Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands					3 Months Ended			9 Months Ended
	Sep. 30, 2015		Dec. 31, 2014		Sep. 30, 2015		Sep. 30, 2014	Sep. 30, 2015		Sep. 30, 2014
Available-for-sale Securities [Abstract]
Derivative, Notional Amount	$ 8,052,133		$ 6,837,571		$ 8,052,133			$ 8,052,133
Investment Securities [Member]
Schedule of Available-for-sale and Trading Securities [Line Items]
Proceeds from sale of available-for-sale investment securities					8,700		$ 39,700	83,735		$ 770,149
Available-for-sale Securities, Gross Realized Gains					100		$ 100	100		$ 600
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	2,024,185		1,932,236		2,024,185			2,024,185
Unamortized Premium/(Discount)	6,679		7,471		6,679			6,679
Available-for-sale Securities, Amortized Cost Basis	2,030,864		1,939,707		2,030,864			2,030,864
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	7,916		9,003		7,916			7,916
Available-for-sale Securities, Unrealized Losses	(7,151)		(10,211)		(7,151)			(7,151)
Available-for-sale Securities, Fair Value Disclosure	2,031,629		1,938,499		2,031,629			2,031,629
Trading Securities [Abstract]
Trading, at fair value	550		689		550			550
Marketable Securities [Abstract]
Marketable Securities Unpaid Principal Balance	2,026,510		1,935,104		2,026,510			2,026,510
Marketable Securities, Amortized Cost	2,033,189		1,942,575		2,033,189			2,033,189
Marketable Securities, Unrealized Gains	7,916		9,003		7,916			7,916
Marketable Securities, Unrealized Losses	(8,926)		(12,390)		(8,926)			(8,926)
Marketable Securities	$ 2,032,179		1,939,188		$ 2,032,179			$ 2,032,179
Floating rate auction-rate certificates backed by Government guaranteed student loans
Marketable Securities [Abstract]
Other-Than-Temporary Impairment Loss Sales Price, As Percent of Par	99.63%				99.63%			99.63%
USDA Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Fair Value Disclosure	$ 1,854,422		1,731,222		$ 1,854,422			$ 1,854,422
Trading Securities [Abstract]
Trading, at fair value	31,936		40,310		31,936			31,936
Marketable Securities [Abstract]
Marketable Securities	1,886,358		1,771,532		1,886,358			1,886,358
Available-for-sale Securities [Member] | Investment Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	6,679		7,471		6,679			6,679
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			46,500
Available-for-sale Securities, Amortized Cost Basis	46,500		46,600		46,500			46,500
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)			(1,576)
Available-for-sale Securities, Fair Value Disclosure	44,924		40,576		44,924			44,924
Marketable Securities [Abstract]
Other than Temporary Impairment Losses, Investments					100			100
Floating Interest Rate [Member] | Asset-backed Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	81,868		100,730		81,868			81,868
Available-for-sale Securities, Amortized Cost Basis	81,582		100,656		81,582			81,582
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	44		283		44			44
Available-for-sale Securities, Unrealized Losses	(508)		(37)		(508)			(508)
Available-for-sale Securities, Fair Value Disclosure	81,118		100,902		81,118			81,118
Trading Securities [Abstract]
Trading Securities - Amount Outstanding	2,325		2,868		2,325			2,325
Trading Securities, Debt, Amortized Cost	2,325		2,868		2,325			2,325
Trading Securities, Unrealized Gains	0		0
Trading Securities, Unrealized Losses	(1,775)		(2,179)
Trading, at fair value	550		689		550			550
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			0
Available-for-sale Securities, Unrealized Losses	(9)		0		(9)			(9)
Available-for-sale Securities, Fair Value Disclosure	9,991		10,091		9,991			9,991
Floating Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	856,089		605,053		856,089			856,089
Available-for-sale Securities, Amortized Cost Basis	859,806		608,484		859,806			859,806
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	3,588		4,712		3,588			3,588
Available-for-sale Securities, Unrealized Losses	(1,170)		(443)		(1,170)			(1,170)
Available-for-sale Securities, Fair Value Disclosure	862,224		612,753		862,224			862,224
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	(3,888)		(3,680)		(3,888)			(3,888)
Available-for-sale Securities, Fair Value Disclosure	66,112		66,320		66,112			66,112
Floating Interest Rate [Member] | Taxable Municipal Bonds [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance			75,000
Available-for-sale Securities, Amortized Cost Basis			74,990
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax			0
Available-for-sale Securities, Unrealized Losses			(11)
Available-for-sale Securities, Fair Value Disclosure			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)	(286)		(74)		(286)			(286)
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)	3,717		3,431		3,717			3,717
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)
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			10,000
Available-for-sale Securities, Amortized Cost Basis	9,998		29,990		9,998			9,998
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	7		35		7			7
Available-for-sale Securities, Unrealized Losses	0		0		0			0
Available-for-sale Securities, Fair Value Disclosure	10,005		30,025		10,005			10,005
Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	728		853	[1]	728			728
Available-for-sale Securities, Amortized Cost Basis	3,951		4,395	[1]	3,951			3,951
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	4,022		3,807	[1]	4,022			4,022
Available-for-sale Securities, Unrealized Losses	0		0	[1]	0			0
Available-for-sale Securities, Fair Value Disclosure	7,973	[2]	8,202	[1]	7,973	[2]		7,973	[2]
Derivative, Notional Amount	152,400		152,400		152,400			152,400
Fixed Interest Rate [Member] | USDA Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	380,806		18,806		380,806			380,806
Available-for-sale Securities, Amortized Cost Basis	380,703		18,936		380,703			380,703
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	83		3		83			83
Available-for-sale Securities, Unrealized Losses	0		0		0			0
Available-for-sale Securities, Fair Value Disclosure	380,786		18,939		380,786			380,786
Fixed Interest Rate [Member] | US Treasury Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	568,194		975,194		568,194			568,194
Available-for-sale Securities, Amortized Cost Basis	568,324		975,656		568,324			568,324
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	172		72		172			172
Available-for-sale Securities, Unrealized Losses	0		(16)		0			0
Available-for-sale Securities, Fair Value Disclosure	568,496		975,712		568,496			568,496
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Corporate Debt Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	(2)		(10)		(2)			(2)
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	3,223		3,542	[1]	3,223			3,223
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | USDA Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	(103)		130		(103)			(103)
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | US Treasury Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	130		462		130			130
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			$ 7,200

[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 $152.4 million.