Investment Securities - Summary of Investment Securities by Maturity (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity [Abstract]
Amortized Cost	$ 381,012
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total, fair value	376,453
US government agencies
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity [Abstract]
Within one year, amortized cost	6,323
One to five years, amortized cost	16,464
Amortized Cost	22,787
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Within one year, fair value	6,290
One to five years, fair value	16,142
Total, fair value	22,432
Residential mortgage-backed securities
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity [Abstract]
One to five years, amortized cost	4,691
Five to ten years, amortized cost	28,998
After 10 years, amortized cost	324,536
Amortized Cost	358,225
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
One to five years, fair value	4,526
Five to ten years, fair value	28,878
After 10 years, fair value	320,617
Total, fair value	$ 354,021