Securities (Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Available-for-sale:
Available-for-sale, amortized cost	$ 443,086	$ 383,631
Available-for-sale, gross unrealized gains	5,382	4,282
Available-for-sale, gross unrealized losses	(1,010)	(3,554)
Total available-for-sale securities	447,458	384,359
Held-to-maturity:
Total held-to-maturity securities	2,197,629	2,133,804
Held-to-maturity, gross unrealized gains	26,655	17,605
Held-to-maturity, gross unrealized losses	(8,840)	(20,038)
Held-to-maturity securities, fair value	2,215,444	2,131,371
GSE issuance REMICs and CMOs
Available-for-sale:
Available-for-sale, amortized cost	329,635	266,946
Available-for-sale, gross unrealized gains	4,821	3,608
Available-for-sale, gross unrealized losses	(472)	(1,556)
Total available-for-sale securities	333,984	268,998
Held-to-maturity:
Total held-to-maturity securities	1,411,120	1,575,402
Held-to-maturity, gross unrealized gains	18,691	14,536
Held-to-maturity, gross unrealized losses	(6,804)	(14,041)
Held-to-maturity securities, fair value	1,423,007	1,575,897
Non-GSE issuance REMICs and CMOs
Available-for-sale:
Available-for-sale, amortized cost	3,548	5,071
Available-for-sale, gross unrealized gains	16	34
Available-for-sale, gross unrealized losses	(8)	(1)
Total available-for-sale securities	3,556	5,104
Held-to-maturity:
Total held-to-maturity securities	200	2,482
Held-to-maturity, gross unrealized gains		31
Held-to-maturity, gross unrealized losses	(5)	(7)
Held-to-maturity securities, fair value	195	2,506
GSE pass-through certificates
Available-for-sale:
Available-for-sale, amortized cost	11,205	12,919
Available-for-sale, gross unrealized gains	531	640
Available-for-sale, gross unrealized losses	(2)	(2)
Total available-for-sale securities	11,734	13,557
Held-to-maturity:
Total held-to-maturity securities	258,825	281,685
Held-to-maturity, gross unrealized gains	2,946	2,442
Held-to-maturity, gross unrealized losses	(1,649)	(3,877)
Held-to-maturity securities, fair value	260,122	280,250
Total residential mortgage-backed securities
Available-for-sale:
Available-for-sale, amortized cost	344,388	284,936
Available-for-sale, gross unrealized gains	5,368	4,282
Available-for-sale, gross unrealized losses	(482)	(1,559)
Total available-for-sale securities	349,274	287,659
Held-to-maturity:
Total held-to-maturity securities	1,670,145	1,859,569
Held-to-maturity, gross unrealized gains	21,637	17,009
Held-to-maturity, gross unrealized losses	(8,458)	(17,925)
Held-to-maturity securities, fair value	1,683,324	1,858,653
Obligations of GSEs
Available-for-sale:
Available-for-sale, amortized cost	98,683	98,680
Available-for-sale, gross unrealized gains	14
Available-for-sale, gross unrealized losses	(515)	(1,982)
Total available-for-sale securities	98,182	96,698
Held-to-maturity:
Total held-to-maturity securities	207,355	119,336
Held-to-maturity, gross unrealized gains	659	42
Held-to-maturity, gross unrealized losses	(295)	(1,523)
Held-to-maturity securities, fair value	207,719	117,855
Corporate debt securities
Held-to-maturity:
Total held-to-maturity securities	30,000
Held-to-maturity, gross unrealized gains	0
Held-to-maturity, gross unrealized losses	(75)
Held-to-maturity securities, fair value	29,925
Fannie Mae stock
Available-for-sale:
Available-for-sale, amortized cost	15	15
Available-for-sale, gross unrealized gains	0
Available-for-sale, gross unrealized losses	(13)	(13)
Total available-for-sale securities	2	2
Multi-family mortgage-backed securities:
Held-to-maturity:
Total held-to-maturity securities	289,674	154,381
Held-to-maturity, gross unrealized gains	4,359	554
Held-to-maturity, gross unrealized losses	(12)	(590)
Held-to-maturity securities, fair value	294,021	154,345
Other
Held-to-maturity:
Total held-to-maturity securities	455	518
Held-to-maturity securities, fair value	$ 455	$ 518