Fair Value of Assets and Liabilities - Significant unobservable inputs used to value Level 3 FCR liability (Details)	Jun. 30, 2018	Dec. 31, 2017
Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Reversal rate	0.860	0.855
Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Reversal rate	0.983	0.980
Weighted average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Reversal rate	0.897	0.890