Securities (Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Available-for-sale:
Available-for-sale, amortized cost	$ 384,020	$ 418,644
Available-for-sale, gross unrealized gains	5,335	2,872
Available-for-sale, gross unrealized losses	(477)	(4,718)
Total available-for-sale securities	388,878	416,798
Held-to-maturity:
Total held-to-maturity securities	2,443,503	2,296,799
Held-to-maturity, gross unrealized gains	32,582	11,146
Held-to-maturity, gross unrealized losses	(6,627)	(21,853)
Held-to-maturity securities, fair value	2,469,458	2,286,092
GSE issuance REMICs and CMOs
Available-for-sale:
Available-for-sale, amortized cost	297,400	331,099
Available-for-sale, gross unrealized gains	4,846	2,374
Available-for-sale, gross unrealized losses	(459)	(2,934)
Total available-for-sale securities	301,787	330,539
Held-to-maturity:
Total held-to-maturity securities	1,305,818	1,361,907
Held-to-maturity, gross unrealized gains	20,083	8,135
Held-to-maturity, gross unrealized losses	(3,049)	(14,128)
Held-to-maturity securities, fair value	1,322,852	1,355,914
Non-GSE issuance REMICs and CMOs
Available-for-sale:
Available-for-sale, amortized cost	2,612	3,048
Available-for-sale, gross unrealized gains	7	13
Available-for-sale, gross unrealized losses	(2)	(7)
Total available-for-sale securities	2,617	3,054
Held-to-maturity:
Total held-to-maturity securities	197	198
Held-to-maturity, gross unrealized gains	0	0
Held-to-maturity, gross unrealized losses	(7)	(5)
Held-to-maturity securities, fair value	190	193
GSE pass-through certificates
Available-for-sale:
Available-for-sale, amortized cost	10,292	10,781
Available-for-sale, gross unrealized gains	454	485
Available-for-sale, gross unrealized losses	(2)	(2)
Total available-for-sale securities	10,744	11,264
Held-to-maturity:
Total held-to-maturity securities	250,658	260,707
Held-to-maturity, gross unrealized gains	3,420	1,535
Held-to-maturity, gross unrealized losses	(881)	(3,413)
Held-to-maturity securities, fair value	253,197	258,829
Total residential mortgage-backed securities
Available-for-sale:
Available-for-sale, amortized cost	310,304	344,928
Available-for-sale, gross unrealized gains	5,307	2,872
Available-for-sale, gross unrealized losses	(463)	(2,943)
Total available-for-sale securities	315,148	344,857
Held-to-maturity:
Total held-to-maturity securities	1,556,673	1,622,812
Held-to-maturity, gross unrealized gains	23,503	9,670
Held-to-maturity, gross unrealized losses	(3,937)	(17,546)
Held-to-maturity securities, fair value	1,576,239	1,614,936
Obligations of GSEs
Available-for-sale:
Available-for-sale, amortized cost	73,701	73,701
Available-for-sale, gross unrealized gains	28	0
Available-for-sale, gross unrealized losses	0	(1,762)
Total available-for-sale securities	73,729	71,939
Held-to-maturity:
Total held-to-maturity securities	192,541	178,967
Held-to-maturity, gross unrealized gains	483	220
Held-to-maturity, gross unrealized losses	(87)	(480)
Held-to-maturity securities, fair value	192,937	178,707
Fannie Mae stock
Available-for-sale:
Available-for-sale, amortized cost	15	15
Available-for-sale, gross unrealized gains	0	0
Available-for-sale, gross unrealized losses	(14)	(13)
Total available-for-sale securities	1	2
GSE issuance REMICs
Held-to-maturity:
Total held-to-maturity securities	613,877	434,587
Held-to-maturity, gross unrealized gains	8,381	1,255
Held-to-maturity, gross unrealized losses	(171)	(2,334)
Held-to-maturity securities, fair value	622,087	433,508
Corporate Debt securities
Held-to-maturity:
Total held-to-maturity securities	80,000	60,000
Held-to-maturity, gross unrealized gains	214	0
Held-to-maturity, gross unrealized losses	(2,432)	(1,493)
Held-to-maturity securities, fair value	77,782	58,507
Other
Held-to-maturity:
Total held-to-maturity securities	412	433
Held-to-maturity, gross unrealized gains	1	1
Held-to-maturity, gross unrealized losses	0	0
Held-to-maturity securities, fair value	$ 413	$ 434