Investments - Unrealized Gain (Loss) on Investment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Gain (Loss) on Securities [Line Items]
Change in investment gains (losses) on	$ 1,958,165	$ (1,434,918)	$ 921,421
Fixed maturities available for sale
Gain (Loss) on Securities [Line Items]
Change in investment gains (losses) on	1,946,910	(1,429,763)	916,413
Other investments
Gain (Loss) on Securities [Line Items]
Change in investment gains (losses) on	$ 11,255	$ (5,155)	$ 5,008