Investments - Carrying value and estimated fair value (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Debt securities:
Amortized Cost	$ 507,477
Equity securities:
Amortized Cost	507,477	$ 548,640
Gross Unrealized Gains	10,931	7,871
Gross Unrealized Losses	(3,331)	(8,762)
Available-for-sale securities	515,077	547,749
U.S. Government agencies
Debt securities:
Amortized Cost	61,253	69,005
Gross Unrealized Gains	598	242
Gross Unrealized Losses	(47)	(277)
Estimated Fair Value	61,804	68,970
Obligations of states and political subdivisions
Debt securities:
Amortized Cost	204,841	288,543
Gross Unrealized Gains	8,660	6,109
Gross Unrealized Losses	(516)	(4,353)
Estimated Fair Value	212,985	290,299
U.S. Government agencies collateralized by residential mortgage obligations
Debt securities:
Amortized Cost	190,028	181,785
Gross Unrealized Gains	696	484
Gross Unrealized Losses	(2,375)	(4,048)
Estimated Fair Value	188,349	178,221
Private label mortgage backed securities
Debt securities:
Amortized Cost	43,855	1,807
Gross Unrealized Gains	977	1,036
Gross Unrealized Losses	(379)	0
Estimated Fair Value	44,453	2,843
Other equity securities
Equity securities:
Amortized Cost	7,500	7,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(14)	(84)
Estimated Fair Value	$ 7,486	$ 7,416

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