,070
Securities available for sale, After ten years, Amortized Cost	519,543
Securities available for sale, Mortgage backed securities, Amortized Cost	2,626,688
Securities available for sale Total, Amortized Cost	3,474,049	3,515,401
Securities available for sale, Due in one year or less, Estimated Fair Value	54,879
Securities available for sale, After one year through five years, Estimated Fair Value	16,242
Securities available for sale, After five years through ten years, Estimated Fair Value	251,220
Securities available for sale, After ten years, Estimated Fair Value	532,253
Securities available for sale, Mortgage backed securities, Estimated Fair Value	2,550,868
Securities available for sale Total, Estimated Fair Value	$ 3,405,462	$ 3,499,519