Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Gain (Loss) on Investments [Line Items]
Amortized Cost	$ 597,967	$ 537,848
Gross Unrealized Gains	11,694	7,896
Gross Unrealized Losses	4,154	9,543
Fair Value	605,507	536,201
Amortized Cost	544,470	559,378
Gross Unrealized Gains	8,821	5,282
Gross Unrealized Losses	1,670	3,813
Fair Value	551,621	560,847
Amortized Cost	1,142,437	1,097,226
Gross Unrealized Gains	20,515	13,178
Gross Unrealized Losses	5,824	13,356
Fair Value	1,157,128	1,097,048
U.S. Treasury
Gain (Loss) on Investments [Line Items]
Amortized Cost	15,918	15,914
Gross Unrealized Gains	79	80
Gross Unrealized Losses	14	21
Fair Value	15,983	15,973
U.S. Government-sponsored agency securities
Gain (Loss) on Investments [Line Items]
Amortized Cost	3,355	3,550
Gross Unrealized Gains	27	12
Gross Unrealized Losses		17
Fair Value	3,382	3,545
State and municipal
Gain (Loss) on Investments [Line Items]
Amortized Cost	244,298	231,005
Gross Unrealized Gains	6,291	3,878
Gross Unrealized Losses	1,451	3,896
Fair Value	249,138	230,987
Amortized Cost	144,787	145,941
Gross Unrealized Gains	1,977	62
Gross Unrealized Losses	28	91
Fair Value	146,736	145,912
U.S. Government-sponsored mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Amortized Cost	326,329	279,299
Gross Unrealized Gains	4,869	3,926
Gross Unrealized Losses	661	1,973
Fair Value	330,537	281,252
Amortized Cost	399,683	413,437
Gross Unrealized Gains	6,844	5,220
Gross Unrealized Losses	1,642	3,722
Fair Value	404,885	414,935
Corporate obligations
Gain (Loss) on Investments [Line Items]
Amortized Cost	6,361	6,374
Gross Unrealized Gains	428
Gross Unrealized Losses	2,028	3,636
Fair Value	4,761	2,738
Equity securities
Gain (Loss) on Investments [Line Items]
Amortized Cost	1,706	1,706
Fair Value	$ 1,706	$ 1,706