Fair Value of Assets and Liabilities (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	$ 3,721,377	$ 3,683,005
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	1,270	16,832
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		202
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	13,151	17,480
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	1,339,876	1,214,518
Corporate bonds [Member]
Fair value measurement of assets and liabilities measured at fair value
Total assets measured at fair value on a recurring basis	442,797	288,795
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		1,708
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	1,925,553	2,160,302
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
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
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	3,721,377	3,683,005
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	1,270	16,832
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		202
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	13,151	17,480
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	1,339,876	1,214,518
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	442,797	288,795
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		1,708
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	1,925,553	2,160,302
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
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
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