Available-for-Sale Securities - Company's Securities' Gross Unrealized Losses and Fair Values, Aggregated by Length of Time Individual Securities Have Been in a Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Securities With Gross Unrealized Losses Less Than 12 Months, Fair Value	$ 2,299	$ 47,693
Securities With Gross Unrealized Losses Less Than 12 Months, Unrealized Loss	(16)	(384)
Securities With Gross Unrealized Losses 12 Months or More, Fair Value	24,552
Securities With Gross Unrealized Losses 12 Months or More, Unrealized Loss	(359)
Securities With Gross Unrealized Losses, Total Fair Value	26,851	47,693
Securities With Gross Unrealized Losses, Total Unrealized Loss	(375)	(384)
U.S. Government and Federal Agencies
Schedule Of Available For Sale Securities [Line Items]
Securities With Gross Unrealized Losses Less Than 12 Months, Fair Value		41,559
Securities With Gross Unrealized Losses Less Than 12 Months, Unrealized Loss		(347)
Securities With Gross Unrealized Losses 12 Months or More, Fair Value	24,552
Securities With Gross Unrealized Losses 12 Months or More, Unrealized Loss	(359)
Securities With Gross Unrealized Losses, Total Fair Value	24,552	41,559
Securities With Gross Unrealized Losses, Total Unrealized Loss	(359)	(347)
FNMA Certificates
Schedule Of Available For Sale Securities [Line Items]
Securities With Gross Unrealized Losses Less Than 12 Months, Fair Value	1,094	3,489
Securities With Gross Unrealized Losses Less Than 12 Months, Unrealized Loss	(15)	(5)
Securities With Gross Unrealized Losses, Total Fair Value	1,094	3,489
Securities With Gross Unrealized Losses, Total Unrealized Loss	(15)	(5)
GNMA Certificates
Schedule Of Available For Sale Securities [Line Items]
Securities With Gross Unrealized Losses Less Than 12 Months, Fair Value	1,205	2,645
Securities With Gross Unrealized Losses Less Than 12 Months, Unrealized Loss	(1)	(32)
Securities With Gross Unrealized Losses, Total Fair Value	1,205	2,645
Securities With Gross Unrealized Losses, Total Unrealized Loss	$ (1)	$ (32)