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	Jun. 30, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 250,329	$ 16,844
Less than 12 Months Gross Unrealized Losses	7,889	80
12 Months or Longer Fair Value	717	194
12 Months or Longer Gross Unrealized Losses	5,519	5,881
Total Fair Value	251,046	17,038
Total Gross Unrealized Losses	13,408	5,961
State and municipal
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	66,388	4,524
Less than 12 Months Gross Unrealized Losses	3,180	19
Total Fair Value	66,388	4,524
Total Gross Unrealized Losses	3,180	19
U.S. Government-sponsored mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	183,941	12,320
Less than 12 Months Gross Unrealized Losses	4,709	61
Total Fair Value	183,941	12,320
Total Gross Unrealized Losses	4,709	61
Corporate obligations
Investments, Unrealized Loss Position [Line Items]
12 Months or Longer Fair Value	717	194
12 Months or Longer Gross Unrealized Losses	5,519	5,881
Total Fair Value	717	194
Total Gross Unrealized Losses	$ 5,519	$ 5,881