Investment Securities - Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized Cost
Available for sale securities, due in one year or less, amortized cost	$ 7,315	$ 209,129
Available for sale securities, due after one year through five years, amortized cost	156,115	180,657
Available for sale securities, due after five years through ten years, amortized cost	24,596	7,258
Available for sale securities, due after ten years, amortized cost	12,110	14,058
Available for sale securities, subtotal, amortized cost	200,136	411,102
Amortized Cost	584,382	745,328
Fair Value
Available for sale securities, due in one year or less, fair value	7,313	209,099
Available for sale securities, due after one year through five years, fair value	156,142	177,972
Available for sale securities, due after five years through ten years, fair value	24,692	7,268
Available for sale securities, due after ten years, fair value	12,273	13,961
Available for sale securities, subtotal, fair value	200,420	408,300
Fair Value	588,119	737,442
Mortgage-related securities
Amortized Cost
Available for sale securities, no stated maturity, amortized cost	384,246	334,226
Amortized Cost	345,611	294,076
Fair Value
Available for sale securities, no stated maturity, fair value	387,699	329,142
Fair Value	$ 348,975	$ 289,890