Securities Securities - (Premium Amortization and Discount Accretion Included in Income on Securities) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Premium amortization	$ (126,210)	$ (120,785)	$ (108,483)
Discount accretion	2,425	5,227	7,955
Net (premium amortization) discount accretion	$ (123,785)	$ (115,558)	$ (100,528)

Amount of accretion of purchase discount on nonoperating securities.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.8)
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

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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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