Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Available-for-sale:
Amortized Cost	$ 418,644	$ 383,631
Gross Unrealized Gains	2,872	4,282
Gross Unrealized Losses	(4,718)	(3,554)
Estimated Fair Value	416,798	384,359
Held-to-maturity:
Total held-to-maturity securities	2,296,799	2,133,804
Gross Unrealized Gains	11,146	17,605
Gross Unrealized Losses	(21,853)	(20,038)
Estimated Fair Value	2,286,092	2,131,371
GSE issuance REMICs and CMOs
Available-for-sale:
Amortized Cost	331,099	266,946
Gross Unrealized Gains	2,374	3,608
Gross Unrealized Losses	(2,934)	(1,556)
Estimated Fair Value	330,539	268,998
Held-to-maturity:
Total held-to-maturity securities	1,361,907	1,575,402
Gross Unrealized Gains	8,135	14,536
Gross Unrealized Losses	(14,128)	(14,041)
Estimated Fair Value	1,355,914	1,575,897
Non-GSE issuance REMICs and CMOs
Available-for-sale:
Amortized Cost	3,048	5,071
Gross Unrealized Gains	13	34
Gross Unrealized Losses	(7)	(1)
Estimated Fair Value	3,054	5,104
Held-to-maturity:
Total held-to-maturity securities	198	2,482
Gross Unrealized Gains	0	31
Gross Unrealized Losses	(5)	(7)
Estimated Fair Value	193	2,506
GSE pass-through certificates
Available-for-sale:
Amortized Cost	10,781	12,919
Gross Unrealized Gains	485	640
Gross Unrealized Losses	(2)	(2)
Estimated Fair Value	11,264	13,557
Held-to-maturity:
Total held-to-maturity securities	260,707	281,685
Gross Unrealized Gains	1,535	2,442
Gross Unrealized Losses	(3,413)	(3,877)
Estimated Fair Value	258,829	280,250
Total residential mortgage-backed securities
Available-for-sale:
Amortized Cost	344,928	284,936
Gross Unrealized Gains	2,872	4,282
Gross Unrealized Losses	(2,943)	(1,559)
Estimated Fair Value	344,857	287,659
Held-to-maturity:
Total held-to-maturity securities	1,622,812	1,859,569
Gross Unrealized Gains	9,670	17,009
Gross Unrealized Losses	(17,546)	(17,925)
Estimated Fair Value	1,614,936	1,858,653
Obligations of GSEs
Available-for-sale:
Amortized Cost	73,701	98,680
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,762)	(1,982)
Estimated Fair Value	71,939	96,698
Held-to-maturity:
Total held-to-maturity securities	178,967	119,336
Gross Unrealized Gains	220	42
Gross Unrealized Losses	(480)	(1,523)
Estimated Fair Value	178,707	117,855
Fannie Mae stock
Available-for-sale:
Amortized Cost	15	15
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(13)	(13)
Estimated Fair Value	2	2
GSE issuance REMICs
Held-to-maturity:
Total held-to-maturity securities	434,587	154,381
Gross Unrealized Gains	1,255	554
Gross Unrealized Losses	(2,334)	(590)
Estimated Fair Value	433,508	154,345
Corporate debt securities
Held-to-maturity:
Total held-to-maturity securities	60,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(1,493)
Estimated Fair Value	58,507
Other
Held-to-maturity:
Total held-to-maturity securities	433	518
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 434	$ 518