Quarterly report pursuant to Section 13 or 15(d)

Schedule of Unrealized Loss on Investments (Details)

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Schedule of Unrealized Loss on Investments (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]    
Available-for-Sale Securities, Number of Positions 1,732 1,493
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Positions 863 250
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Positions 35 47
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value $ 8,461,815 $ 2,548,995
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Percent Agency 97.50% 95.80%
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses (405,056) (18,610)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 62,024 52,689
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses (7,569) (9,628)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 8,523,839 2,601,684
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses (412,625) (28,238)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value $ 8,247,874 $ 2,440,929