Securities (Estimated Fair Values of Securities With Gross Unrealized Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Available-for-sale:
Available-for-sale, less than twelve months, estimated fair value	$ 20,075	$ 214,138
Available-for-sale, less than twelve months, gross unrealized losses	(133)	(3,327)
Available-for-sale, twelve months or longer, estimated fair value	24,125	47,506
Available-for-sale, twelve months or longer, gross unrealized losses	(344)	(1,391)
Available-for-sale, total, estimated fair value	44,200	261,644
Available-for-sale, total, gross unrealized losses	(477)	(4,718)
Held-to-maturity:
Held-to-maturity, less than twelve months, estimated fair value	222,904	895,094
Held-to-maturity securities, continuous unrealized loss position, less than 12 months, accumulated loss	(2,839)	(8,865)
Held-to-maturity, twelve months or longer, estimated fair value	339,471	396,576
Held-to-maturity securities, continuous unrealized loss position, 12 months or longer, accumulated loss	(3,788)	(12,988)
Held-to-maturity, total, estimated fair value	562,375	1,291,670
Held-to-maturity, total, gross unrealized losses	(6,627)	(21,853)
GSE issuance REMICs and CMOs
Available-for-sale:
Available-for-sale, less than twelve months, estimated fair value	19,950	189,364
Available-for-sale, less than twelve months, gross unrealized losses	(131)	(2,934)
Available-for-sale, twelve months or longer, estimated fair value	23,962	0
Available-for-sale, twelve months or longer, gross unrealized losses	(328)	0
Available-for-sale, total, estimated fair value	43,912	189,364
Available-for-sale, total, gross unrealized losses	(459)	(2,934)
Held-to-maturity:
Held-to-maturity, less than twelve months, estimated fair value	51,103	395,659
Held-to-maturity securities, continuous unrealized loss position, less than 12 months, accumulated loss	(149)	(3,972)
Held-to-maturity, twelve months or longer, estimated fair value	234,606	289,645
Held-to-maturity securities, continuous unrealized loss position, 12 months or longer, accumulated loss	(2,900)	(10,156)
Held-to-maturity, total, estimated fair value	285,709	685,304
Held-to-maturity, total, gross unrealized losses	(3,049)	(14,128)
Non-GSE issuance REMICs and CMOs
Available-for-sale:
Available-for-sale, less than twelve months, estimated fair value	109	75
Available-for-sale, less than twelve months, gross unrealized losses	(1)	(2)
Available-for-sale, twelve months or longer, estimated fair value	63	64
Available-for-sale, twelve months or longer, gross unrealized losses	(1)	(5)
Available-for-sale, total, estimated fair value	172	139
Available-for-sale, total, gross unrealized losses	(2)	(7)
Held-to-maturity:
Held-to-maturity, less than twelve months, estimated fair value	0	0
Held-to-maturity securities, continuous unrealized loss position, less than 12 months, accumulated loss	0	0
Held-to-maturity, twelve months or longer, estimated fair value	190	193
Held-to-maturity securities, continuous unrealized loss position, 12 months or longer, accumulated loss	(7)	(5)
Held-to-maturity, total, estimated fair value	190	193
Held-to-maturity, total, gross unrealized losses	(7)	(5)
GSE pass-through certificates
Available-for-sale:
Available-for-sale, less than twelve months, estimated fair value	16	97
Available-for-sale, less than twelve months, gross unrealized losses	(1)	(1)
Available-for-sale, twelve months or longer, estimated fair value	99	103
Available-for-sale, twelve months or longer, gross unrealized losses	(1)	(1)
Available-for-sale, total, estimated fair value	115	200
Available-for-sale, total, gross unrealized losses	(2)	(2)
Held-to-maturity:
Held-to-maturity, less than twelve months, estimated fair value	0	56,503
Held-to-maturity securities, continuous unrealized loss position, less than 12 months, accumulated loss	0	(586)
Held-to-maturity, twelve months or longer, estimated fair value	104,675	106,738
Held-to-maturity securities, continuous unrealized loss position, 12 months or longer, accumulated loss	(881)	(2,827)
Held-to-maturity, total, estimated fair value	104,675	163,241
Held-to-maturity, total, gross unrealized losses	(881)	(3,413)
Obligations of GSEs
Available-for-sale:
Available-for-sale, less than twelve months, estimated fair value		24,602
Available-for-sale, less than twelve months, gross unrealized losses		(390)
Available-for-sale, twelve months or longer, estimated fair value		47,337
Available-for-sale, twelve months or longer, gross unrealized losses		(1,372)
Available-for-sale, total, estimated fair value		71,939
Available-for-sale, total, gross unrealized losses	0	(1,762)
Held-to-maturity:
Held-to-maturity, less than twelve months, estimated fair value	24,888	107,824
Held-to-maturity securities, continuous unrealized loss position, less than 12 months, accumulated loss	(87)	(480)
Held-to-maturity, twelve months or longer, estimated fair value	0	0
Held-to-maturity securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0	0
Held-to-maturity, total, estimated fair value	24,888	107,824
Held-to-maturity, total, gross unrealized losses	(87)	(480)
Fannie Mae stock
Available-for-sale:
Available-for-sale, less than twelve months, estimated fair value	0	0
Available-for-sale, less than twelve months, gross unrealized losses	0	0
Available-for-sale, twelve months or longer, estimated fair value	1	2
Available-for-sale, twelve months or longer, gross unrealized losses	(14)	(13)
Available-for-sale, total, estimated fair value	1	2
Available-for-sale, total, gross unrealized losses	(14)	(13)
GSE issuance REMICs
Held-to-maturity:
Held-to-maturity, less than twelve months, estimated fair value	79,344	276,601
Held-to-maturity securities, continuous unrealized loss position, less than 12 months, accumulated loss	(171)	(2,334)
Held-to-maturity, twelve months or longer, estimated fair value	0	0
Held-to-maturity securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0	0
Held-to-maturity, total, estimated fair value	79,344	276,601
Held-to-maturity, total, gross unrealized losses	(171)	(2,334)
Corporate Debt securities
Held-to-maturity:
Held-to-maturity, less than twelve months, estimated fair value	67,569	58,507
Held-to-maturity securities, continuous unrealized loss position, less than 12 months, accumulated loss	(2,432)	(1,493)
Held-to-maturity, twelve months or longer, estimated fair value	0	0
Held-to-maturity securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0	0
Held-to-maturity, total, estimated fair value	67,569	58,507
Held-to-maturity, total, gross unrealized losses	$ (2,432)	$ (1,493)