Securities - Amortized cost, gross unrealized gains and losses and fair value of securities held to maturity (Details) - USD ($) $ in Thousands		Sep. 30, 2016	Dec. 31, 2015
Held-to-maturity securities:
Amortized Cost		$ 344,917	$ 364,696
Gross Unrealized Gains		12,812	8,635
Gross Unrealized Losses		179	1,673
Held-to-Maturity, Fair value		357,550	371,658
State and political subdivisions
Held-to-maturity securities:
Amortized Cost		57,911	62,073
Gross Unrealized Gains		3,545	3,211
Gross Unrealized Losses		61
Held-to-Maturity, Fair value		61,395	65,284
Mortgage-backed securities
Held-to-maturity securities:
Amortized Cost	[1]	287,006	302,623
Gross Unrealized Gains	[1]	9,267	5,424
Gross Unrealized Losses	[1]	118	1,673
Held-to-Maturity, Fair value	[1]	$ 296,155	$ 306,374

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