Fair Value of Financial Instruments (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government and federal agency [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	$ 0	$ 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government sponsored enterprises [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | State and local governments and other issues [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Collateralized debt obligations [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Residential Mortgage Backed Securities [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	0	0
Significant Other Observable Inputs (Level 2) | U.S. Government and federal agency [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	210	211
Significant Other Observable Inputs (Level 2) | U.S. Government sponsored enterprises [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	36,121	41,518
Significant Other Observable Inputs (Level 2) | State and local governments and other issues [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	987,591	657,421
Significant Other Observable Inputs (Level 2) | Collateralized debt obligations [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	0	0
Significant Other Observable Inputs (Level 2) | Residential Mortgage Backed Securities [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	1,754,341	1,689,946
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	2,778,263	2,389,096
Significant Unobservable Inputs (Level 3) | U.S. Government and federal agency [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	0	0
Significant Unobservable Inputs (Level 3) | U.S. Government sponsored enterprises [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	0	0
Significant Unobservable Inputs (Level 3) | State and local governments and other issues [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	0	0
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	5,953	6,595
Significant Unobservable Inputs (Level 3) | Residential Mortgage Backed Securities [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	199	156
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	6,152	6,751
U.S. Government and federal agency [Member] | Fair Value, Measurements, Recurring [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	210	211
U.S. Government sponsored enterprises [Member] | Fair Value, Measurements, Recurring [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	36,121	41,518
State and local governments and other issues [Member] | Fair Value, Measurements, Recurring [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	987,591	657,421
Collateralized debt obligations [Member] | Fair Value, Measurements, Recurring [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	5,953	6,595
Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	1,754,540	1,690,102
Fair Value, Measurements, Recurring [Member]
Major class of assets measured at fair value on recurring basis
Total financial assets	$ 2,784,415	$ 2,395,847