Fair Value Measurement - Information by Credit Spread (Details)	Dec. 31, 2018	Dec. 31, 2017
Based on actual collateral specific spreads [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Gross spread percentage	20.00%	14.00%
Based on market indices [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Gross spread percentage	33.00%	48.00%
Provided by the CDS counterparty [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Gross spread percentage	47.00%	38.00%
Credit Default Swap [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Gross spread percentage	100.00%	100.00%