Investment Securities - Schedule of Investment Securities (Details) - USD ($)					3 Months Ended			6 Months Ended
	Jun. 30, 2015		Dec. 31, 2014		Jun. 30, 2015		Jun. 30, 2014	Jun. 30, 2015		Jun. 30, 2014
Available-for-sale Securities [Abstract]
Derivative, Notional Amount	$ 7,542,090,000		$ 6,837,571,000		$ 7,542,090,000			$ 7,542,090,000
Investment Securities [Member]
Schedule of Available-for-sale and Trading Securities [Line Items]
Proceeds from sale of available-for-sale investment securities					0		$ 720,500,000	74,998,000		$ 730,527,000
Available-for-sale Securities, Gross Realized Gains					0		$ 100,000	6,000		$ 200,000
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	1,847,737,000		1,932,236,000		1,847,737,000			1,847,737,000
Unamortized Premium/(Discount)	6,506,000		7,471,000		6,506,000			6,506,000
Available-for-sale Securities, Amortized Cost Basis	1,854,243,000		1,939,707,000		1,854,243,000			1,854,243,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	8,506,000		9,003,000		8,506,000			8,506,000
Available-for-sale Securities, Unrealized Losses	(10,850,000)		(10,211,000)		(10,850,000)			(10,850,000)
Available-for-sale Securities, Fair Value Disclosure	1,851,899,000		1,938,499,000		1,851,899,000			1,851,899,000
Trading Securities [Abstract]
Trading, at fair value	606,000		689,000		606,000			606,000
Marketable Securities [Abstract]
Marketable Securities Unpaid Principal Balance	1,850,168,000		1,935,104,000		1,850,168,000			1,850,168,000
Marketable Securities, Amortized Cost	1,856,674,000		1,942,575,000		1,856,674,000			1,856,674,000
Marketable Securities, Unrealized Gains	8,506,000		9,003,000		8,506,000			8,506,000
Marketable Securities, Unrealized Losses	(12,675,000)		(12,390,000)		(12,675,000)			(12,675,000)
Marketable Securities	1,852,505,000		1,939,188,000		1,852,505,000			1,852,505,000
USDA Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Fair Value Disclosure	1,825,406,000		1,731,222,000		1,825,406,000			1,825,406,000
Trading Securities [Abstract]
Trading, at fair value	33,770,000		40,310,000		33,770,000			33,770,000
Marketable Securities [Abstract]
Marketable Securities	1,859,176,000		1,771,532,000		1,859,176,000			1,859,176,000
Available-for-sale Securities [Member] | Investment Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	6,506,000		7,471,000		6,506,000			6,506,000
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	46,600,000		46,600,000		46,600,000			46,600,000
Available-for-sale Securities, Amortized Cost Basis	46,600,000		46,600,000		46,600,000			46,600,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0		0		0			0
Available-for-sale Securities, Unrealized Losses	(6,418,000)		(6,024,000)		(6,418,000)			(6,418,000)
Available-for-sale Securities, Fair Value Disclosure	40,182,000		40,576,000		40,182,000			40,182,000
Floating Interest Rate [Member] | Asset-backed Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	91,382,000		100,730,000		91,382,000			91,382,000
Available-for-sale Securities, Amortized Cost Basis	91,065,000		100,656,000		91,065,000			91,065,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	182,000		283,000		182,000			182,000
Available-for-sale Securities, Unrealized Losses	(69,000)		(37,000)		(69,000)			(69,000)
Available-for-sale Securities, Fair Value Disclosure	91,178,000		100,902,000		91,178,000			91,178,000
Trading Securities [Abstract]
Trading Securities - Amount Outstanding	2,431,000		2,868,000		2,431,000			2,431,000
Trading Securities, Debt, Amortized Cost	2,431,000		2,868,000		2,431,000			2,431,000
Trading Securities, Unrealized Gains	0		0
Trading Securities, Unrealized Losses	(1,825,000)		(2,179,000)
Trading, at fair value	606,000		689,000		606,000			606,000
Floating Interest Rate [Member] | Corporate Debt Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	10,000,000		10,000,000		10,000,000			10,000,000
Available-for-sale Securities, Amortized Cost Basis	10,000,000		10,000,000		10,000,000			10,000,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	62,000		91,000		62,000			62,000
Available-for-sale Securities, Unrealized Losses	(2,000)		0		(2,000)			(2,000)
Available-for-sale Securities, Fair Value Disclosure	10,060,000		10,091,000		10,060,000			10,060,000
Floating Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	703,990,000		605,053,000		703,990,000			703,990,000
Available-for-sale Securities, Amortized Cost Basis	707,255,000		608,484,000		707,255,000			707,255,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	4,111,000		4,712,000		4,111,000			4,111,000
Available-for-sale Securities, Unrealized Losses	(306,000)		(443,000)		(306,000)			(306,000)
Available-for-sale Securities, Fair Value Disclosure	711,060,000		612,753,000		711,060,000			711,060,000
Floating Interest Rate [Member] | GSE Subordinated Debt [Member}
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	70,000,000		70,000,000		70,000,000			70,000,000
Available-for-sale Securities, Amortized Cost Basis	70,000,000		70,000,000		70,000,000			70,000,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0		0		0			0
Available-for-sale Securities, Unrealized Losses	(4,028,000)		(3,680,000)		(4,028,000)			(4,028,000)
Available-for-sale Securities, Fair Value Disclosure	65,972,000		66,320,000		65,972,000			65,972,000
Floating Interest Rate [Member] | Taxable Municipal Bonds [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance			75,000,000
Available-for-sale Securities, Amortized Cost Basis			74,990,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax			0
Available-for-sale Securities, Unrealized Losses			(11,000)
Available-for-sale Securities, Fair Value Disclosure			74,979,000
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)	(317,000)		(74,000)		(317,000)			(317,000)
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)	3,265,000		3,431,000		3,265,000			3,265,000
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)			(10,000)
Fixed Interest Rate [Member] | Corporate Debt Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	15,000,000		30,000,000		15,000,000			15,000,000
Available-for-sale Securities, Amortized Cost Basis	14,997,000		29,990,000		14,997,000			14,997,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	4,000		35,000		4,000			4,000
Available-for-sale Securities, Unrealized Losses	(16,000)		0		(16,000)			(16,000)
Available-for-sale Securities, Fair Value Disclosure	14,985,000		30,025,000		14,985,000			14,985,000
Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	765,000		853,000	[1]	765,000			765,000
Available-for-sale Securities, Amortized Cost Basis	4,095,000		4,395,000	[1]	4,095,000			4,095,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	3,952,000		3,807,000	[1]	3,952,000			3,952,000
Available-for-sale Securities, Unrealized Losses	0		0	[1]	0			0
Available-for-sale Securities, Fair Value Disclosure	8,047,000	[2]	8,202,000	[1]	8,047,000	[2]		8,047,000	[2]
Derivative, Notional Amount	152,400,000		152,400,000		152,400,000			152,400,000
Fixed Interest Rate [Member] | USDA Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	297,806,000		18,806,000		297,806,000			297,806,000
Available-for-sale Securities, Amortized Cost Basis	297,777,000		18,936,000		297,777,000			297,777,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	68,000		3,000		68,000			68,000
Available-for-sale Securities, Unrealized Losses	0		0		0			0
Available-for-sale Securities, Fair Value Disclosure	297,845,000		18,939,000		297,845,000			297,845,000
Fixed Interest Rate [Member] | US Treasury Securities [Member]
Available-for-sale Securities [Abstract]
Available-for-Sale Securities Unpaid Principal Balance	612,194,000		975,194,000		612,194,000			612,194,000
Available-for-sale Securities, Amortized Cost Basis	612,454,000		975,656,000		612,454,000			612,454,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	127,000		72,000		127,000			127,000
Available-for-sale Securities, Unrealized Losses	(11,000)		(16,000)		(11,000)			(11,000)
Available-for-sale Securities, Fair Value Disclosure	612,570,000		975,712,000		612,570,000			612,570,000
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Corporate Debt Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	(3,000)		(10,000)		(3,000)			(3,000)
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	3,330,000		3,542,000	[1]	3,330,000			3,330,000
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | USDA Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	(29,000)		130,000		(29,000)			(29,000)
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | US Treasury Securities [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	260,000		462,000		260,000			260,000
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Interest-Only-Strip [Member]
Available-for-sale Securities [Abstract]
Unamortized Premium/(Discount)	$ 7,200,000		$ 7,300,000		$ 7,200,000			$ 7,200,000

[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.