Fair Value Measurement - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) Security	Dec. 31, 2018
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Percentage of CDS contracts which are fair valued using minimum premium	0.10%	17.00%
Recurring [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of fixed maturity securities valued using model processes | Security	131
Fixed maturity securities | $	$ 1,428
Measurement Input, Default Rate [Member] | Total [Member] | Recurring [Member] | Level 3 [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	2.34%	2.47%
Measurement Input, Default Rate [Member] | Total [Member] | Recurring [Member] | Level 3 [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	2.64%	2.89%