Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 366,487	$ 350,029
Gross unrealized gains	8,169	4,484
Gross unrealized losses	(230)	(1,075)
Estimated fair value	374,426	353,438
U. S. treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	6,008	6,517
Gross unrealized gains	22	1
Gross unrealized losses	0	(1)
Estimated fair value	6,030	6,517
Common stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	458	580
Gross unrealized gains	544	906
Gross unrealized losses	0	0
Estimated fair value	1,002	1,486
U. S. federal agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	27,188	30,064
Gross unrealized gains	167	43
Gross unrealized losses	(6)	(187)
Estimated fair value	27,349	29,920
Municipal obligations, tax-exempt [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	160,122	135,341
Gross unrealized gains	3,907	2,671
Gross unrealized losses	(181)	(71)
Estimated fair value	163,848	137,941
Municipal obligations, taxable [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	71,680	81,999
Gross unrealized gains	1,949	472
Gross unrealized losses	(1)	(581)
Estimated fair value	73,628	81,890
Agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	91,331	85,829
Gross unrealized gains	1,580	391
Gross unrealized losses	(42)	(235)
Estimated fair value	92,869	85,985
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	9,700	9,699
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	$ 9,700	$ 9,699