Securities Securities - Premium Amortization and Discount Accretion Included in Income on Securities (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Premium amortization	$ (24,586)	$ (22,762)	$ (72,733)	$ (67,321)
Discount accretion	1,783	2,497	6,278	8,250
Net (premium amortization) discount accretion	$ (22,803)	$ (20,265)	$ (66,455)	$ (59,071)

Amount of accretion of purchase discount on nonoperating securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.8)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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Amount of amortization of purchase premium on nonoperating securities.

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Amount of accretion (amortization) of purchase discount (premium) on nonoperating securities.

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