Investment Securities - Investments' Gross Unrealized Losses and Fair Value Aggregated by Investment Category and Length of Time in Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 468,331	$ 16,844
Less than 12 Months Gross Unrealized Losses	9,132	80
12 Months or Longer Fair Value	8,801	194
12 Months or Longer Gross Unrealized Losses	4,224	5,881
Total Fair Value	477,132	17,038
Total Gross Unrealized Losses	13,356	5,961
U.S. Treasury
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	4,875
Less than 12 Months Gross Unrealized Losses	21
Total Fair Value	4,875
Total Gross Unrealized Losses	21
U.S. Government-sponsored agency securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	3,433
Less than 12 Months Gross Unrealized Losses	17
Total Fair Value	3,433
Total Gross Unrealized Losses	17
State and municipal
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	129,109	4,524
Less than 12 Months Gross Unrealized Losses	3,931	19
12 Months or Longer Fair Value	767
12 Months or Longer Gross Unrealized Losses	56
Total Fair Value	129,876	4,524
Total Gross Unrealized Losses	3,987	19
U.S. Government-sponsored mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	330,914	12,320
Less than 12 Months Gross Unrealized Losses	5,163	61
12 Months or Longer Fair Value	5,323
12 Months or Longer Gross Unrealized Losses	532
Total Fair Value	336,237	12,320
Total Gross Unrealized Losses	5,695	61
Corporate obligations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value
Less than 12 Months Gross Unrealized Losses
12 Months or Longer Fair Value	2,711	194
12 Months or Longer Gross Unrealized Losses	3,636	5,881
Total Fair Value	2,711	194
Total Gross Unrealized Losses	$ 3,636	$ 5,881