Securities Available for Sale and Held to Maturity - Gains and Losses as a Result of Calls and Disposition of Securities (Details) - Available for Sale [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gains (losses) on Investments [Abstract]
Proceeds from calls and sales of securities	$ 27,682	$ 24,693	$ 0
Gross realized gains	16	32	0
Gross realized losses	$ (1)	$ (342)	$ 0