Accounting Policies - Components of Other Comprehensive Income Loss and Related Tax Impact (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 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		$ 9,525		$ 56,107		$ 28,328		$ 53,385
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
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		4,691		34,202		16,987		29,038
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		500		500		1,000		1,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(3,807)		(2,901)		(8,317)		(5,635)
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
Other comprehensive income, net of tax		884		31,301		8,670		23,403
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		(2,309)		(3,488)		(2,192)		(8,710)
Other Comprehensive Income (Loss) before Reclassifications, Tax		(809)		(1,221)		(766)		(3,048)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		(1,500)		(2,267)		(1,426)		(5,662)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		299		332		633		631
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, net of tax		(1,201)		(1,935)		(793)		(5,031)
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		6,191		36,469		18,413		34,700
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(2,725)		(2,585)		(5,302)		(4,961)
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
Other comprehensive income, net of tax		3,466		33,884		13,111		29,739
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		(1,381)		(648)		(3,648)		(1,305)
Other Comprehensive Income (Loss), before Tax		(2,125)		(997)		(5,612)		(2,008)
Other Comprehensive Income (Loss), Tax		(744)		(349)		(1,964)		(703)
Other comprehensive income, net of tax		(1,381)		(648)		(3,648)		(1,305)
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		(4,186)	[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 from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(2,721)	[1]	(2,611)	[1]	(5,294)	[2]	(5,161)	[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	[3]					0		9
Reclassification from AOCI, Current Period, Tax	[3]					0		3
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[3]					0		6
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		(6)	[4]	39	[4]	(13)	[5]	298	[5]
Reclassification from AOCI, Current Period, Tax		(2)	[4]	13	[4]	(5)	[5]	104	[5]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(4)	[4]	26	[4]	(8)	[5]	194	[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		(2,125)	[6]	(997)	[6]	(5,612)	[7]	(2,008)	[7]
Reclassification from AOCI, Current Period, Tax		(744)	[6]	(349)	[6]	(1,964)	[7]	(703)	[7]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(1,381)	[6]	(648)	[6]	(3,648)	[7]	(1,305)	[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		461	[8]	512	[8]	973	[9]	971	[9]
Reclassification from AOCI, Current Period, Tax		162	[8]	180	[8]	340	[9]	340	[9]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		$ 299	[8]	$ 332	[8]	$ 633	[9]	$ 631	[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.