Schedule of Agency Securities and MBS Portfolio Includes MBS not Issued or Guaranteed by U.S. Government Agency or U.S. Government Sponsored Entity (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	$ 17,354,770	$ 17,533,042
Gross Unrealized Loss	(35,531)	(120,019)
Gross Unrealized Gain	267,771	229,509
Mortgage-backed securities, at fair value (including pledged assets of $16,538,214 and $17,049,670 at September 30, 2014 and December 31, 2013, respectively)	17,587,010	17,642,532
Agency Securities
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	17,279,316
Gross Unrealized Loss	(35,531)
Gross Unrealized Gain	267,623
Mortgage-backed securities, at fair value (including pledged assets of $16,538,214 and $17,049,670 at September 30, 2014 and December 31, 2013, respectively)	17,511,408
Non-Agency ARMs
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	75,454
Gross Unrealized Gain	148
Mortgage-backed securities, at fair value (including pledged assets of $16,538,214 and $17,049,670 at September 30, 2014 and December 31, 2013, respectively)	75,602
Adjustable Rate Residential Mortgage
Schedule Of Available For Sale Securities [Line Items]
Mortgage-backed securities, at fair value (including pledged assets of $16,538,214 and $17,049,670 at September 30, 2014 and December 31, 2013, respectively)	16,310,376	16,401,789
Fixed-Rate
Schedule Of Available For Sale Securities [Line Items]
Mortgage-backed securities, at fair value (including pledged assets of $16,538,214 and $17,049,670 at September 30, 2014 and December 31, 2013, respectively)	1,276,634	1,240,743
Fannie Mae Certificates
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	10,315,029	10,427,055
Gross Unrealized Loss	(13,737)	(46,669)
Gross Unrealized Gain	189,066	171,680
Mortgage-backed securities, at fair value (including pledged assets of $16,538,214 and $17,049,670 at September 30, 2014 and December 31, 2013, respectively)	10,490,358	10,552,066
Fannie Mae Certificates | Adjustable Rate Residential Mortgage
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	9,203,741	9,620,743
Gross Unrealized Loss	(13,737)	(44,871)
Gross Unrealized Gain	183,889	167,848
Mortgage-backed securities, at fair value (including pledged assets of $16,538,214 and $17,049,670 at September 30, 2014 and December 31, 2013, respectively)	9,373,893	9,743,720
Fannie Mae Certificates | Fixed-Rate
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	1,111,288	806,312
Gross Unrealized Loss		(1,798)
Gross Unrealized Gain	5,177	3,832
Mortgage-backed securities, at fair value (including pledged assets of $16,538,214 and $17,049,670 at September 30, 2014 and December 31, 2013, respectively)	1,116,465	808,346
Freddie Mac Certificates
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	6,964,287	7,009,751
Gross Unrealized Loss	(21,794)	(72,352)
Gross Unrealized Gain	78,557	57,829
Mortgage-backed securities, at fair value (including pledged assets of $16,538,214 and $17,049,670 at September 30, 2014 and December 31, 2013, respectively)	7,021,050	6,995,228
Freddie Mac Certificates | Adjustable Rate Residential Mortgage
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	6,805,885	6,671,013
Gross Unrealized Loss	(21,794)	(70,752)
Gross Unrealized Gain	76,790	57,808
Mortgage-backed securities, at fair value (including pledged assets of $16,538,214 and $17,049,670 at September 30, 2014 and December 31, 2013, respectively)	6,860,881	6,658,069
Freddie Mac Certificates | Fixed-Rate
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	158,402	338,738
Gross Unrealized Loss		(1,600)
Gross Unrealized Gain	1,767	21
Mortgage-backed securities, at fair value (including pledged assets of $16,538,214 and $17,049,670 at September 30, 2014 and December 31, 2013, respectively)	160,169	337,159
Ginnie Mae Certificates
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost		96,236
Gross Unrealized Loss		(998)
Mortgage-backed securities, at fair value (including pledged assets of $16,538,214 and $17,049,670 at September 30, 2014 and December 31, 2013, respectively)		95,238
Ginnie Mae Certificates | Fixed-Rate
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost		96,236
Gross Unrealized Loss		(998)
Mortgage-backed securities, at fair value (including pledged assets of $16,538,214 and $17,049,670 at September 30, 2014 and December 31, 2013, respectively)		$ 95,238