Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Available for sale securities
Amortized Cost	$ 552,032		$ 537,848
Gross Unrealized Gains	17,170		7,896
Gross Unrealized Losses	1,206		9,543
Investment securities available for sale	567,996		536,201
Held to maturity securities
Amortized Cost	621,818		559,378
Gross Unrealized Gains	13,782		5,282
Gross Unrealized Losses	1,045		3,813
Investment securities held to maturity - fair value	634,555		560,847
Amortized Cost	1,173,850		1,097,226
Gross Unrealized Gains	30,952		13,178
Gross Unrealized Losses	2,251		13,356
Fair Value	1,202,551		1,097,048
U.S. Treasury
Available for sale securities
Amortized Cost			15,914
Gross Unrealized Gains			80
Gross Unrealized Losses			21
Investment securities available for sale			15,973
U.S. Government-sponsored agency securities
Available for sale securities
Amortized Cost	100		3,550
Gross Unrealized Gains	9		12
Gross Unrealized Losses			17
Investment securities available for sale	109		3,545
State and municipal
Available for sale securities
Amortized Cost	225,402		231,005
Gross Unrealized Gains	10,601		3,878
Gross Unrealized Losses	206		3,896
Investment securities available for sale	235,797		230,987
Held to maturity securities
Amortized Cost	191,660		145,941
Gross Unrealized Gains	4,935		62
Gross Unrealized Losses	64		91
Investment securities held to maturity - fair value	196,531		145,912
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Amortized Cost	323,254		279,299
Gross Unrealized Gains	6,560		3,926
Gross Unrealized Losses	283		1,973
Investment securities available for sale	329,531		281,252
Held to maturity securities
Amortized Cost	430,158		413,437
Gross Unrealized Gains	8,847		5,220
Gross Unrealized Losses	981		3,722
Investment securities held to maturity - fair value	438,024		414,935
Corporate obligations
Available for sale securities
Amortized Cost	1,570		6,374
Gross Unrealized Gains
Gross Unrealized Losses	717		3,636
Investment securities available for sale	853		2,738
Held to maturity securities
Amortized Cost		1,500
Equity securities
Available for sale securities
Amortized Cost	1,706		1,706
Investment securities available for sale	$ 1,706		$ 1,706