Annual report pursuant to Section 13 and 15(d)

Available-for-Sale Securities, at Fair Value Schedule of Unrealized Loss on Investments (Details)

v3.8.0.1
Available-for-Sale Securities, at Fair Value Schedule of Unrealized Loss on Investments (Details)
$ in Thousands
Dec. 31, 2017
USD ($)
position
Dec. 31, 2016
USD ($)
position
Schedule of Available-for-sale Securities [Line Items]    
Available-for-Sale Securities, Number of Positions | position 1,435 1,238
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | position 253 251
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | position 234 125
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Percent Agency 98.50% 96.00%
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value $ 12,198,870 $ 6,404,134
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses (65,313) (203,922)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 2,464,544 504,978
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses (114,724) (39,955)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 14,663,414 6,909,112
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses (180,037) (243,877)
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 $ 12,010,451 $ 6,147,713