Investments - Schedule of Components of Net Investment Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Derivative gains (losses)	$ 49,620	$ 31,520	$ 40,243	$ (106,629)	$ 60,080	$ 3,296	$ 17,828	$ 42,003
Total investment income									$ 419,614	$ 558,744	$ 350,430
Less investment expenses									2,412	3,252	1,353
Net investment income									417,202	555,492	349,077
Debt and equity securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income									373,479	403,372	399,645
Mortgage loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income									13,162	12,595	12,066
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income									3,361	3,539	3,185
Derivative gains (losses)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Derivative gains (losses)									14,754	123,207	(80,004)
Short term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income									2,160	2,974	2,249
Other investment income
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income									$ 12,698	$ 13,057	$ 13,289