Fair Value Disclosures - Fair Value Measurements, Recurring and Nonrecurring (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	$ 8,007,000		$ 8,007,000			$ 7,093,000
Derivative Liability	(29,000,000)		(29,000,000)			1,673,000
Loans and Leases Receivable, Net Amount	5,360,900,000		5,360,900,000			5,266,786,000
Real estate owned, at lower of cost or fair value	128,000		128,000			139,000
Cumulative Effect of New Accounting Principle in Period of Adoption						27,000		$ 0
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	8,007,000		8,007,000			7,093,000
Assets, Fair Value Disclosure	8,199,550,000		8,199,550,000			7,707,927,000
Derivative Liability	17,841,000		17,841,000			26,599,000
Financial and Nonfinancial Liabilities, Fair Value Disclosure	17,841,000		17,841,000			26,599,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0		0			0
Assets, Fair Value Disclosure	838,089,000		838,089,000			767,424,000
Derivative Liability	10,000		10,000			36,000
Financial and Nonfinancial Liabilities, Fair Value Disclosure	10,000		10,000			36,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	8,007,000		8,007,000			7,093,000
Assets, Fair Value Disclosure	1,374,959,000		1,374,959,000			1,431,927,000
Derivative Liability	17,831,000		17,831,000			26,563,000
Financial and Nonfinancial Liabilities, Fair Value Disclosure	17,831,000		17,831,000			26,563,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(13,435,000)	$ (1,171,000)	(45,298,000)	$ 5,082,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(13,287,000)	(1,676,000)	10,505,000	12,825,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	713,702,000	234,644,000	1,751,973,000	1,054,554,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	26,321,000	40,844,000	105,628,000	126,939,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	689,721,000	31,392,000	1,134,113,000	362,780,000
Derivative Asset	0		0			0
Assets, Fair Value Disclosure	5,986,502,000		5,986,502,000			5,508,576,000
Derivative Liability	0		0			0
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0		0			0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	5,986,502,000	5,481,586,000	5,986,502,000	5,481,586,000	$ 6,015,564,000	5,508,576,000	$ 5,322,025,000	4,891,803,000
Cumulative Effect of New Accounting Principle in Period of Adoption						487,000
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	445,000		445,000			508,000
Loans and Leases Receivable, Net Amount	317,000		317,000			508,000
Real estate owned, at lower of cost or fair value	128,000		128,000
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	0		0			0
Loans and Leases Receivable, Net Amount	0		0			0
Real estate owned, at lower of cost or fair value	0		0
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	0		0			0
Loans and Leases Receivable, Net Amount	0		0			0
Real estate owned, at lower of cost or fair value	0		0
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	445,000		445,000			508,000
Loans and Leases Receivable, Net Amount	317,000		317,000			508,000
Real estate owned, at lower of cost or fair value	128,000		128,000
Farmer Mac Guaranteed Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(13,432,000)	(1,171,000)	(43,230,000)	5,166,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(13,238,000)	(1,594,000)	10,462,000	14,286,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	687,381,000	193,800,000	1,646,345,000	927,615,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	689,213,000	29,851,000	1,128,674,000	357,006,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	5,957,304,000	5,443,746,000	5,957,304,000	5,443,746,000	5,985,806,000	5,471,914,000	5,282,562,000	4,853,685,000
Cumulative Effect of New Accounting Principle in Period of Adoption						487,000
Farmer Mac Guaranteed Securities [Member] | Available-for-sale Securities [Member] | Institutional Credit [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	5,957,304,000		5,957,304,000			5,471,914,000
Farmer Mac Guaranteed Securities [Member] | Available-for-sale Securities [Member] | Institutional Credit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
Farmer Mac Guaranteed Securities [Member] | Available-for-sale Securities [Member] | Institutional Credit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
Farmer Mac Guaranteed Securities [Member] | Available-for-sale Securities [Member] | Institutional Credit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(13,432,000)	(1,171,000)	(43,230,000)	5,166,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(13,238,000)	(1,594,000)	10,462,000	14,286,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	687,381,000	193,800,000	1,646,345,000	927,615,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	689,213,000	29,851,000	1,128,674,000	357,006,000
Investments, Fair Value Disclosure	5,957,304,000		5,957,304,000			5,471,914,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	5,957,304,000	5,443,746,000	5,957,304,000	5,443,746,000	5,985,806,000	5,471,914,000	5,282,562,000	4,853,685,000
Cumulative Effect of New Accounting Principle in Period of Adoption						487,000
Farmer Mac Guaranteed Securities [Member] | Available-for-sale Securities [Member] | USDA Guarantees [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	5,957,304,000		5,957,304,000			5,471,914,000
Farmer Mac Guaranteed Securities [Member] | Available-for-sale Securities [Member] | USDA Guarantees [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
Farmer Mac Guaranteed Securities [Member] | Available-for-sale Securities [Member] | USDA Guarantees [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
Farmer Mac Guaranteed Securities [Member] | Available-for-sale Securities [Member] | USDA Guarantees [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	5,957,304,000		5,957,304,000			5,471,914,000
Investment Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						2,215,405,000
Investment Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						767,424,000
Investment Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						1,424,834,000
Investment Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						23,147,000
Investment Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	2,224,002,000		2,224,002,000
Investment Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	838,089,000		838,089,000
Investment Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	1,366,952,000		1,366,952,000
Investment Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	0	(2,092,000)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(49,000)	(82,000)	43,000	(1,461,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	111,000	2,137,000	334,000
Investments, Fair Value Disclosure	18,961,000		18,961,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	18,961,000	22,976,000	18,961,000	22,976,000	19,010,000	23,147,000	23,169,000	17,730,000
Cumulative Effect of New Accounting Principle in Period of Adoption						0
USDA Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	10,237,000		10,237,000			13,515,000
USDA Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
USDA Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
USDA Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities, Unrealized Gains		34,000	100,000	42,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(3,000)	0	24,000	(84,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	26,321,000	40,844,000	105,628,000	126,939,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	26,321,000	40,844,000	105,628,000	126,939,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	508,000	1,430,000	3,302,000	5,440,000
Investments, Fair Value Disclosure	10,237,000		10,237,000			13,515,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	10,237,000	14,864,000	10,237,000	14,864,000	10,748,000	13,515,000	16,294,000	20,388,000
Cumulative Effect of New Accounting Principle in Period of Adoption						0
USDA Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	26,321,000	40,844,000	105,628,000	126,939,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	26,321,000	40,844,000	105,628,000	126,939,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	0	0	0	0	0	0	0	0
Cumulative Effect of New Accounting Principle in Period of Adoption						0
USDA Securities [Member] | Trading Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	10,237,000		10,237,000			13,515,000
USDA Securities [Member] | Trading Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
USDA Securities [Member] | Trading Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
USDA Securities [Member] | Trading Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(3,000)	0	24,000	(84,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	508,000	1,430,000	3,302,000	5,440,000
Investments, Fair Value Disclosure	10,237,000		10,237,000			13,515,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	10,237,000	14,864,000	10,237,000	14,864,000	10,748,000	13,515,000	16,294,000	20,388,000
Cumulative Effect of New Accounting Principle in Period of Adoption						0
Floating Interest Rate [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	18,961,000		18,961,000			18,814,000
Floating Interest Rate [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
Floating Interest Rate [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
Floating Interest Rate [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(49,000)	0	147,000	788,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	0
Investments, Fair Value Disclosure	18,961,000		18,961,000			18,814,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	18,961,000	18,518,000	18,961,000	18,518,000	$ 19,010,000	18,814,000	18,518,000	17,730,000
Cumulative Effect of New Accounting Principle in Period of Adoption						0
Floating Interest Rate [Member] | Asset-backed Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	30,056,000		30,056,000			34,210,000
Floating Interest Rate [Member] | Asset-backed Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
Floating Interest Rate [Member] | Asset-backed Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	30,056,000		30,056,000			34,210,000
Floating Interest Rate [Member] | Asset-backed Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
Floating Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	1,336,474,000		1,336,474,000			1,290,187,000
Floating Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
Floating Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	1,336,474,000		1,336,474,000			1,290,187,000
Floating Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
Floating Interest Rate [Member] | Senior Agency Debt [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						99,951,000
Floating Interest Rate [Member] | Senior Agency Debt [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						0
Floating Interest Rate [Member] | Senior Agency Debt [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						99,951,000
Floating Interest Rate [Member] | Senior Agency Debt [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						0
Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	422,000		422,000			4,819,000
Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	422,000		422,000			486,000
Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		0	(2,092,000)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		(82,000)	(104,000)	(2,249,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		111,000	2,137,000	334,000
Investments, Fair Value Disclosure	0		0			4,333,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	0	$ 4,458,000	0	$ 4,458,000		4,333,000	$ 4,651,000	$ 0
Cumulative Effect of New Accounting Principle in Period of Adoption						0
Fixed Interest Rate [Member] | US Treasury Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	838,089,000		838,089,000			767,424,000
Fixed Interest Rate [Member] | US Treasury Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	838,089,000		838,089,000			767,424,000
Fixed Interest Rate [Member] | US Treasury Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0		0			0
Fixed Interest Rate [Member] | US Treasury Securities [Member] | Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	$ 0		$ 0			$ 0