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	Mar. 31, 2014	Dec. 31, 2013
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 319,120	$ 468,331
Less than 12 Months Gross Unrealized Losses	3,186	9,132
12 Months or Longer Fair Value	18,316	8,801
12 Months or Longer Gross Unrealized Losses	2,638	4,224
Total Fair Value	337,436	477,132
Total Gross Unrealized Losses	5,824	13,356
U.S. Treasury
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	4,889	4,875
Less than 12 Months Gross Unrealized Losses	14	21
Total Fair Value	4,889	4,875
Total Gross Unrealized Losses	14	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	59,027	129,109
Less than 12 Months Gross Unrealized Losses	1,410	3,931
12 Months or Longer Fair Value	1,028	767
12 Months or Longer Gross Unrealized Losses	69	56
Total Fair Value	60,055	129,876
Total Gross Unrealized Losses	1,479	3,987
U.S. Government-sponsored mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	255,204	330,914
Less than 12 Months Gross Unrealized Losses	1,762	5,163
12 Months or Longer Fair Value	14,098	5,323
12 Months or Longer Gross Unrealized Losses	541	532
Total Fair Value	269,302	336,237
Total Gross Unrealized Losses	2,303	5,695
Corporate obligations
Investments, Unrealized Loss Position [Line Items]
12 Months or Longer Fair Value	3,190	2,711
12 Months or Longer Gross Unrealized Losses	2,028	3,636
Total Fair Value	3,190	2,711
Total Gross Unrealized Losses	$ 2,028	$ 3,636