Investment Securities - Investments' Gross Unrealized Losses and Fair Value Aggregated by Investment Category and Length of Time in Continuous Unrealized Loss Position (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 282,863	$ 16,844
Less than 12 Months Gross Unrealized Losses	9,633	80
12 Months or Longer Fair Value	692	194
12 Months or Longer Gross Unrealized Losses	5,853	5,881
Total Fair Value	283,555	17,038
Total Gross Unrealized Losses	15,486	5,961
U.S. Government-sponsored agency securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,439
Less than 12 Months Gross Unrealized Losses	9
Total Fair Value	1,439
Total Gross Unrealized Losses	9
State and municipal
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	76,681	4,524
Less than 12 Months Gross Unrealized Losses	3,467	19
12 Months or Longer Fair Value	239
12 Months or Longer Gross Unrealized Losses	2
Total Fair Value	76,920	4,524
Total Gross Unrealized Losses	3,469	19
U.S. Government-sponsored mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	204,743	12,320
Less than 12 Months Gross Unrealized Losses	6,157	61
Total Fair Value	204,743	12,320
Total Gross Unrealized Losses	6,157	61
Corporate obligations
Investments, Unrealized Loss Position [Line Items]
12 Months or Longer Fair Value	453	194
12 Months or Longer Gross Unrealized Losses	5,851	5,881
Total Fair Value	453	194
Total Gross Unrealized Losses	$ 5,851	$ 5,881