Investments - Carrying value and estimated fair value (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Debt securities:
Amortized Cost	$ 520,905
Equity securities:
Amortized Cost	520,905	$ 548,640
Gross Unrealized Gains	10,755	7,871
Gross Unrealized Losses	(3,533)	(8,762)
Available-for-sale securities	528,127	547,749
U.S. Government agencies
Debt securities:
Amortized Cost	63,921	69,005
Gross Unrealized Gains	531	242
Gross Unrealized Losses	(31)	(277)
Estimated Fair Value	64,421	68,970
Obligations of states and political subdivisions
Debt securities:
Amortized Cost	217,112	288,543
Gross Unrealized Gains	8,506	6,109
Gross Unrealized Losses	(608)	(4,353)
Estimated Fair Value	225,010	290,299
U.S. Government agencies collateralized by residential mortgage obligations
Debt securities:
Amortized Cost	193,644	181,785
Gross Unrealized Gains	696	484
Gross Unrealized Losses	(2,437)	(4,048)
Estimated Fair Value	191,903	178,221
Private label mortgage backed securities
Debt securities:
Amortized Cost	38,728	1,807
Gross Unrealized Gains	1,022	1,036
Gross Unrealized Losses	(436)	0
Estimated Fair Value	39,314	2,843
Other equity securities
Equity securities:
Amortized Cost	7,500	7,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(21)	(84)
Estimated Fair Value	$ 7,479	$ 7,416

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