Fair Value Measurement Fair Value Measurement - Information by Credit Spread (Details)	Mar. 31, 2019	Dec. 31, 2018
Based on actual collateral specific spreads
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross spread percentage	20.00%	20.00%
Based on market indices
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross spread percentage	31.00%	33.00%
Provided by the CDS counterparty
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross spread percentage	49.00%	47.00%
Total [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross spread percentage	100.00%	100.00%