Investments - Realized Investment Gains (Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2019
Gain (Loss) on Securities [Line Items]
Net investment gains (losses)	$ (2,289)	$ 153,340	$ (12,543)
Investment gain realized on transfer	135,300
Reinsurance block of in-force fixed and variable annuity business				$ 2,900,000
Fixed maturity securities
Gain (Loss) on Securities [Line Items]
Net investment gains (losses)	9,392	141,448	(5,713)
Equity securities
Gain (Loss) on Securities [Line Items]
Net investment gains (losses)	1,843	15,975	(10,649)
Short-term and other investments
Gain (Loss) on Securities [Line Items]
Net investment gains (losses)	$ (13,524)	$ (4,083)	$ 3,819