Investment Securities - Securities Available for Sale Classified by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less, amortized cost	$ 209,129	$ 211,019
Due after one year through five years, amortized cost	180,657	126,452
Due after five years through ten years, amortized cost	7,258	23,147
Due after ten years, amortized cost	14,058	15,439
Subtotal, amortized cost	411,102	376,057
Amortized Cost	745,328	692,824
Fair Value
Due in one year or less, fair value	209,099	211,019
Due after one year through five years, fair value	177,972	124,797
Due after five years through ten years, fair value	7,268	22,804
Due after ten years, fair value	13,961	15,421
Subtotal, fair value	408,300	374,041
Fair Value	737,442	689,202
Mortgage-backed securities
Amortized Cost
Securities with no stated maturity, amortized cost	334,226	313,554
Amortized Cost	294,076	275,958
Fair Value
Securities with no stated maturity, fair value	329,142	311,652
Fair Value	289,890	274,990
Mutual funds with no stated maturity
Amortized Cost
Securities with no stated maturity, amortized cost	0	3,213
Fair Value
Securities with no stated maturity, fair value	$ 0	$ 3,509