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	$ 1,283,214	$ 1,236,077
Cash and cash equivalents	548,588	416,840	$ 255,256	$ 212,569
Accrued investment income	89,822	88,278
Reinsurance recoverables	42,921,981	34,682,127
Receivables from parent and affiliates	286,694	289,580
Liabilities:
Cash collateral for loaned securities	7,759	11,063
Fair Value
Assets:
Commercial mortgage and other loans	1,288,280	1,214,350
Policy loans	1,283,214	1,236,077
Cash and cash equivalents	79,880	199,894
Accrued investment income	89,822	88,278
Reinsurance recoverables	210,484	0
Receivables from parent and affiliates	160,926	154,938
Other assets	31,912	28,950
Total assets	3,144,518	2,922,487
Liabilities:
Policyholders’ account balances - investment contracts	1,520,725	1,479,069
Cash collateral for loaned securities	7,759	11,063
Payables to parent and affiliates	180,261	229,345
Other liabilities	368,759	372,997
Total liabilities	2,077,504	2,092,474
Carrying Amount
Assets:
Commercial mortgage and other loans	1,230,059	1,209,150
Policy loans	1,283,214	1,236,077
Cash and cash equivalents	79,880	199,894
Accrued investment income	89,822	88,278
Reinsurance recoverables	209,265	0
Receivables from parent and affiliates	160,925	154,938
Other assets	31,912	28,950
Total assets	3,085,077	2,917,287
Liabilities:
Policyholders’ account balances - investment contracts	1,519,506	1,486,929
Cash collateral for loaned securities	7,759	11,063
Payables to parent and affiliates	180,261	229,345
Other liabilities	368,759	372,997
Total liabilities	2,076,285	2,100,334
Level 1 | Fair Value
Assets:
Commercial mortgage and other loans	0	0
Policy loans	0	0
Cash and cash equivalents	79,880	56,894
Accrued investment income	0	0
Reinsurance recoverables	0	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	79,880	56,894
Liabilities:
Policyholders’ account balances - investment contracts	0	0
Cash collateral for loaned securities	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	89,822	88,278
Reinsurance recoverables	0	0
Receivables from parent and affiliates	160,926	154,938
Other assets	31,912	28,950
Total assets	282,660	415,166
Liabilities:
Policyholders’ account balances - investment contracts	1,242,610	1,206,747
Cash collateral for loaned securities	7,759	11,063
Payables to parent and affiliates	180,261	229,345
Other liabilities	368,759	372,997
Total liabilities	1,799,389	1,820,152
Level 3 | Fair Value
Assets:
Commercial mortgage and other loans	1,288,280	1,214,350
Policy loans	1,283,214	1,236,077
Cash and cash equivalents	0	0
Accrued investment income	0	0
Reinsurance recoverables	210,484	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	2,781,978	2,450,427
Liabilities:
Policyholders’ account balances - investment contracts	278,115	272,322
Cash collateral for loaned securities	0	0
Payables to parent and affiliates	0	0
Other liabilities	0	0
Total liabilities	$ 278,115	$ 272,322