Fair Value Measurements (Details 3)	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Changes in the fair value of Level 3 assets measured on a recurring basis
Total gains or losses (unrealized): included in earnings	$ 0	$ 0
Number of transfers between Level 1 and Level 2 | item	0	0
Number of transfers between Level 2 and Level 3 | item	0	0
U.S. States and political subdivisions securities
Changes in the fair value of Level 3 assets measured on a recurring basis
Balance at the beginning of the period	$ 2,699,000	$ 4,528,000
Total gains or losses (realized/unrealized): included in earnings	0	0
Total gains or losses (realized/unrealized): included in other comprehensive income	31,000	(24,000)
Purchases	1,437,000	1,100,000
Issuances	0	0
Sales	0	0
Settlements	0	0
Maturities	(2,012,000)	(2,905,000)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance at the end of the period	2,155,000	2,699,000
Foreign government and other securities
Changes in the fair value of Level 3 assets measured on a recurring basis
Balance at the beginning of the period	807,000	809,000
Total gains or losses (realized/unrealized): included in earnings	0	0
Total gains or losses (realized/unrealized): included in other comprehensive income	3,000	(2,000)
Purchases	500,000	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Maturities	(600,000)	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance at the end of the period	710,000	807,000
Investment securities available-for-sale
Changes in the fair value of Level 3 assets measured on a recurring basis
Balance at the beginning of the period	3,506,000	5,337,000
Total gains or losses (realized/unrealized): included in earnings	0	0
Total gains or losses (realized/unrealized): included in other comprehensive income	34,000	(26,000)
Purchases	1,937,000	1,100,000
Issuances	0	0
Sales	0	0
Settlements	0	0
Maturities	(2,612,000)	(2,905,000)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance at the end of the period	$ 2,865,000	$ 3,506,000