Fair Value of Assets and Liabilities (Financial Instruments where Carrying Amounts and Fair Values May Differ) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Assets:
Policy loans	$ 1,455,484	$ 505,367
Cash and cash equivalents	1,845,278	2,397,627
Accrued investment income	308,908	219,635
Reinsurance recoverables	36,860,011	37,096,562	[1]
Receivables from parent and affiliates	231,879	224,921
Liabilities [Abstract]
Cash collateral for loaned securities	184,001	86,750
Fair Value
Assets:
Commercial mortgage and other loans	5,093,398	4,602,177
Policy loans	1,455,484	505,367
Short-term investments	45,002	26,331
Cash and cash equivalents	700,364	965,445
Accrued investment income	308,908	219,635
Reinsurance recoverables	22,899	25,127
Receivables from parent and affiliates	87,180	76,846
Other assets	1,467,171	824,882
Total assets	9,180,406	7,245,810
Liabilities [Abstract]
Policyholders’ account balances - investment contracts	5,704,706	4,333,271
Cash collateral for loaned securities	184,001	86,750
Short-term debt to affiliates	297,280	120,325
Long-term debt to affiliates		173,905
Payables to parent and affiliates	31,254	41,654
Other liabilities	1,985,732	1,302,865
Total liabilities	8,202,973	6,058,770
Carrying Amount
Assets:
Commercial mortgage and other loans	5,492,869	4,928,680
Policy loans	1,455,484	505,367
Short-term investments	45,002	26,331
Cash and cash equivalents	700,364	965,445
Accrued investment income	308,908	219,635
Reinsurance recoverables	24,942	27,183
Receivables from parent and affiliates	87,180	76,846
Other assets	1,467,171	824,882
Total assets	9,581,920	7,574,369
Liabilities [Abstract]
Policyholders’ account balances - investment contracts	5,726,015	4,351,945
Cash collateral for loaned securities	184,001	86,750
Short-term debt to affiliates	304,317	126,250
Long-term debt to affiliates		185,563
Payables to parent and affiliates	31,254	41,654
Other liabilities	1,985,732	1,302,866
Total liabilities	8,231,319	6,095,028
Level 1 | Fair Value
Assets:
Commercial mortgage and other loans	0	0
Policy loans	0	0
Short-term investments	45,002	26,331
Cash and cash equivalents	560,365	675,445
Accrued investment income	0	0
Reinsurance recoverables	0	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	605,367	701,776
Liabilities [Abstract]
Policyholders’ account balances - investment contracts	0	0
Cash collateral for loaned securities	0	0
Short-term debt to affiliates	0	0
Long-term debt to affiliates		0
Payables to parent and affiliates	0	0
Other liabilities	0	0
Total liabilities	0	0
Level 2 | Fair Value
Assets:
Commercial mortgage and other loans	0	0
Policy loans	0	0
Short-term investments	0	0
Cash and cash equivalents	139,999	290,000
Accrued investment income	308,908	219,635
Reinsurance recoverables	0	0
Receivables from parent and affiliates	87,180	76,846
Other assets	89,142	94,200
Total assets	625,229	680,681
Liabilities [Abstract]
Policyholders’ account balances - investment contracts	1,017,396	1,192,271
Cash collateral for loaned securities	184,001	86,750
Short-term debt to affiliates	297,280	120,325
Long-term debt to affiliates		173,905
Payables to parent and affiliates	31,254	41,654
Other liabilities	1,953,309	1,269,615
Total liabilities	3,483,240	2,884,520
Level 3 | Fair Value
Assets:
Commercial mortgage and other loans	5,093,398	4,602,177
Policy loans	1,455,484	505,367
Short-term investments	0	0
Cash and cash equivalents	0	0
Accrued investment income	0	0
Reinsurance recoverables	22,899	25,127
Receivables from parent and affiliates	0	0
Other assets	1,378,029	730,682
Total assets	7,949,810	5,863,353
Liabilities [Abstract]
Policyholders’ account balances - investment contracts	4,687,310	3,141,000
Cash collateral for loaned securities	0	0
Short-term debt to affiliates	0	0
Long-term debt to affiliates		0
Payables to parent and affiliates	0	0
Other liabilities	32,423	33,250
Total liabilities	$ 4,719,733	$ 3,174,250

[1] Prior period amounts adjusted for the implementation of Accounting Standard Update ("ASU") 2018-12: Targeted Improvements to the Accounting for Long-Duration Contracts.