Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands					12 Months Ended
	Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Available-for-sale Securities [Abstract]
Derivative, Notional Amount	$ 8,429,964		$ 6,837,571		$ 8,429,964		$ 6,837,571
Investment Securities [Member]
Schedule of Available-for-sale and Trading Securities [Line Items]
Proceeds from sale of available-for-sale investment securities					83,735		770,149		$ 366,562
Available-for-sale Securities, Gross Realized Gains					100		600		$ 3,100
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	2,773,111		1,932,236		2,773,111		1,932,236
Unamortized Premium/(Discount)	5,936		7,471		5,936		7,471
Available-for-sale Securities, Amortized Cost Basis	2,779,047		1,939,707		2,779,047		1,939,707
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	6,891		9,003		6,891		9,003
Available-for-sale Securities, Unrealized Losses	(10,913)		(10,211)		(10,913)		(10,211)
Available-for-sale Securities, Fair Value Disclosure	2,775,025		1,938,499		2,775,025		1,938,499
Trading Securities [Abstract]
Trading, at fair value	491		689		491		689
Marketable Securities [Abstract]
Marketable Securities Unpaid Principal Balance	2,775,322		1,935,104		2,775,322		1,935,104
Marketable Securities, Amortized Cost	2,781,258		1,942,575		2,781,258		1,942,575
Marketable Securities, Unrealized Gains	6,891		9,003		6,891		9,003
Marketable Securities, Unrealized Losses	(12,633)		(12,390)		(12,633)		(12,390)
Marketable Securities	2,775,516		1,939,188		2,775,516		1,939,188
USDA Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Fair Value Disclosure	1,888,344		1,731,222		1,888,344		1,731,222
Trading Securities [Abstract]
Trading, at fair value	28,975		40,310		28,975		40,310
Marketable Securities [Abstract]
Marketable Securities	1,917,319		1,771,532		1,917,319		1,771,532
Available-for-sale Securities [Member] | Investment Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	5,936		7,471		5,936		7,471
Floating Interest Rate [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	46,500		46,600		46,500		46,600
Available-for-sale Securities, Amortized Cost Basis	46,500		46,600		46,500		46,600
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0		0		0		0
Available-for-sale Securities, Unrealized Losses	(1,576)		(6,024)		(1,576)		(6,024)
Available-for-sale Securities, Fair Value Disclosure	44,924		40,576		44,924		40,576
Floating Interest Rate [Member] | Asset-backed Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	74,744		100,730		74,744		100,730
Available-for-sale Securities, Amortized Cost Basis	74,491		100,656		74,491		100,656
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	14		283		14		283
Available-for-sale Securities, Unrealized Losses	(776)		(37)		(776)		(37)
Available-for-sale Securities, Fair Value Disclosure	73,729		100,902		73,729		100,902
Trading Securities [Abstract]
Trading Securities - Amount Outstanding	2,211		2,868		2,211		2,868
Trading Securities, Debt, Amortized Cost	2,211		2,868		2,211		2,868
Trading Securities, Unrealized Gains	0		0
Trading Securities, Unrealized Losses	(1,720)		(2,179)
Trading, at fair value	491		689		491		689
Floating Interest Rate [Member] | Corporate Debt Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	10,000		10,000		10,000		10,000
Available-for-sale Securities, Amortized Cost Basis	10,000		10,000		10,000		10,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0		91		0		91
Available-for-sale Securities, Unrealized Losses	(9)		0		(9)		0
Available-for-sale Securities, Fair Value Disclosure	9,991		10,091		9,991		10,091
Floating Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	1,353,495		605,053		1,353,495		605,053
Available-for-sale Securities, Amortized Cost Basis	1,357,010		608,484		1,357,010		608,484
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	2,768		4,712		2,768		4,712
Available-for-sale Securities, Unrealized Losses	(4,319)		(443)		(4,319)		(443)
Available-for-sale Securities, Fair Value Disclosure	1,355,459		612,753		1,355,459		612,753
Floating Interest Rate [Member] | GSE Subordinated Debt [Member}
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	70,000		70,000		70,000		70,000
Available-for-sale Securities, Amortized Cost Basis	70,000		70,000		70,000		70,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0		0		0		0
Available-for-sale Securities, Unrealized Losses	(3,751)		(3,680)		(3,751)		(3,680)
Available-for-sale Securities, Fair Value Disclosure	66,249		66,320		66,249		66,320
Floating Interest Rate [Member] | Taxable Municipal Bonds [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance			75,000				75,000
Available-for-sale Securities, Amortized Cost Basis			74,990				74,990
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax			0				0
Available-for-sale Securities, Unrealized Losses			(11)				(11)
Available-for-sale Securities, Fair Value Disclosure			74,979				74,979
Floating Interest Rate [Member] | Trading Securities [Member] | Asset-backed Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	0		0		0		0
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	0		0		0		0
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | Asset-backed Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	(253)		(74)		(253)		(74)
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | Corporate Debt Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	0		0		0		0
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	3,515		3,431		3,515		3,431
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | GSE Subordinated Debt [Member}
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	0		0		0		0
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | Taxable Municipal Bonds [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)			(10)				(10)
Fixed Interest Rate [Member] | Corporate Debt Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	10,000		30,000		10,000		30,000
Available-for-sale Securities, Amortized Cost Basis	9,999		29,990		9,999		29,990
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0		35		0		35
Available-for-sale Securities, Unrealized Losses	(5)		0		(5)		0
Available-for-sale Securities, Fair Value Disclosure	9,994		30,025		9,994		30,025
Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	692		853	[1]	692		853	[1]
Available-for-sale Securities, Amortized Cost Basis	3,809		4,395	[1]	3,809		4,395	[1]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	4,095		3,807	[1]	4,095		3,807	[1]
Available-for-sale Securities, Unrealized Losses	0		0	[1]	0		0	[1]
Available-for-sale Securities, Fair Value Disclosure	7,904	[2]	8,202	[1]	7,904	[2]	8,202	[1]
Derivative, Notional Amount	148,500		152,400		148,500		152,400
Fixed Interest Rate [Member] | USDA Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	214,000		18,806		214,000		18,806
Available-for-sale Securities, Amortized Cost Basis	213,975		18,936		213,975		18,936
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	12		3		12		3
Available-for-sale Securities, Unrealized Losses	0		0		0		0
Available-for-sale Securities, Fair Value Disclosure	213,987		18,939		213,987		18,939
Fixed Interest Rate [Member] | US Treasury Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	993,680		975,194		993,680		975,194
Available-for-sale Securities, Amortized Cost Basis	993,263		975,656		993,263		975,656
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	2		72		2		72
Available-for-sale Securities, Unrealized Losses	(477)		(16)		(477)		(16)
Available-for-sale Securities, Fair Value Disclosure	992,788		975,712		992,788		975,712
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Corporate Debt Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	(1)		(10)		(1)		(10)
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	3,117		3,542	[1]	3,117		3,542	[1]
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | USDA Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	(25)		130		(25)		130
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | US Treasury Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	(417)		462		(417)		462
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Interest-Only-Strip [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	$ 7,200		$ 7,300		$ 7,200		$ 7,300

[1] Fair value includes $7.3 million of an interest-only security with a notional amount of $152.4 million.

[2] Fair value includes $7.2 million of an interest-only security with a notional amount of $148.5 million.