Fair Value Measurement (Details 1) (Recurring [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	$ 3,353,553	$ 2,987,040
Level 2 [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	3,262,715	2,922,774
Issues	853
Level 2 [Member] | U.S. government agency debentures [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	123,069
Issues	7
Level 2 [Member] | U.S. States and political subdivisions [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	357,731
Issues	523
Level 2 [Member] | Residential mortgage-backed securities issued by U.S. Government Sponsored Enterprises [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	1,460,343
Issues	191
Level 2 [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	1,137,834
Issues	105
Level 2 [Member] | Non-agency [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	2
Issues	1
Level 2 [Member] | US Government and Government Agencies and Authorities [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	129,127
Issues	12
Level 2 [Member] | Corporate bonds [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	54,609
Issues	14
Level 3 [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	87,539	60,344
Issues	13
Level 3 [Member] | U.S. government agency debentures [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	0
Issues	0
Level 3 [Member] | U.S. States and political subdivisions [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	0
Issues	0
Level 3 [Member] | Residential mortgage-backed securities issued by U.S. Government Sponsored Enterprises [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	0
Issues	0
Level 3 [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	0
Issues	1
Level 3 [Member] | Non-agency [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	43,305
Issues	4
Level 3 [Member] | US Government and Government Agencies and Authorities [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	0
Issues	0
Level 3 [Member] | Corporate bonds [Member]
Fair Value Measurements From Third Party Pricing Service or Independent Brokers [Line Items]
Available for sale securities	$ 44,234
Issues	8