Accounting Policies - Components of Other Comprehensive Income Loss and Related Tax Impact (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, before Tax	$ 3,536		$ 4,225		$ 31,863		$ 57,610
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	1,238		1,479		11,152		20,164
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	2,298		2,746		20,711		37,446
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	2,201		3,273		19,189		32,310
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	400		500		1,400		1,500
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(3,834)		(2,093)		(12,152)		(7,727)
Other Comprehensive Income (Loss), before Tax	(2,512)		1,815		10,826		37,821
Other Comprehensive Income (Loss), Tax	(879)		635		3,789		13,238
Other comprehensive income, net of tax	(1,633)		1,180		7,037		24,583
Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, before Tax	(150)		810		(2,342)		(7,901)
Other Comprehensive Income (Loss) before Reclassifications, Tax	(53)		283		(820)		(2,765)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(97)		527		(1,522)		(5,136)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	262		342		894		974
Other Comprehensive Income (Loss), before Tax	253		1,336		(966)		(6,403)
Other Comprehensive Income (Loss), Tax	88		467		(338)		(2,241)
Other comprehensive income, net of tax	165		869		(628)		(4,162)
Available-for-sale Securities [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	2,298		2,746		20,711		37,446
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(2,875)		(2,388)		(8,177)		(7,349)
Other Comprehensive Income (Loss), before Tax	(886)		552		19,283		46,305
Other Comprehensive Income (Loss), Tax	(309)		194		6,749		16,208
Other comprehensive income, net of tax	(577)		358		12,534		30,097
Held-to-maturity Securities [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0		0		0		0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(1,221)		(47)		(4,869)		(1,352)
Other Comprehensive Income (Loss), before Tax	(1,879)		(73)		(7,491)		(2,081)
Other Comprehensive Income (Loss), Tax	(658)		(26)		(2,622)		(729)
Other comprehensive income, net of tax	(1,221)		(47)		(4,869)		(1,352)
Gain (Loss) on Derivative Instruments [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Available-for-sale Securities [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(4,326)	[1]	(3,652)	[1]	(12,470)	[2]	(11,591)	[2]
Reclassification from AOCI, Current Period, Tax	(1,514)	[1]	(1,278)	[1]	(4,365)	[2]	(4,056)	[2]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(2,812)	[1]	(2,374)	[1]	(8,105)	[2]	(7,535)	[2]
Gain (Loss) on Investments [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Available-for-sale Securities [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(89)		0		(89)	[3]	9	[3]
Reclassification from AOCI, Current Period, Tax	(31)		0		(31)	[3]	3	[3]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(58)	[4]	0	[4]	(58)	[3]	6	[3]
Other Income [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Available-for-sale Securities [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(7)	[4]	(21)	[4]	(21)	[5]	277	[5]
Reclassification from AOCI, Current Period, Tax	(2)	[4]	(7)	[4]	(7)	[5]	97	[5]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(5)	[4]	(14)	[4]	(14)	[5]	180	[5]
Interest Income [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Held-to-maturity Securities [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(1,879)	[6]	(73)	[6]	(7,491)	[7]	(2,081)	[7]
Reclassification from AOCI, Current Period, Tax	(658)	[6]	(26)	[6]	(2,622)	[7]	(729)	[7]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(1,221)	[6]	(47)	[6]	(4,869)	[7]	(1,352)	[7]
Interest Expense [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	403	[8]	526	[8]	1,376	[9]	1,498	[9]
Reclassification from AOCI, Current Period, Tax	141	[8]	184	[8]	482	[9]	524	[9]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	$ 262	[8]	$ 342	[8]	$ 894	[9]	$ 974	[9]

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

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

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

[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 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.

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

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