Accounting Policies - Changes in Accumulated Other Comprehensive Income, Net of Tax (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, before Tax		$ 9,525		$ 56,107		$ 28,328		$ 53,385
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOCI Beginning Balance		41,544		(18,917)		33,758		(11,019)
Other comprehensive income/(loss) before reclassifications		4,691		34,202		16,987		29,038
Amounts reclassified from AOCI		(3,807)		(2,901)		(8,317)		(5,635)
Net other comprehensive income/(loss)		884		31,301		8,670		23,403
AOCI Ending Balance		42,428		12,384		42,428		12,384
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax		3,334		19,638		9,915		18,685
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax		6,191		36,469		18,413		34,700
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		500		500		1,000		1,000
Other Comprehensive Income (Loss), before Tax		1,360		48,157		13,339		36,006
Other Comprehensive Income (Loss), Tax		476		16,856		4,669		12,603
Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] | Available-for-sale Securities [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOCI Beginning Balance		(4,742)		(14,180)		(14,387)		(10,035)
Other comprehensive income/(loss) before reclassifications		6,191		36,469		18,413		34,700
Amounts reclassified from AOCI		(2,725)		(2,585)		(5,302)		(4,961)
Net other comprehensive income/(loss)		3,466		33,884		13,111		29,739
AOCI Ending Balance		(1,276)		19,704		(1,276)		19,704
Other Comprehensive Income (Loss), before Tax		5,333		52,130		20,170		45,753
Other Comprehensive Income (Loss), Tax		1,867		18,246		7,059		16,014
Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] | Held-to-maturity Securities [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOCI Beginning Balance		43,485		(1,133)		45,752		(476)
Other comprehensive income/(loss) before reclassifications		0		0		0		0
Amounts reclassified from AOCI		(1,381)		(648)		(3,648)		(1,305)
Net other comprehensive income/(loss)		(1,381)		(648)		(3,648)		(1,305)
AOCI Ending Balance		42,104		(1,781)		42,104		(1,781)
Other Comprehensive Income (Loss), before Tax		(2,125)		(997)		(5,612)		(2,008)
Other Comprehensive Income (Loss), Tax		(744)		(349)		(1,964)		(703)
Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOCI Beginning Balance		2,801		(3,604)		2,393		(508)
Other comprehensive income/(loss) before reclassifications		(1,500)		(2,267)		(1,426)		(5,662)
Amounts reclassified from AOCI		299		332		633		631
Net other comprehensive income/(loss)		(1,201)		(1,935)		(793)		(5,031)
AOCI Ending Balance		1,600		(5,539)		1,600		(5,539)
Other Comprehensive Income (Loss), before Tax		(1,848)		(2,976)		(1,219)		(7,739)
Other Comprehensive Income (Loss), Tax		(647)		(1,041)		(426)		(2,708)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		(2,309)		(3,488)		(2,192)		(8,710)
Other Comprehensive Income (Loss) before Reclassifications, Tax		(809)		(1,221)		(766)		(3,048)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Gain (Loss) on Derivative Instruments [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] | Available-for-sale Securities [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amounts reclassified from AOCI		(2,721)	[1]	(2,611)	[1]	(5,294)	[2]	(5,161)	[2]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		(4,186)	[1]	(4,016)	[1]	(8,145)	[2]	(7,939)	[2]
Reclassification from AOCI, Current Period, Tax		(1,465)	[1]	(1,405)	[1]	(2,851)	[2]	(2,778)	[2]
Reclassification out of Accumulated Other Comprehensive Income [Member] | Other Income [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] | Available-for-sale Securities [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amounts reclassified from AOCI		(4)	[3]	26	[3]	(8)	[4]	194	[4]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		(6)	[3]	39	[3]	(13)	[4]	298	[4]
Reclassification from AOCI, Current Period, Tax		(2)	[3]	13	[3]	(5)	[4]	104	[4]
Reclassification out of Accumulated Other Comprehensive Income [Member] | Interest Income [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] | Held-to-maturity Securities [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amounts reclassified from AOCI		(1,381)	[5]	(648)	[5]	(3,648)	[6]	(1,305)	[6]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		(2,125)	[5]	(997)	[5]	(5,612)	[6]	(2,008)	[6]
Reclassification from AOCI, Current Period, Tax		(744)	[5]	(349)	[5]	(1,964)	[6]	(703)	[6]
Reclassification out of Accumulated Other Comprehensive Income [Member] | Interest Expense [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amounts reclassified from AOCI		299	[7]	332	[7]	633	[8]	631	[8]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		461	[7]	512	[7]	973	[8]	971	[8]
Reclassification from AOCI, Current Period, Tax		$ 162	[7]	$ 180	[7]	340	[8]	340	[8]
Reclassification out of Accumulated Other Comprehensive Income [Member] | Gain (Loss) on Investments [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] | Available-for-sale Securities [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amounts reclassified from AOCI	[9]					0		6
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[9]					0		9
Reclassification from AOCI, Current Period, Tax	[9]					$ 0		$ 3

[1] (1) Relates to the amortization of unrealized gains on hedged items prior to the application of fair value hedge accounting.

[2] Relates to the amortization of unrealized gains on hedged items prior to the application of fair value hedge accounting.

[3] Represents amortization of deferred gains related to certain available-for-sale USDA Securities and Farmer Mac Guaranteed USDA Securities.

[4] Represents amortization of deferred gains related to certain available-for-sale USDA Securities and Farmer Mac Guaranteed USDA Securities.

[5] Relates to the amortization of unrealized gains or losses prior to the reclassification of these securities from available-for-sale to held-to-maturity. The amortization of unrealized gains or losses reported in AOCI for held-to-maturity securities will be offset by the amortization of the premium or discount created from the transfer into held-to-maturity securities, which occurred at fair value. These unrealized gains or losses will be recorded over the remaining life of the security with no impact on future net income.

[6] Relates to the amortization of unrealized gains or losses prior to the reclassification of these securities from available-for-sale to held-to-maturity. The amortization of unrealized gains or losses reported in AOCI for held-to-maturity securities will be offset by the amortization of the premium or discount created from the transfer into held-to-maturity securities, which occurred at fair value. These unrealized gains or losses will be recorded over the remaining life of the security with no impact on future net income.

[7] Relates to the recognition of unrealized gains and losses on cash flow hedges recorded in AOCI.

[8] Relates to the recognition of unrealized gains and losses on cash flow hedges recorded in AOCI.

[9] Represents unrealized losses on sales of available-for-sale investment securities.