Income Taxes Income Taxes - AMT Credit and Foreign Tax Credit Carryover (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Tax Cuts and Jobs Act of 2017, Reclassification of Deferred Income Tax Assets Into Income Taxes Receivable Payable	$ 218,000,000	$ 235,000,000
Foreign Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards	$ 18,000,000	$ 23,000,000