Securities (Estimated Fair Values of Securities With Gross Unrealized Losses) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Available-for-sale:
Less than twelve months, estimated fair value	$ 112	$ 214,138
Less than twelve months, gross unrealized losses	(2)	(3,327)
Twelve months or longer, estimated fair value	23,086	47,506
Twelve months or longer, gross unrealized losses	(90)	(1,391)
Total, estimated fair value	23,198	261,644
Total, gross unrealized losses	(92)	(4,718)
Held-to-maturity:
Less than twelve months, estimated fair value	123,095	895,094
Less than twelve months, gross unrealized losses	(2,160)	(8,865)
Twelve months or longer, estimated fair value	227,732	396,576
Twelve months or longer, gross unrealized losses	(1,411)	(12,988)
Total, estimated fair value	350,827	1,291,670
Total, gross unrealized losses	(3,571)	(21,853)
GSE issuance REMICs and CMOs
Available-for-sale:
Less than twelve months, estimated fair value	0	189,364
Less than twelve months, gross unrealized losses	0	(2,934)
Twelve months or longer, estimated fair value	22,913	0
Twelve months or longer, gross unrealized losses	(75)	0
Total, estimated fair value	22,913	189,364
Total, gross unrealized losses	(75)	(2,934)
Held-to-maturity:
Less than twelve months, estimated fair value	9,800	395,659
Less than twelve months, gross unrealized losses	(1)	(3,972)
Twelve months or longer, estimated fair value	157,856	289,645
Twelve months or longer, gross unrealized losses	(1,130)	(10,156)
Total, estimated fair value	167,656	685,304
Total, gross unrealized losses	(1,131)	(14,128)
Non-GSE issuance REMICs and CMOs
Available-for-sale:
Less than twelve months, estimated fair value	38	75
Less than twelve months, gross unrealized losses	(1)	(2)
Twelve months or longer, estimated fair value	79	64
Twelve months or longer, gross unrealized losses	(1)	(5)
Total, estimated fair value	117	139
Total, gross unrealized losses	(2)	(7)
Held-to-maturity:
Less than twelve months, estimated fair value	0	0
Less than twelve months, gross unrealized losses	0	0
Twelve months or longer, estimated fair value	189	193
Twelve months or longer, gross unrealized losses	(7)	(5)
Total, estimated fair value	189	193
Total, gross unrealized losses	(7)	(5)
GSE pass-through certificates
Available-for-sale:
Less than twelve months, estimated fair value	74	97
Less than twelve months, gross unrealized losses	(1)	(1)
Twelve months or longer, estimated fair value	92	103
Twelve months or longer, gross unrealized losses	(1)	(1)
Total, estimated fair value	166	200
Total, gross unrealized losses	(2)	(2)
Held-to-maturity:
Less than twelve months, estimated fair value	0	56,503
Less than twelve months, gross unrealized losses	0	(586)
Twelve months or longer, estimated fair value	69,687	106,738
Twelve months or longer, gross unrealized losses	(274)	(2,827)
Total, estimated fair value	69,687	163,241
Total, gross unrealized losses	(274)	(3,413)
Obligations of GSEs
Available-for-sale:
Less than twelve months, estimated fair value		24,602
Less than twelve months, gross unrealized losses		(390)
Twelve months or longer, estimated fair value		47,337
Twelve months or longer, gross unrealized losses		(1,372)
Total, estimated fair value		71,939
Total, gross unrealized losses	0	(1,762)
Held-to-maturity:
Less than twelve months, estimated fair value		107,824
Less than twelve months, gross unrealized losses		(480)
Twelve months or longer, estimated fair value		0
Twelve months or longer, gross unrealized losses		0
Total, estimated fair value		107,824
Total, gross unrealized losses	0	(480)
GSE issuance REMICs
Held-to-maturity:
Less than twelve months, estimated fair value	45,407	276,601
Less than twelve months, gross unrealized losses	(47)	(2,334)
Twelve months or longer, estimated fair value	0	0
Twelve months or longer, gross unrealized losses	0	0
Total, estimated fair value	45,407	276,601
Total, gross unrealized losses	(47)	(2,334)
Fannie Mae stock
Available-for-sale:
Less than twelve months, estimated fair value	0	0
Less than twelve months, gross unrealized losses	0	0
Twelve months or longer, estimated fair value	2	2
Twelve months or longer, gross unrealized losses	(13)	(13)
Total, estimated fair value	2	2
Total, gross unrealized losses	(13)	(13)
Corporate Debt securities
Held-to-maturity:
Less than twelve months, estimated fair value	67,888	58,507
Less than twelve months, gross unrealized losses	(2,112)	(1,493)
Twelve months or longer, estimated fair value	0	0
Twelve months or longer, gross unrealized losses	0	0
Total, estimated fair value	67,888	58,507
Total, gross unrealized losses	$ (2,112)	$ (1,493)