Held-to-Maturity Securities (Continuous Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 5,766,200	$ 8,597,524
Held To Maturity Securities Continuous Unrealized Loss Position Less Than 12 Months Accumulated Loss	(70,064)	(117,176)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,122,601	1,193,241
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(40,403)	(57,375)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	6,888,801	9,790,765
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(110,467)	(174,551)
US Government-sponsored Enterprises Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	32,786
Held To Maturity Securities Continuous Unrealized Loss Position Less Than 12 Months Accumulated Loss	(3)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	32,786
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(3)
Other Than Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	32,786
Held To Maturity Securities Continuous Unrealized Loss Position Less Than 12 Months Accumulated Loss	(3)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	32,786
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(3)
Single Family, Mortgage-backed Securities, Other US Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,538,654	2,151,584
Held To Maturity Securities Continuous Unrealized Loss Position Less Than 12 Months Accumulated Loss	(15,951)	(23,151)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	1,538,654	2,151,584
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(15,951)	(23,151)
Collateralized Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	5,733,414
Held To Maturity Securities Continuous Unrealized Loss Position Less Than 12 Months Accumulated Loss	(70,061)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,122,601
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(40,403)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	6,856,015
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(110,464)
Single Family [Member] \| Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	3,912,027	4,548,897
Held To Maturity Securities Continuous Unrealized Loss Position Less Than 12 Months Accumulated Loss	(53,875)	(90,119)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,122,601	1,193,241
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(40,403)	(57,375)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	5,034,628	5,742,138
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(94,278)	(147,494)
Multifamily [Member] \| Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	282,733	1,897,043
Held To Maturity Securities Continuous Unrealized Loss Position Less Than 12 Months Accumulated Loss	(235)	(3,906)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	282,733	1,897,043
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (235)	$ (3,906)

Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity in a continuous loss position.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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This item represents the fair value of securities categorized as held-to-maturity securities that have been in a continuous unrealized loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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