Commitments and Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Total costs to remediate IRC Section 7702 issue	$ 12.3	$ 14.4
Toll charges and fees to remediate IRC Section 7702 issue	12.0
Cost of claim and reserve liabilities to remediate IRC Section 7702 issue	0.3
Decrease in total cost to remediate Section 7702 and 72(s) issue during the period	2.1
Minimum [Member]
Loss Contingencies [Line Items]
Cost Range to Remediate IRC Section 7702 Issue	5.9
Minimum [Member] | IRC Section 7702 remediation costs [Member]
Loss Contingencies [Line Items]
Loss contingency, range of possible loss	1.0
Maximum [Member]
Loss Contingencies [Line Items]
Cost Range to Remediate IRC Section 7702 Issue	48.2
Maximum [Member] | IRC Section 7702 remediation costs [Member]
Loss Contingencies [Line Items]
Loss contingency, range of possible loss	$ 2.0