Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available for sale, Amortized Cost
Due in one year or less	$ 959
Due after one year through five years	26,825
Due after five years through ten years	8,646
Due after ten years	190,764
Agency mortgage-backed securities	888,129
Amortized Cost	1,115,323	$ 733,279
Available for sale, Estimated Fair Value
Due in one year or less	978
Due after one year through five years	27,355
Due after five years through ten years	8,945
Due after ten years	196,018
Agency mortgage-backed securities	902,021
Securities available-for-sale	1,135,317	721,369
Held to maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	4,167
Due after five years through ten years	130,653
Due after ten years	0
Agency mortgage-backed securities	46,346
Amortized Cost	181,166	65,679
Held to maturity, Estimated Fair Value
Due in one year or less	0
Due after one year through five years	4,234
Due after five years through ten years	130,684
Due after ten years	0
Agency mortgage-backed securities	47,021
Held to maturity, fair value	$ 181,939	$ 63,934