Investment Securities - Amortized Cost and Estimated Fair Value of Restricted Equity Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Restricted Equity Securities [Line Items]
Amortized Cost	$ 11,947	$ 10,358	$ 13,363
Unrealized Gains	0	0	0
Unrealized Losses	0	0	0
Fair Value (Estimated)	11,947	10,358	13,363
Federal Home Loan Bank Stock
Restricted Equity Securities [Line Items]
Amortized Cost	10,910	9,321	12,326
Unrealized Gains	0	0	0
Unrealized Losses	0	0	0
Fair Value (Estimated)	10,910	9,321	12,326
Federal Reserve Bank Stock
Restricted Equity Securities [Line Items]
Amortized Cost	1,037	1,037	1,037
Unrealized Gains	0	0	0
Unrealized Losses	0	0	0
Fair Value (Estimated)	$ 1,037	$ 1,037	$ 1,037