Fair Value Measurement Fair Value Measurement - Information by Credit Spread (Details)	Mar. 31, 2018	Dec. 31, 2017
Based on actual collateral specific spreads
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross spread percentage	13.00%	14.00%
Based on market indices
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross spread percentage	50.00%	48.00%
Provided by the CDS counterparty
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross spread percentage	37.00%	38.00%
Total [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross spread percentage	100.00%	100.00%