Investment Securities Held to Maturity - Schedule of Amortized Costs of Held to Maturity Debt Securities Aggregated by Credit Quality Indicator (Detail) $ in Thousands	Mar. 31, 2020 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, amortized cost	$ 195,958
Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, amortized cost	67,841
Mortgage Backed Securities [Member] | Not Rated [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, amortized cost	67,841	[1]
Municipal Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, amortized cost	128,117
Municipal Securities [Member] | AAA/AA/A [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, amortized cost	126,737
Municipal Securities [Member] | Not Rated [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, amortized cost	$ 1,380	[1]

[1] All unrated mortgage-backed securities are FNMA and GNMA. The federal backing of these securities result in negligible credit risk.