Investment Securities - Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Available for sale securities, due in one year or less, amortized cost	$ 12,265	$ 211,019
Available for sale securities, due after one year through five years, amortized cost	176,700	126,452
Available for sale securities, due after five years through ten years, amortized cost	24,459	23,147
Available for sale securities, due after ten years, amortized cost	0	15,439
Available for sale securities, subtotal, amortized cost	213,424	376,057
Amortized Cost	543,240	692,824
Fair Value
Available for sale securities, due in one year or less, fair value	12,237	211,019
Available for sale securities, due after one year through five years, fair value	171,374	124,797
Available for sale securities, due after five years through ten years, fair value	23,839	22,804
Available for sale securities, due after ten years, fair value	0	15,421
Available for sale securities, subtotal, fair value	207,450	374,041
Fair Value	528,064	689,202
Mortgage-backed securities
Amortized Cost
Available for sale securities, no stated maturity, amortized cost	329,816	313,554
Amortized Cost	292,005	275,958
Fair Value
Available for sale securities, no stated maturity, fair value	320,614	311,652
Fair Value	284,421	274,990
Mutual funds with no stated maturity
Amortized Cost
Available for sale securities, no stated maturity, amortized cost	0	3,213
Fair Value
Available for sale securities, no stated maturity, fair value	$ 0	$ 3,509