Fair Value Measurement - Information by Credit Spread (Details)	Dec. 31, 2017	Dec. 31, 2016
Based on actual collateral specific spreads [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Gross spread percentage	14.00%	7.00%
Based on market indices [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Gross spread percentage	48.00%	77.00%
Provided by the CDS counterparty [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Gross spread percentage	38.00%	16.00%
Credit Default Swap [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Gross spread percentage	100.00%	100.00%