Investment Securities - Expected Maturities of Available-for-Sale Securities (Details) $ in Thousands	Sep. 30, 2018 USD ($)
Amortized cost
Amortized cost	$ 1,018,464
Fair value
Fair value	985,421
Debt Securities excluding Mortgage-based securities and CMOs
Amortized cost
One year or less	1,397
More than one year to five years	50,124
More than five years to ten years	59,371
More than ten years	5,000
Amortized cost	115,892
Fair value
One year or less	1,397
More than one year to five years	49,004
More than five years to ten years	57,890
More than ten years	4,444
Fair value	112,735
Mortgage-backed securities and collateralized mortgage obligations
Amortized cost
Amortized cost	902,572
Fair value
Fair value	$ 872,686