Accounting Policies - Components of Other Comprehensive Income Loss and Related Tax Impact (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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		$ 8,433	$ (9,270)	$ 59,887
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax		2,952	(3,244)	20,960
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax		5,481	(6,026)	38,927
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		55,062	(7,181)	53,065
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		2,000	1,200	200
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(10,285)	(19,371)	(21,330)
Other Comprehensive Income (Loss), before Tax		68,889	(40,850)	48,823
Other Comprehensive Income (Loss), Tax		24,112	(14,298)	17,088
Other comprehensive income, net of tax		44,777	(26,552)	31,735
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		2,443	(1,776)	(559)
Other Comprehensive Income (Loss) before Reclassifications, Tax		855	(621)	(195)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		1,588	(1,155)	(364)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		1,313	803	138
Other Comprehensive Income (Loss), before Tax		4,463	(541)	(347)
Other Comprehensive Income (Loss), Tax		1,562	(189)	(121)
Other comprehensive income, net of tax		2,901	(352)	(226)
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		5,481	(6,026)	38,927
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(9,833)	(13,725)	(12,939)
Other Comprehensive Income (Loss), before Tax		(6,694)	(30,387)	39,980
Other Comprehensive Income (Loss), Tax		(2,342)	(10,636)	13,992
Other comprehensive income, net of tax		(4,352)	(19,751)	25,988
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, before Tax	[1]	73,835	0	22,311
Other Comprehensive Income (Loss) before Reclassifications, Tax	[1]	25,842	0	7,809
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	[1]	47,993	0	14,502
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(1,765)	(6,449)	(8,529)
Other Comprehensive Income (Loss), before Tax		71,120	(9,922)	9,190
Other Comprehensive Income (Loss), Tax		24,892	(3,473)	3,217
Other comprehensive income, net of tax		46,228	(6,449)	5,973
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		(15,375)	(20,125)	(19,213)
Reclassification from AOCI, Current Period, Tax		(5,381)	(7,044)	(6,725)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[2]	(9,994)	(13,081)	(12,488)
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		9	(10)	239
Reclassification from AOCI, Current Period, Tax		3	(4)	84
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[3]	6	(6)	155
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		239	(982)	(933)
Reclassification from AOCI, Current Period, Tax		84	(344)	(327)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[4]	155	(638)	(606)
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		(2,715)	(9,922)	(13,121)
Reclassification from AOCI, Current Period, Tax		(950)	(3,473)	(4,592)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[5]	(1,765)	(6,449)	(8,529)
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	[6]	2,020	1,235	212
Reclassification from AOCI, Current Period, Tax	[6]	707	432	74
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[6]	$ 1,313	$ 803	$ 138

[1] Represents the accumulated unrealized gain on the USDA Securities and the Farmer Mac Guaranteed Securities transferred from available-for-sale to held-to-maturity.

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

[3] (2) Represents unrealized gains and 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] 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.

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