Accounting Policies - Components of Other Comprehensive Income Loss and Related Tax Impact (Details) - USD ($) $ in Thousands		3 Months Ended				9 Months Ended
		Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2016		Sep. 30, 2015
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		$ 4,225		$ (51,373)		$ 57,610		$ (24,594)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax		1,479		(17,981)		20,164		(8,609)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax		2,746		(33,392)		37,446		(15,985)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		3,273		(35,739)		32,310		(17,799)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		(500)		(400)		(1,500)		(800)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(2,093)		(4,808)		(7,727)		(15,548)
Other Comprehensive Income (Loss), before Tax		1,815		(62,380)		37,821		(51,305)
Other Comprehensive Income (Loss), Tax		635		(21,833)		13,238		(17,958)
Other comprehensive income, net of tax		1,180		(40,547)		24,583		(33,347)
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		810		(3,611)		(7,901)		(2,791)
Other Comprehensive Income (Loss) before Reclassifications, Tax		283		(1,264)		(2,765)		(977)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		527		(2,347)		(5,136)		(1,814)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		342		270		974		506
Other Comprehensive Income (Loss), before Tax		1,336		(3,195)		(6,403)		(2,012)
Other Comprehensive Income (Loss), Tax		467		(1,118)		(2,241)		(704)
Other comprehensive income, net of tax		869		(2,077)		(4,162)		(1,308)
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		2,746		(33,392)		37,446		(15,985)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(2,388)		(3,624)		(7,349)		(10,249)
Other Comprehensive Income (Loss), before Tax		552		(56,949)		46,305		(40,363)
Other Comprehensive Income (Loss), Tax		194		(19,933)		16,208		(14,129)
Other comprehensive income, net of tax		358		(37,016)		30,097		(26,234)
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		(47)		(1,454)		(1,352)		(5,805)
Other Comprehensive Income (Loss), before Tax		(73)		(2,236)		(2,081)		(8,930)
Other Comprehensive Income (Loss), Tax		(26)		(782)		(729)		(3,125)
Other comprehensive income, net of tax		(47)		(1,454)		(1,352)		(5,805)
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	[1]	0		(4)
Reclassification from AOCI, Current Period, Tax	[1]	0		(2)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]	0		(2)
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		(3,652)	[2]	(5,038)	[2]	(11,591)	[3]	(14,852)	[3]
Reclassification from AOCI, Current Period, Tax		(1,278)	[2]	(1,763)	[2]	(4,056)	[3]	(5,198)	[3]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(2,374)	[2]	(3,275)	[2]	(7,535)	[3]	(9,654)	[3]
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	[4]					9		(10)
Reclassification from AOCI, Current Period, Tax	[4]					3		(5)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[4]					6		(5)
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		(21)	[5]	(534)	[5]	277	[6]	(907)	[6]
Reclassification from AOCI, Current Period, Tax		(7)	[5]	(187)	[5]	97	[6]	(317)	[6]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(14)	[5]	(347)	[5]	180	[6]	(590)	[6]
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		(73)	[7]	(2,236)	[7]	(2,081)	[8]	(8,930)	[8]
Reclassification from AOCI, Current Period, Tax		(26)	[7]	(782)	[7]	(729)	[8]	(3,125)	[8]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(47)	[7]	(1,454)	[7]	(1,352)	[8]	(5,805)	[8]
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		526	[9]	416	[9]	1,498	[10]	779	[10]
Reclassification from AOCI, Current Period, Tax		184	[9]	146	[9]	524	[10]	273	[10]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		$ 342	[9]	$ 270	[9]	$ 974	[10]	$ 506	[10]

[1] Represents unrealized gains and losses on sales of available-for-sale investment securities.

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

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

[4] (2) Represents unrealized gains and losses on sales of available-for-sale investment securities.

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

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

[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 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.

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

[10] (5) Relates to the recognition of unrealized gains and losses on cash flow hedges recorded in AOCI.