Fair Value of Assets and Liabilities (Financial Instruments where Carrying Amounts and Fair Values May Differ) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Assets:
Policy loans	$ 12,352	$ 12,805
Cash and cash equivalents	5,923,320	4,503,534	$ 394,262	$ 1,639,939
Accrued investment income	98,662	90,895
Reinsurance recoverables	710,596	572,102
Receivables from parent and affiliates	15,101	46,381
Liabilities:
Cash collateral for loaned securities	0	384
Short-term debt	0	140,569
Long-term debt	653,596	787,596
Reinsurance payables	227,948	232,937
Fair Value
Assets:
Commercial mortgage and other loans	1,381,743	1,339,707
Policy loans	12,352	12,805
Short-term investments	245,000	7,750
Cash and cash equivalents	1,840,944	811,175
Accrued investment income	98,662	90,895
Reinsurance recoverables	57,595	55,236
Receivables from parent and affiliates	11,934	9,188
Other assets	14,845	3,735
Total assets	3,663,075	2,330,491
Liabilities:
Policyholders’ account balances - investment contracts	1,259,338	560,548
Cash collateral for loaned securities	0	384
Short-term debt	0	139,843
Long-term debt	683,860	791,670
Reinsurance payables	51,267	55,236
Payables to parent and affiliates	48,380	30,846
Other liabilities	413,737	554,162
Separate account liabilities - investment contracts	58	71
Total liabilities	2,456,640	2,132,760
Carrying Amount
Assets:
Commercial mortgage and other loans	1,335,206	1,353,478
Policy loans	12,352	12,805
Short-term investments	245,000	7,750
Cash and cash equivalents	1,840,944	811,175
Accrued investment income	98,662	90,895
Reinsurance recoverables	57,191	55,236
Receivables from parent and affiliates	11,934	9,188
Other assets	14,845	3,735
Total assets	3,616,134	2,344,262
Liabilities:
Policyholders’ account balances - investment contracts	1,251,639	565,903
Cash collateral for loaned securities	0	384
Short-term debt	0	140,569
Long-term debt	653,596	787,596
Reinsurance payables	51,267	55,236
Payables to parent and affiliates	48,380	30,846
Other liabilities	413,737	554,162
Separate account liabilities - investment contracts	58	71
Total liabilities	2,418,677	2,134,767
Level 1 | Fair Value
Assets:
Commercial mortgage and other loans	0	0
Policy loans	0	0
Short-term investments	245,000	7,750
Cash and cash equivalents	585,944	136,175
Accrued investment income	0	0
Reinsurance recoverables	0	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	830,944	143,925
Liabilities:
Policyholders’ account balances - investment contracts	0	0
Cash collateral for loaned securities	0	0
Short-term debt	0	0
Long-term debt	0	0
Reinsurance payables	0	0
Payables to parent and affiliates	0	0
Other liabilities	0	0
Separate account liabilities - investment contracts	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	1,255,000	675,000
Accrued investment income	98,662	90,895
Reinsurance recoverables	0	0
Receivables from parent and affiliates	11,934	9,188
Other assets	14,845	3,735
Total assets	1,380,441	778,818
Liabilities:
Policyholders’ account balances - investment contracts	0	0
Cash collateral for loaned securities	0	384
Short-term debt	0	139,843
Long-term debt	683,860	791,670
Reinsurance payables	0	0
Payables to parent and affiliates	48,380	30,846
Other liabilities	413,737	554,162
Separate account liabilities - investment contracts	58	71
Total liabilities	1,146,035	1,516,976
Level 3 | Fair Value
Assets:
Commercial mortgage and other loans	1,381,743	1,339,707
Policy loans	12,352	12,805
Short-term investments	0	0
Cash and cash equivalents	0	0
Accrued investment income	0	0
Reinsurance recoverables	57,595	55,236
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	1,451,690	1,407,748
Liabilities:
Policyholders’ account balances - investment contracts	1,259,338	560,548
Cash collateral for loaned securities	0	0
Short-term debt	0	0
Long-term debt	0	0
Reinsurance payables	51,267	55,236
Payables to parent and affiliates	0	0
Other liabilities	0	0
Separate account liabilities - investment contracts	0	0
Total liabilities	$ 1,310,605	$ 615,784