Fair Value Measurement - Narrative (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2021 USD ($) Security Trust	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Percentage of CDS contracts fair valued using minimum premium (de minimis)		51.00%
Fair Value Measured at Net Asset Value Per Share | Healthcare Private Equity Fund
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Long-term purchase commitment, amount	$ 117,000,000
Recurring | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of fixed maturity securities valued using model processes | Security	209
Fixed maturity securities	$ 1,318,000,000
CDR | Total | Recurring | Level 3 | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	0.11%	0.19%
CDR | Total | Recurring | Level 3 | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	2.52%	1.33%
AGC And AGM
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of custodial trusts | Trust	4
Share value, amount	$ 200,000,000
Maximum amount	$ 50,000,000