Fair Value of Assets and Liabilities (Financial Instruments where Carrying Amounts and Fair Values May Differ) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets:
Policy loans	$ 1,314,064	$ 1,236,077
Cash and cash equivalents	563,199	416,840	$ 212,569	$ 96,157
Accrued investment income	89,448	88,278
Reinsurance recoverables	40,710,159	34,682,127
Receivables from parent and affiliates	271,981	289,580
Liabilities:
Cash collateral for loaned securities	7,529	11,063
Short-term debt	2,845	0
Fair Value
Assets:
Commercial mortgage and other loans	1,288,625	1,214,350
Policy loans	1,314,064	1,236,077
Cash and cash equivalents	15,557	199,894
Accrued investment income	89,448	88,278
Reinsurance recoverables	212,368	0
Receivables from parent and affiliates	149,415	154,938
Other assets	22,505	28,950
Total Assets	3,091,982	2,922,487
Liabilities:
Policyholder account balances - investment contracts	1,541,910	1,479,069
Cash collateral for loaned securities	7,529	11,063
Short-term debt	2,845	0
Payables to parent and affiliates	216,842	229,345
Other liabilities	387,109	372,997
Total liabilities	2,156,235	2,092,474
Carrying Amount
Assets:
Commercial mortgage and other loans	1,239,885	1,209,150
Policy loans	1,314,064	1,236,077
Cash and cash equivalents	15,557	199,894
Accrued investment income	89,448	88,278
Reinsurance recoverables	211,813	0
Receivables from parent and affiliates	149,415	154,938
Other assets	22,505	28,950
Total Assets	3,042,687	2,917,287
Liabilities:
Policyholder account balances - investment contracts	1,541,355	1,486,929
Cash collateral for loaned securities	7,529	11,063
Short-term debt	2,845	0
Payables to parent and affiliates	216,842	229,345
Other liabilities	387,109	372,997
Total liabilities	2,155,680	2,100,334
Level 1 | Fair Value
Assets:
Commercial mortgage and other loans	0	0
Policy loans	0	0
Cash and cash equivalents	15,557	56,894
Accrued investment income	0	0
Reinsurance recoverables	0	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total Assets	15,557	56,894
Liabilities:
Policyholder account balances - investment contracts	0	0
Cash collateral for loaned securities	0	0
Short-term debt	0	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
Cash and cash equivalents	0	143,000
Accrued investment income	89,448	88,278
Reinsurance recoverables	0	0
Receivables from parent and affiliates	149,415	154,938
Other assets	22,505	28,950
Total Assets	261,368	415,166
Liabilities:
Policyholder account balances - investment contracts	1,264,128	1,206,747
Cash collateral for loaned securities	7,529	11,063
Short-term debt	2,845	0
Payables to parent and affiliates	216,842	229,345
Other liabilities	387,109	372,997
Total liabilities	1,878,453	1,820,152
Level 3 | Fair Value
Assets:
Commercial mortgage and other loans	1,288,625	1,214,350
Policy loans	1,314,064	1,236,077
Cash and cash equivalents	0	0
Accrued investment income	0	0
Reinsurance recoverables	212,368	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total Assets	2,815,057	2,450,427
Liabilities:
Policyholder account balances - investment contracts	277,782	272,322
Cash collateral for loaned securities	0	0
Short-term debt	0	0
Payables to parent and affiliates	0	0
Other liabilities	0	0
Total liabilities	$ 277,782	$ 272,322