Investment Securities - Expected Maturities of Available-for-Sale Securities (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Amortized cost
Amortized cost	$ 870,446
Fair value
Fair value	849,055
Debt Securities excluding Mortgage-based securities and CMOs
Amortized cost
One year or less	1,104
More than one year to five years	45,121
More than five years to ten years	59,359
More than ten years	10,000
Amortized cost	115,584
Fair value
One year or less	1,105
More than one year to five years	44,385
More than five years to ten years	59,204
More than ten years	9,162
Fair value	113,856
Mortgage-backed securities and collateralized mortgage obligations
Amortized cost
Amortized cost	754,862
Fair value
Fair value	$ 735,199