Other Comprehensive Income (Loss) - Reclassification of Available-for-Sale Securities to Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification of Available-for-Sale Securities to Held-to-maturity Roll Forward [Roll Forward]
Beginning balance	$ 181,321	$ 172,521
Other comprehensive income (loss)	(2,933)	60
Ending balance	181,372	174,853
Accumulated Net Gain (Loss) on Securities Transferred from Available-for-Sale to Held-to-Maturity
Reclassification of Available-for-Sale Securities to Held-to-maturity Roll Forward [Roll Forward]
Beginning balance	(174)	(129)
Amortization of net unrealized losses	10	6
Related deferred taxes (at a Federal Income Tax rate of 21% for March 31, 2018 and 35% for March 31, 2017)	2	2
Other comprehensive income (loss)	(8)	(4)
Ending balance	$ (182)	$ (133)