Income Taxes (Summary of Tax Years Subject to Examination by Major Taxing Jurisdictions) (Details)	12 Months Ended
	Dec. 31, 2017
Federal [Member] | Earliest Tax Year [Member]
Income Tax Examination [Line Items]
Tax years subject to examination	2013
Federal [Member] | Latest Tax Year [Member]
Income Tax Examination [Line Items]
Tax years subject to examination	2017
State and Local [Member] | Arkansas [Member] | Earliest Tax Year [Member]
Income Tax Examination [Line Items]
Tax years subject to examination	2014
State and Local [Member] | Arkansas [Member] | Latest Tax Year [Member]
Income Tax Examination [Line Items]
Tax years subject to examination	2017
State and Local [Member] | Michigan [Member] | Earliest Tax Year [Member]
Income Tax Examination [Line Items]
Tax years subject to examination	2013
State and Local [Member] | Michigan [Member] | Latest Tax Year [Member]
Income Tax Examination [Line Items]
Tax years subject to examination	2017
State and Local [Member] | Minnesota [Member] | Earliest Tax Year [Member]
Income Tax Examination [Line Items]
Tax years subject to examination	2013
State and Local [Member] | Minnesota [Member] | Latest Tax Year [Member]
Income Tax Examination [Line Items]
Tax years subject to examination	2017
State and Local [Member] | Idaho [Member] | Earliest Tax Year [Member]
Income Tax Examination [Line Items]
Tax years subject to examination	2014
State and Local [Member] | Idaho [Member] | Latest Tax Year [Member]
Income Tax Examination [Line Items]
Tax years subject to examination	2017