Securities (Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Available-for-sale:
Available-for-sale, amortized cost	$ 429,396	$ 383,631
Available-for-sale, gross unrealized gains	4,258	4,282
Available-for-sale, gross unrealized losses	(4,909)	(3,554)
Total available-for-sale securities	428,745	384,359
Held-to-maturity:
Total held-to-maturity securities	2,156,882	2,133,804
Held-to-maturity, gross unrealized losses	(21,587)	(20,038)
Held-to-maturity, gross unrealized gains	16,796	17,605
Held-to-maturity securities, fair value	2,152,091	2,131,371
GSE issuance REMICs and CMOs
Available-for-sale:
Available-for-sale, amortized cost	314,975	266,946
Available-for-sale, gross unrealized gains	3,663	3,608
Available-for-sale, gross unrealized losses	(2,238)	(1,556)
Total available-for-sale securities	316,400	268,998
Held-to-maturity:
Total held-to-maturity securities	1,450,045	1,575,402
Held-to-maturity, gross unrealized losses	(15,281)	(14,041)
Held-to-maturity, gross unrealized gains	13,315	14,536
Held-to-maturity securities, fair value	1,448,079	1,575,897
Non-GSE issuance REMICs and CMOs
Available-for-sale:
Available-for-sale, amortized cost	3,986	5,071
Available-for-sale, gross unrealized gains	22	34
Available-for-sale, gross unrealized losses	(2)	(1)
Total available-for-sale securities	4,006	5,104
Held-to-maturity:
Total held-to-maturity securities	298	2,482
Held-to-maturity, gross unrealized losses	(7)	(7)
Held-to-maturity, gross unrealized gains		31
Held-to-maturity securities, fair value	291	2,506
GSE pass-through certificates
Available-for-sale:
Available-for-sale, amortized cost	11,738	12,919
Available-for-sale, gross unrealized gains	573	640
Available-for-sale, gross unrealized losses	(2)	(2)
Total available-for-sale securities	12,309	13,557
Held-to-maturity:
Total held-to-maturity securities	260,948	281,685
Held-to-maturity, gross unrealized losses	(2,837)	(3,877)
Held-to-maturity, gross unrealized gains	2,009	2,442
Held-to-maturity securities, fair value	260,120	280,250
Total residential mortgage-backed securities
Available-for-sale:
Available-for-sale, amortized cost	330,699	284,936
Available-for-sale, gross unrealized gains	4,258	4,282
Available-for-sale, gross unrealized losses	(2,242)	(1,559)
Total available-for-sale securities	332,715	287,659
Held-to-maturity:
Total held-to-maturity securities	1,711,291	1,859,569
Held-to-maturity, gross unrealized losses	(18,125)	(17,925)
Held-to-maturity, gross unrealized gains	15,324	17,009
Held-to-maturity securities, fair value	1,708,490	1,858,653
Obligations of GSEs
Available-for-sale:
Available-for-sale, amortized cost	98,682	98,680
Available-for-sale, gross unrealized losses	(2,654)	(1,982)
Total available-for-sale securities	96,028	96,698
Held-to-maturity:
Total held-to-maturity securities	207,344	119,336
Held-to-maturity, gross unrealized losses	(2,909)	(1,523)
Held-to-maturity, gross unrealized gains	21	42
Held-to-maturity securities, fair value	204,456	117,855
Fannie Mae stock
Available-for-sale:
Available-for-sale, amortized cost	15	15
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	237,771	154,381
Held-to-maturity, gross unrealized losses	(553)	(590)
Held-to-maturity, gross unrealized gains	1,451	554
Held-to-maturity securities, fair value	238,669	154,345
Other
Held-to-maturity:
Total held-to-maturity securities	476	518
Held-to-maturity securities, fair value	$ 476	$ 518