Fair Value of Assets and Liabilities (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	$ 3,586,355	$ 3,126,743
Interest rate swap [Member]
Fair value measurement of assets and liabilities measured at fair value
Total liabilities measured at fair value on a recurring basis	17,352	8,906
U.S. government and federal agency [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	204	208
U.S. government sponsored enterprises [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	19,860	31,155
State and local governments [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	1,237,878	1,064,655
Corporate bonds [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	243,479	62,237
Collateralized debt obligations [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	2,762	5,366
Residential mortgage-backed securities [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	2,082,172	1,963,122
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Interest rate swap [Member]
Fair value measurement of assets and liabilities measured at fair value
Total liabilities measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. government and federal agency [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. government sponsored enterprises [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | State and local governments [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Corporate bonds [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Collateralized debt obligations [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Residential mortgage-backed securities [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	0	0
Significant Other Observable Inputs (Level 2) [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	3,586,355	3,126,743
Significant Other Observable Inputs (Level 2) [Member] | Interest rate swap [Member]
Fair value measurement of assets and liabilities measured at fair value
Total liabilities measured at fair value on a recurring basis	17,352	8,906
Significant Other Observable Inputs (Level 2) [Member] | U.S. government and federal agency [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	204	208
Significant Other Observable Inputs (Level 2) [Member] | U.S. government sponsored enterprises [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	19,860	31,155
Significant Other Observable Inputs (Level 2) [Member] | State and local governments [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	1,237,878	1,064,655
Significant Other Observable Inputs (Level 2) [Member] | Corporate bonds [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	243,479	62,237
Significant Other Observable Inputs (Level 2) [Member] | Collateralized debt obligations [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	2,762	5,366
Significant Other Observable Inputs (Level 2) [Member] | Residential mortgage-backed securities [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	2,082,172	1,963,122
Significant Unobservable Inputs (Level 3) [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) [Member] | Interest rate swap [Member]
Fair value measurement of assets and liabilities measured at fair value
Total liabilities measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) [Member] | U.S. government and federal agency [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) [Member] | U.S. government sponsored enterprises [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) [Member] | State and local governments [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) [Member] | Corporate bonds [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) [Member] | Collateralized debt obligations [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) [Member] | Residential mortgage-backed securities [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	$ 0	$ 0