Securities - Amortized cost, gross unrealized gains and losses and fair value of securities held to maturity (Details) - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015
Held-to-maturity securities:
Amortized Cost		$ 341,951	$ 364,696
Gross Unrealized Gains		14,796	8,635
Gross Unrealized Losses		7	1,673
Held-to-Maturity, Fair value		356,740	371,658
State and political subdivisions
Held-to-maturity securities:
Amortized Cost		61,547	62,073
Gross Unrealized Gains		4,782	3,211
Gross Unrealized Losses		1
Held-to-Maturity, Fair value		66,328	65,284
Mortgage-backed securities
Held-to-maturity securities:
Amortized Cost	[1]	280,404	302,623
Gross Unrealized Gains	[1]	10,014	5,424
Gross Unrealized Losses	[1]	6	1,673
Held-to-Maturity, Fair value	[1]	$ 290,412	$ 306,374

[1] All are residential type mortgage-backed securities or U.S. government agency commercial mortgage backed securities.