Investment Securities - Amortized Cost and Approximate Fair Values of Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Amortized Cost	$ 537,848	$ 498,104
Gross Unrealized Gains	7,896	21,184
Gross Unrealized Losses	9,543	5,945
Fair Value	536,201	513,343
Amortized Cost	559,378	361,020
Gross Unrealized Gains	5,282	17,170
Gross Unrealized Losses	3,813	16
Fair Value	560,847	378,174
Amortized Cost	1,097,226	859,124
Gross Unrealized Gains	13,178	38,354
Gross Unrealized Losses	13,356	5,961
Fair Value	1,097,048	891,517
U.S. Treasury
Gain (Loss) on Investments [Line Items]
Amortized Cost	15,914
Gross Unrealized Gains	80
Gross Unrealized Losses	21
Fair Value	15,973
Government Agency and Municipal Bonds and Notes
Gain (Loss) on Investments [Line Items]
Amortized Cost	3,550	4,475
Gross Unrealized Gains	12	165
Gross Unrealized Losses	17
Fair Value	3,545	4,640
State and municipal
Gain (Loss) on Investments [Line Items]
Amortized Cost	231,005	148,187
Gross Unrealized Gains	3,878	10,025
Gross Unrealized Losses	3,896	18
Fair Value	230,987	158,194
Amortized Cost	145,941	117,227
Gross Unrealized Gains	62	5,489
Gross Unrealized Losses	91	1
Fair Value	145,912	122,715
U.S. Government-sponsored mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Amortized Cost	279,299	337,631
Gross Unrealized Gains	3,926	10,994
Gross Unrealized Losses	1,973	46
Fair Value	281,252	348,579
Amortized Cost	413,437	243,793
Gross Unrealized Gains	5,220	11,681
Gross Unrealized Losses	3,722	15
Fair Value	414,935	255,459
Corporate obligations
Gain (Loss) on Investments [Line Items]
Amortized Cost	6,374	6,105
Gross Unrealized Losses	3,636	5,881
Fair Value	2,738	224
Equity securities
Gain (Loss) on Investments [Line Items]
Amortized Cost	1,706	1,706
Fair Value	$ 1,706	$ 1,706