Investment Securities - Expected Maturities of Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized cost
Amortized cost	$ 264,184	$ 239,618
Fair value
Fair value	251,417	236,125
U.S. government and agency obligations
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	5,000
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	4,915
Amortized cost
More than one year to five years	8,404
Amortized cost	23,404	8,402
Fair value
More than one year to five years	7,997
Fair value	22,857	8,344
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	10,000
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	9,945
Mortgage-backed securities and collateralized mortgage obligations
Amortized cost
Amortized cost	240,780	231,216
Fair value
Fair value	$ 228,560	$ 227,781