Securities (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Securities
Available-for-sale, less than twelve months, estimated fair value	$ 45,226	$ 334,474
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	555	14,657
Available-for-sale, less than twelve months, gross unrealized losses	(892)	(43,112)
Available-for-sale, twelve months or longer, estimated fair value	147,718	205
Available-for-sale, twelve months or longer, gross unrealized losses	(2,999)	(14)
Available-for-sale, total, estimated fair value	192,944	334,679
Available-for-sale, total, gross unrealized losses	(3,554)	(14,671)
Held-to-maturity, less than twelve months, estimated fair value	222,216	1,031,627
Held-to-maturity, twelve months or longer, estimated fair value	744,508	151,609
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	19,146	8,315
Held-to-maturity, total, estimated fair value	966,724	1,183,236
Held-to-maturity, total, gross unrealized losses	(20,038)	(51,427)
Held-to-maturity, amortized cost	(2,133,804)	(1,849,526)
GSE issuance REMICs and CMOs
Securities
Available-for-sale, less than twelve months, estimated fair value	20,587	243,149
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	159	7,134
Available-for-sale, less than twelve months, gross unrealized losses	(302)	(25,611)
Available-for-sale, twelve months or longer, estimated fair value	75,444
Available-for-sale, twelve months or longer, gross unrealized losses	(1,397)
Available-for-sale, total, estimated fair value	96,031	243,149
Available-for-sale, total, gross unrealized losses	(1,556)	(7,134)
Held-to-maturity, less than twelve months, estimated fair value	121,861	719,715
Held-to-maturity, twelve months or longer, estimated fair value	500,348	151,581
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	13,739	8,314
Held-to-maturity, total, estimated fair value	622,209	871,296
Held-to-maturity, total, gross unrealized losses	(14,041)	(33,925)
Held-to-maturity, amortized cost	(1,575,402)	(1,474,506)
Non-GSE issuance REMICs and CMOs
Securities
Available-for-sale, less than twelve months, gross unrealized losses		(10)
Available-for-sale, twelve months or longer, estimated fair value	96	132
Available-for-sale, twelve months or longer, gross unrealized losses	(1)	(1)
Available-for-sale, total, estimated fair value	96	132
Available-for-sale, total, gross unrealized losses	(1)	(1)
Held-to-maturity, less than twelve months, estimated fair value		392
Held-to-maturity, twelve months or longer, estimated fair value	294
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	7
Held-to-maturity, total, estimated fair value	294	392
Held-to-maturity, total, gross unrealized losses	(7)	(10)
Held-to-maturity, amortized cost	(2,482)	(3,833)
GSE pass-through certificates
Securities
Available-for-sale, less than twelve months, estimated fair value	53	172
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	1
Available-for-sale, less than twelve months, gross unrealized losses		(10,088)
Available-for-sale, twelve months or longer, estimated fair value	64	70
Available-for-sale, twelve months or longer, gross unrealized losses	(1)	(1)
Available-for-sale, total, estimated fair value	117	242
Available-for-sale, total, gross unrealized losses	(2)	(2)
Held-to-maturity, less than twelve months, estimated fair value		230,795
Held-to-maturity, twelve months or longer, estimated fair value	164,453	28
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,877	1
Held-to-maturity, total, estimated fair value	164,453	230,823
Held-to-maturity, total, gross unrealized losses	(3,877)	(10,089)
Held-to-maturity, amortized cost	(281,685)	(282,473)
Multi-family mortgage-backed securities
Securities
Available-for-sale, less than twelve months, gross unrealized losses	(590)
Held-to-maturity, less than twelve months, estimated fair value	100,355
Held-to-maturity, total, estimated fair value	100,355
Held-to-maturity, total, gross unrealized losses	(590)
Held-to-maturity, amortized cost	(154,381)
Obligations of GSEs
Securities
Available-for-sale, less than twelve months, estimated fair value	24,586	91,153
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	395	7,522
Available-for-sale, less than twelve months, gross unrealized losses		(7,403)
Available-for-sale, twelve months or longer, estimated fair value	72,112
Available-for-sale, twelve months or longer, gross unrealized losses	(1,587)
Available-for-sale, total, estimated fair value	96,698	91,153
Available-for-sale, total, gross unrealized losses	(1,982)	(7,522)
Held-to-maturity, less than twelve months, estimated fair value		80,725
Held-to-maturity, twelve months or longer, estimated fair value	79,413
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,523
Held-to-maturity, total, estimated fair value	79,413	80,725
Held-to-maturity, total, gross unrealized losses	(1,523)	(7,403)
Held-to-maturity, amortized cost	(119,336)	(88,128)
Fannie Mae stock
Securities
Available-for-sale, twelve months or longer, estimated fair value	2	3
Available-for-sale, twelve months or longer, gross unrealized losses	(13)	(12)
Available-for-sale, total, estimated fair value	2	3
Available-for-sale, total, gross unrealized losses	$ (13)	$ (12)