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1. Basis of Presentation and Use of Estimates Immaterial Errors Related to Presentation (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2012
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Net Unrealized Holding Gain (Loss) Net Of Income Tax [Member]
        
Quantifying Misstatement in Current Year Financial Statements [Line Items]        
Quantifying Misstatement in Current Year Financial Statements, Amount $ 27,422 $ 65,448 $ 12,663 $ 15,495
Income Tax On Net Unrealized Holding Gain (Loss) [Member]
        
Quantifying Misstatement in Current Year Financial Statements [Line Items]        
Quantifying Misstatement in Current Year Financial Statements, Amount 15,967 38,108 7,373 9,022
Reclassification Adjustments For Net Holding (Gain) Loss Net Of Income Tax [Member]
        
Quantifying Misstatement in Current Year Financial Statements [Line Items]        
Quantifying Misstatement in Current Year Financial Statements, Amount (4,084) (5,045) 104 (385)
Income Tax on Reclassification Adjustments For Net Holding (Gain) Loss [Member]
        
Quantifying Misstatement in Current Year Financial Statements [Line Items]        
Quantifying Misstatement in Current Year Financial Statements, Amount (2,378) (2,937) 61 (224)
Income Tax on Reclassification Adjustments For Net Holding (Gain) Loss [Member]
        
Quantifying Misstatement in Current Year Financial Statements [Line Items]        
Quantifying Misstatement in Current Year Financial Statements, Amount (4,756) (5,874) 121 (448)
Reclassification Adjustments For Net Holding (Gain) Loss Net Of Income Tax [Member]
        
Quantifying Misstatement in Current Year Financial Statements [Line Items]        
Quantifying Misstatement in Current Year Financial Statements, Amount (8,168) (10,090) 208 (770)
Income Tax On Net Unrealized Holding Gain (Loss) [Member]
        
Quantifying Misstatement in Current Year Financial Statements [Line Items]        
Quantifying Misstatement in Current Year Financial Statements, Amount (4,756) (5,874) 121 (448)
Net Unrealized Holding Gain (Loss) Net Of Income Tax [Member]
        
Quantifying Misstatement in Current Year Financial Statements [Line Items]        
Quantifying Misstatement in Current Year Financial Statements, Amount $ (8,168) $ (10,090) $ 208 $ (770)