Derivative Financial Instruments - CDS notional amount by credit rating and fair value of protection sold (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Credit derivative, term	5 years
Notional amount	$ 3,886,000,000	$ 3,478,000,000
Credit default contracts
Derivative [Line Items]
Notional amount	759,000,000	9,000,000
Fair value	(2,000,000)	0
Single name | Corporate | Credit default contracts
Derivative [Line Items]
Notional amount	9,000,000	9,000,000
Fair value	0	0
Corporate debt | Corporate | Credit default contracts
Derivative [Line Items]
Notional amount	750,000,000
Fair value	(2,000,000)
AAA | Credit default contracts
Derivative [Line Items]
Notional amount	6,000,000	0
AAA | Single name | Corporate | Credit default contracts
Derivative [Line Items]
Notional amount	0	0
AAA | Corporate debt | Corporate | Credit default contracts
Derivative [Line Items]
Notional amount	6,000,000
AA | Credit default contracts
Derivative [Line Items]
Notional amount	12,000,000	0
AA | Single name | Corporate | Credit default contracts
Derivative [Line Items]
Notional amount	0	0
AA | Corporate debt | Corporate | Credit default contracts
Derivative [Line Items]
Notional amount	12,000,000
A | Credit default contracts
Derivative [Line Items]
Notional amount	168,000,000	0
A | Single name | Corporate | Credit default contracts
Derivative [Line Items]
Notional amount	0	0
A | Corporate debt | Corporate | Credit default contracts
Derivative [Line Items]
Notional amount	168,000,000
BBB | Credit default contracts
Derivative [Line Items]
Notional amount	510,000,000	0
BBB | Single name | Corporate | Credit default contracts
Derivative [Line Items]
Notional amount	0	0
BBB | Corporate debt | Corporate | Credit default contracts
Derivative [Line Items]
Notional amount	510,000,000
BB and lower | Credit default contracts
Derivative [Line Items]
Notional amount	63,000,000	9,000,000
BB and lower | Single name | Corporate | Credit default contracts
Derivative [Line Items]
Notional amount	9,000,000	$ 9,000,000
BB and lower | Corporate debt | Corporate | Credit default contracts
Derivative [Line Items]
Notional amount	$ 54,000,000