Investment Securities - Expected Maturities of Available-for-Sale Securities (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Amortized cost
Amortized cost	$ 717,279
Fair value
Fair value	707,383
Debt Securities excluding Mortgage-based securities and CMOs
Amortized cost
One year or less	1,957
More than one year to five years	34,954
More than five years to ten years	54,444
More than ten years	10,000
Amortized cost	101,355
Fair value
One year or less	1,957
More than one year to five years	34,934
More than five years to ten years	54,600
More than ten years	9,280
Fair value	100,771
Mortgage-backed securities and collateralized mortgage obligations
Amortized cost
Amortized cost	615,924
Fair value
Fair value	$ 606,612