Securities (Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Available-for-sale:
Available-for-sale, amortized cost	$ 350,650	$ 418,644
Available-for-sale, gross unrealized gains	5,789	2,872
Total, gross unrealized losses	(92)	(4,718)
Available-for-sale, estimated fair value	356,347	416,798
Held-to-maturity:
Total held-to-maturity securities	2,677,930	2,296,799
Held-to-maturity, gross unrealized gains	42,343	11,146
Total, gross unrealized losses	(3,571)	(21,853)
Held-to-maturity securities, estimated fair value	2,716,702	2,286,092
GSE issuance REMICs and CMOs
Available-for-sale:
Available-for-sale, amortized cost	283,609	331,099
Available-for-sale, gross unrealized gains	5,278	2,374
Total, gross unrealized losses	(75)	(2,934)
Available-for-sale, estimated fair value	288,812	330,539
Held-to-maturity:
Total held-to-maturity securities	1,217,296	1,361,907
Held-to-maturity, gross unrealized gains	22,029	8,135
Total, gross unrealized losses	(1,131)	(14,128)
Held-to-maturity securities, estimated fair value	1,238,194	1,355,914
Non-GSE issuance REMICs and CMOs
Available-for-sale:
Available-for-sale, amortized cost	2,213	3,048
Available-for-sale, gross unrealized gains	6	13
Total, gross unrealized losses	(2)	(7)
Available-for-sale, estimated fair value	2,217	3,054
Held-to-maturity:
Total held-to-maturity securities	196	198
Held-to-maturity, gross unrealized gains	0	0
Total, gross unrealized losses	(7)	(5)
Held-to-maturity securities, estimated fair value	189	193
GSE pass-through certificates
Available-for-sale:
Available-for-sale, amortized cost	9,821	10,781
Available-for-sale, gross unrealized gains	448	485
Total, gross unrealized losses	(2)	(2)
Available-for-sale, estimated fair value	10,267	11,264
Held-to-maturity:
Total held-to-maturity securities	238,763	260,707
Held-to-maturity, gross unrealized gains	4,383	1,535
Total, gross unrealized losses	(274)	(3,413)
Held-to-maturity securities, estimated fair value	242,872	258,829
Total residential mortgage-backed securities
Available-for-sale:
Available-for-sale, amortized cost	295,643	344,928
Available-for-sale, gross unrealized gains	5,732	2,872
Total, gross unrealized losses	(79)	(2,943)
Available-for-sale, estimated fair value	301,296	344,857
Held-to-maturity:
Total held-to-maturity securities	1,456,255	1,622,812
Held-to-maturity, gross unrealized gains	26,412	9,670
Total, gross unrealized losses	(1,412)	(17,546)
Held-to-maturity securities, estimated fair value	1,481,255	1,614,936
Obligations of GSEs
Available-for-sale:
Available-for-sale, amortized cost	54,992	73,701
Available-for-sale, gross unrealized gains	57	0
Total, gross unrealized losses	0	(1,762)
Available-for-sale, estimated fair value	55,049	71,939
Held-to-maturity:
Total held-to-maturity securities	246,274	178,967
Held-to-maturity, gross unrealized gains	870	220
Total, gross unrealized losses	0	(480)
Held-to-maturity securities, estimated fair value	247,144	178,707
Fannie Mae stock
Available-for-sale:
Available-for-sale, amortized cost	15	15
Available-for-sale, gross unrealized gains	0	0
Total, gross unrealized losses	(13)	(13)
Available-for-sale, estimated fair value	2	2
GSE issuance REMICs
Held-to-maturity:
Total held-to-maturity securities	895,011	434,587
Held-to-maturity, gross unrealized gains	14,624	1,255
Total, gross unrealized losses	(47)	(2,334)
Held-to-maturity securities, estimated fair value	909,588	433,508
Corporate Debt securities
Held-to-maturity:
Total held-to-maturity securities	80,000	60,000
Held-to-maturity, gross unrealized gains	436	0
Total, gross unrealized losses	(2,112)	(1,493)
Held-to-maturity securities, estimated fair value	78,324	58,507
Other
Held-to-maturity:
Total held-to-maturity securities	390	433
Held-to-maturity, gross unrealized gains	1	1
Total, gross unrealized losses	0	0
Held-to-maturity securities, estimated fair value	$ 391	$ 434