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Income Taxes (Schedule Of Uncertain Tax Positions) (Details) | 12 Months Ended
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Dec. 31, 2017
Statute of limitations, expiration period from extended due date of federal tax return | 3 years
Statute of limitations, expiration period from extended due date of Iowa tax return | 3 years
Statute of limitations, expiration period from extended due date of Wisconsin tax return | 4 years

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