Investment Securities - Amortized cost, gross unrealized gains and losses and fair value of securities held to maturity (Details 1) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012
Held-to-maturity Securities, Unclassified [Abstract]
Amortized Cost	$ 231,087		$ 244,184		$ 303,636
Gross Unrealized Gains	16,051		16,947		14,854
Gross Unrealized Losses	51
Fair value	247,087		261,131		318,490
State and political subdivisions
Held-to-maturity Securities, Unclassified [Abstract]
Amortized Cost	51,768		51,780		51,893
Gross Unrealized Gains	5,663		5,486		4,413
Gross Unrealized Losses
Fair value	57,431		57,266		56,306
Mortgage-backed securities
Held-to-maturity Securities, Unclassified [Abstract]
Amortized Cost	179,319	[1]	192,404	[1]	251,743	[1]
Gross Unrealized Gains	10,388	[1]	11,461	[1]	10,441	[1]
Gross Unrealized Losses	51	[1]		[1]		[1]
Fair value	$ 189,656	[1]	$ 203,865	[1]	$ 262,184	[1]

[1] All are residential type mortgage-backed securities