Fair Value of Financial Instruments - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	$ 10,770,923	$ 3,356,945
Equity securities	17,744	23,594
Derivatives, index options	132,821	157,588
Other invested assets		2
Total assets	10,921,488	3,538,129
Policyholder account balances	161,351	155,902
Other liabilities	6,202	15,301
Total liabilities	$ 167,553	171,203
Percent of total	100.00%
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	$ 0	0
Equity securities	17,744	23,594
Derivatives, index options	0	0
Other invested assets		2
Total assets	17,744	23,596
Policyholder account balances	0	0
Other liabilities	0	0
Total liabilities	$ 0	0
Percent of total	0.20%
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	$ 10,770,923	3,356,945
Equity securities	0	0
Derivatives, index options	0	0
Other invested assets		0
Total assets	10,770,923	3,356,945
Policyholder account balances	0	0
Other liabilities	0	0
Total liabilities	$ 0	0
Percent of total	98.60%
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	$ 0	0
Equity securities	0	0
Derivatives, index options	132,821	157,588
Other invested assets		0
Total assets	132,821	157,588
Policyholder account balances	161,351	155,902
Other liabilities	6,202	15,301
Total liabilities	$ 167,553	$ 171,203
Percent of total	1.20%
Priced by third-party vendors
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	$ 10,770,923
Equity securities	17,744
Derivatives, index options	132,821
Priced by third-party vendors | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	0
Equity securities	17,744
Derivatives, index options	0
Priced by third-party vendors | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	10,770,923
Equity securities	0
Derivatives, index options	0
Priced by third-party vendors | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	0
Equity securities	0
Derivatives, index options	132,821
Priced internally
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	0
Equity securities	0
Derivatives, index options	0
Priced internally | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	0
Equity securities	0
Derivatives, index options	0
Priced internally | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	0
Equity securities	0
Derivatives, index options	0
Priced internally | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	0
Equity securities	0
Derivatives, index options	$ 0