Accounting Policies - Components of Other Comprehensive Income Loss and Related Tax Impact (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 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		$ 18,804	$ (2,722)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax		6,581	(953)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax		12,223	(1,769)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		12,299	(5,164)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		500
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(4,513)	(2,734)
Other Comprehensive Income (Loss), before Tax		11,980	(12,151)
Other Comprehensive Income (Loss), Tax		4,194	(4,253)
Other comprehensive income, net of tax		7,786	(7,898)
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		117	(5,222)
Other Comprehensive Income (Loss) before Reclassifications, Tax		41	(1,827)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		76	(3,395)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		332	299
Other Comprehensive Income (Loss), before Tax		629	(4,763)
Other Comprehensive Income (Loss), Tax		221	(1,667)
Other comprehensive income, net of tax		408	(3,096)
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		12,223	(1,769)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(2,578)	(2,376)
Other Comprehensive Income (Loss), before Tax		14,838	(6,377)
Other Comprehensive Income (Loss), Tax		5,193	(2,232)
Other comprehensive income, net of tax		9,645	(4,145)
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
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(2,267)	(657)
Other Comprehensive Income (Loss), before Tax		(3,487)	(1,011)
Other Comprehensive Income (Loss), Tax		(1,220)	(354)
Other comprehensive income, net of tax		(2,267)	(657)
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		(3,959)	(3,923)
Reclassification from AOCI, Current Period, Tax		(1,386)	(1,373)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]	(2,573)	(2,550)
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		0	9
Reclassification from AOCI, Current Period, Tax		0	3
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[2]	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		(7)	259
Reclassification from AOCI, Current Period, Tax		(2)	91
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[3]	(5)	168
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		(3,487)	(1,011)
Reclassification from AOCI, Current Period, Tax		(1,220)	(354)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[4]	(2,267)	(657)
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		512	459
Reclassification from AOCI, Current Period, Tax		180	160
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		$ 332	$ 299

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

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

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

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