Fair Value Disclosures - Fair Value Measurements, Recurring and Nonrecurring (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	$ 8,011,000		$ 8,011,000			$ 7,093,000
Derivative Liability	(4,776,000)		(4,776,000)			1,673,000
Loans and Leases Receivable, Net Amount	5,352,584,000		5,352,584,000			5,266,786,000
Real estate owned, at lower of cost or fair value	56,000		56,000			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	8,011,000		8,011,000			7,093,000
Assets, Fair Value Disclosure	8,329,163,000		8,329,163,000			7,707,927,000
Derivative Liability	20,164,000		20,164,000			26,599,000
Financial and Nonfinancial Liabilities, Fair Value Disclosure	20,164,000		20,164,000			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	905,703,000		905,703,000			767,424,000
Derivative Liability	123,000		123,000			36,000
Financial and Nonfinancial Liabilities, Fair Value Disclosure	123,000		123,000			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	8,011,000		8,011,000			7,093,000
Assets, Fair Value Disclosure	1,407,896,000		1,407,896,000			1,431,927,000
Derivative Liability	20,041,000		20,041,000			26,563,000
Financial and Nonfinancial Liabilities, Fair Value Disclosure	20,041,000		20,041,000			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	(12,931,000)	$ 9,550,000	(31,863,000)	$ 6,253,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	2,945,000	1,261,000	23,792,000	14,501,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	348,531,000	247,794,000	1,038,271,000	819,910,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	45,014,000	53,506,000	79,307,000	86,095,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	152,042,000	167,665,000	444,392,000	331,388,000
Derivative Asset	0		0			0
Assets, Fair Value Disclosure	6,015,564,000		6,015,564,000			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	6,015,564,000	5,322,025,000	6,015,564,000	5,322,025,000	$ 5,874,075,000	5,508,576,000	$ 5,284,591,000	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	343,000		343,000			508,000
Loans and Leases Receivable, Net Amount	343,000		343,000			508,000
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
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
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	343,000		343,000			508,000
Loans and Leases Receivable, Net Amount	343,000		343,000			508,000
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	(10,850,000)	9,552,000	(29,798,000)	6,337,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	2,945,000	924,000	23,700,000	15,880,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	303,517,000	194,288,000	958,964,000	733,815,000
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	149,193,000	165,248,000	439,461,000	327,155,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	5,985,806,000	5,282,562,000	5,985,806,000	5,282,562,000	5,839,387,000	5,471,914,000	5,243,046,000	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	5,985,806,000		5,985,806,000			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	(10,850,000)	9,552,000	(29,798,000)	6,337,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	2,945,000	924,000	23,700,000	15,880,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	303,517,000	194,288,000	958,964,000	733,815,000
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	149,193,000	165,248,000	439,461,000	327,155,000
Investments, Fair Value Disclosure	5,985,806,000		5,985,806,000			5,471,914,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	5,985,806,000	5,282,562,000	5,985,806,000	5,282,562,000	5,839,387,000	5,471,914,000	5,243,046,000	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	5,985,806,000		5,985,806,000			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	5,985,806,000		5,985,806,000			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	2,324,598,000		2,324,598,000
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	905,703,000		905,703,000
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	1,399,885,000		1,399,885,000
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	(2,092,000)	0	(2,092,000)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	337,000	92,000	(1,379,000)
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	2,028,000	111,000	2,137,000	223,000
Investments, Fair Value Disclosure	19,010,000		19,010,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	19,010,000	23,169,000	19,010,000	23,169,000	23,130,000	23,147,000	22,943,000	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	10,748,000		10,748,000			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	11,000	100,000	100,000	7,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	11,000	(2,000)	27,000	(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	45,014,000	53,506,000	79,307,000	86,095,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	45,014,000	53,506,000	79,307,000	86,095,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	821,000	2,306,000	2,794,000	4,010,000
Investments, Fair Value Disclosure	10,748,000		10,748,000			13,515,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	10,748,000	16,294,000	10,748,000	16,294,000	11,558,000	13,515,000	18,602,000	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	45,014,000	53,506,000	79,307,000	86,095,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	45,014,000	53,506,000	79,307,000	86,095,000
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	10,748,000		10,748,000			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	11,000	(2,000)	27,000	(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	821,000	2,306,000	2,794,000	4,010,000
Investments, Fair Value Disclosure	10,748,000		10,748,000			13,515,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	10,748,000	16,294,000	10,748,000	16,294,000	11,558,000	13,515,000	18,602,000	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	19,010,000		19,010,000			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)	0	394,000	196,000	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	19,010,000		19,010,000			18,814,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	19,010,000	18,518,000	19,010,000	18,518,000	19,010,000	18,814,000	18,124,000	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	31,321,000		31,321,000			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	31,321,000		31,321,000			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	1,368,125,000		1,368,125,000			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	1,368,125,000		1,368,125,000			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	439,000		439,000			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	439,000		439,000			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	(2,092,000)	0	(2,092,000)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	(57,000)	(104,000)	(2,167,000)
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	2,028,000	111,000	2,137,000	223,000
Investments, Fair Value Disclosure	0		0			4,333,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	0	$ 4,651,000	0	$ 4,651,000	$ 4,120,000	4,333,000	$ 4,819,000	$ 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	905,703,000		905,703,000			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	905,703,000		905,703,000			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