Fair Value Measurements (Details 3) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) item	Mar. 31, 2017 USD ($) 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 levels | 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	$ 2,155	$ 2,699
Total gains or losses (realized/unrealized): included in earnings	0	0
Total gains or losses (realized/unrealized): included in other comprehensive income	0	19
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Maturities	0	(700)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance at the end of the period	2,155	2,018
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	710	807
Total gains or losses (realized/unrealized): included in earnings	0	0
Total gains or losses (realized/unrealized): included in other comprehensive income	(2)	(3)
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Maturities	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance at the end of the period	708	804
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	2,865	3,506
Total gains or losses (realized/unrealized): included in earnings	0	0
Total gains or losses (realized/unrealized): included in other comprehensive income	(2)	16
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Maturities	0	(700)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance at the end of the period	$ 2,863	$ 2,822