Commitments and Contingencies Commitments and Contingencies (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Loss Contingencies [Line Items]
Total costs to remediate IRC Section 7702 issue	$ 7.1	$ 12.5
Toll charges and fees to remediate IRC Section 7702 issue	6.1
Cost of claim and reserve liabilities to remediate IRC Section 7702 issue	1.0
Minimum [Member]
Loss Contingencies [Line Items]
Cost Range to Remediate IRC Section 7702 Issue	4.5	5.9
Minimum [Member] | IRC Section 7702 remediation costs [Member]
Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss	0.5
Maximum [Member]
Loss Contingencies [Line Items]
Cost Range to Remediate IRC Section 7702 Issue	44.0	$ 48.2
Maximum [Member] | IRC Section 7702 remediation costs [Member]
Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss	$ 1.5