Fair Value of Assets and Liabilities (Financial Instruments where Carrying Amounts and Fair Values May Differ) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Assets:
Policy loans	$ 1,217,608	$ 1,161,101
Cash and cash equivalents	255,256	212,569	$ 196,326	$ 96,157
Accrued investment income	87,362	82,341
Receivables from parent and affiliates	312,256	300,116
Liabilities:
Cash collateral for loaned securities	7,585	33,169
Fair Value
Assets:
Commercial mortgage and other loans	1,170,760	1,111,625
Policy loans	1,217,608	1,161,101
Cash and cash equivalents	251,322	184,562
Accrued investment income	87,362	82,341
Receivables from parent and affiliates	185,075	167,545
Other assets	47,029	50,407
Total assets	2,959,156	2,757,581
Liabilities:
Policyholders’ account balances - investment contracts	1,465,725	1,455,018
Cash collateral for loaned securities	7,585	33,169
Short-term debt	1,744	0
Payables to parent and affiliates	166,136	223,550
Other liabilities	377,230	362,592
Total liabilities	2,018,420	2,074,329
Carrying Amount
Assets:
Commercial mortgage and other loans	1,177,395	1,083,419
Policy loans	1,217,608	1,161,101
Cash and cash equivalents	251,322	184,562
Accrued investment income	87,362	82,341
Receivables from parent and affiliates	185,075	167,545
Other assets	47,029	50,407
Total assets	2,965,791	2,729,375
Liabilities:
Policyholders’ account balances - investment contracts	1,474,604	1,458,599
Cash collateral for loaned securities	7,585	33,169
Short-term debt	1,744	0
Payables to parent and affiliates	166,136	223,550
Other liabilities	377,230	362,592
Total liabilities	2,027,299	2,077,910
Level 1 | Fair Value
Assets:
Commercial mortgage and other loans	0	0
Policy loans	0	0
Cash and cash equivalents	46,322	36,562
Accrued investment income	0	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	46,322	36,562
Liabilities:
Policyholders’ 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	205,000	148,000
Accrued investment income	87,362	82,341
Receivables from parent and affiliates	185,075	167,545
Other assets	47,029	50,407
Total assets	524,466	448,293
Liabilities:
Policyholders’ account balances - investment contracts	1,194,005	1,178,583
Cash collateral for loaned securities	7,585	33,169
Short-term debt	1,744	0
Payables to parent and affiliates	166,136	223,550
Other liabilities	377,230	362,592
Total liabilities	1,746,700	1,797,894
Level 3 | Fair Value
Assets:
Commercial mortgage and other loans	1,170,760	1,111,625
Policy loans	1,217,608	1,161,101
Cash and cash equivalents	0	0
Accrued investment income	0	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	2,388,368	2,272,726
Liabilities:
Policyholders’ account balances - investment contracts	271,720	276,435
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	$ 271,720	276,435
Other invested assets | Fair Value | Measurement at NAV per share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cost Method Investments, Fair Value Disclosure		49,000
Other invested assets | Carrying Amount | Measurement at NAV per share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cost Method Investments, Fair Value Disclosure		$ 41,000