Investment Securities - Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized Cost
Available for sale securities, due in one year or less, amortized cost	$ 9,055	$ 209,129
Available for sale securities, due after one year through five years, amortized cost	166,223	180,657
Available for sale securities, due after five years through ten years, amortized cost	10,098	7,258
Available for sale securities, due after ten years, amortized cost	12,547	14,058
Available for sale securities, subtotal, amortized cost	197,923	411,102
Amortized Cost	562,528	745,328
Fair Value
Available for sale securities, due in one year or less, fair value	9,041	209,099
Available for sale securities, due after one year through five years, fair value	165,027	177,972
Available for sale securities, due after five years through ten years, fair value	10,126	7,268
Available for sale securities, due after ten years, fair value	12,390	13,961
Available for sale securities, subtotal, fair value	196,584	408,300
Fair Value	559,983	737,442
Mortgage-related securities
Amortized Cost
Available for sale securities, no stated maturity, amortized cost	364,605	334,226
Amortized Cost	323,610	294,076
Fair Value
Available for sale securities, no stated maturity, fair value	363,399	329,142
Fair Value	$ 322,913	$ 289,890