Agency Securities, Available for Sale (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Other than temporary impairment of Agency Securities									$ 0	$ 0	$ 401,541,000
Available-for-sale securities fair value	$ 12,461,556,000				$ 15,297,529,000				$ 12,461,556,000	$ 15,297,529,000	6,800,000,000
Weighted average coupon rate	3.45%				3.47%				3.45%	3.47%
Payable for unsettled purchases	$ 0				$ 445,292,000				$ 0	$ 445,292,000
Payable for unsettled purchases, fair value					445,527,000					445,527,000
Gross realized gains (losses) from sale proceeds									5,367,123,000	10,595,518,000	14,965,888,000
Realized gain (loss) on sale of Agency Securities	$ (13,108,000)	$ 69,000	$ (5,051,000)	$ 6,544,000	$ 124,000	$ (12,390,000)	$ 11,167,000	$ 69,869,000	$ (11,546,000)	$ 68,770,000	$ (593,498,000)
Available-for-sale Securities | Portfolio Concentration Risk
Schedule of Available-for-sale Securities [Line Items]
Percentage of portfolio invested in agency securities									100.00%	100.00%
25-Year and 30-Year Fixed Rate Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other than temporary impairment of Agency Securities									$ 0