Accounting Policies - Changes in Accumulated Other Comprehensive Income, Net of Tax (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, before Tax		$ 18,804	$ (2,722)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOCI Beginning Balance		33,758	(11,019)
Other comprehensive income/(loss) before reclassifications		12,299	(5,164)
Amounts reclassified from AOCI		(4,513)	(2,734)
Net other comprehensive income/(loss)		7,786	(7,898)
AOCI Ending Balance		41,544	(18,917)
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)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		500
Other Comprehensive Income (Loss), before Tax		11,980	(12,151)
Other Comprehensive Income (Loss), Tax		4,194	(4,253)
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		(14,387)	(10,035)
Other comprehensive income/(loss) before reclassifications		12,223	(1,769)
Amounts reclassified from AOCI		(2,578)	(2,376)
Net other comprehensive income/(loss)		9,645	(4,145)
AOCI Ending Balance		(4,742)	(14,180)
Other Comprehensive Income (Loss), before Tax		14,838	(6,377)
Other Comprehensive Income (Loss), Tax		5,193	(2,232)
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		45,752	(476)
Other comprehensive income/(loss) before reclassifications		0	0
Amounts reclassified from AOCI		(2,267)	(657)
Net other comprehensive income/(loss)		(2,267)	(657)
AOCI Ending Balance		43,485	(1,133)
Other Comprehensive Income (Loss), before Tax		(3,487)	(1,011)
Other Comprehensive Income (Loss), Tax		(1,220)	(354)
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		2,393	(508)
Other comprehensive income/(loss) before reclassifications		76	(3,395)
Amounts reclassified from AOCI		332	299
Net other comprehensive income/(loss)		408	(3,096)
AOCI Ending Balance		2,801	(3,604)
Other Comprehensive Income (Loss), before Tax		629	(4,763)
Other Comprehensive Income (Loss), Tax		221	(1,667)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		117	(5,222)
Other Comprehensive Income (Loss) before Reclassifications, Tax		41	(1,827)
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	[1]	(2,573)	(2,550)
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 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	[2]	(5)	168
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		(7)	259
Reclassification from AOCI, Current Period, Tax		(2)	91
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	[3]	(2,267)	(657)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		(3,487)	(1,011)
Reclassification from AOCI, Current Period, Tax		(1,220)	(354)
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		332	299
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		512	459
Reclassification from AOCI, Current Period, Tax		$ 180	$ 160

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

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

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