Fair Value of Assets and Liabilities (Financial Instruments where Carrying Amounts and Fair Values May Differ) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Assets:
Policy loans	$ 1,169,847	$ 1,161,101
Cash and cash equivalents	81,186	212,569	$ 133,390	$ 96,157
Accrued investment income	83,878	82,341
Receivables from parent and affiliates	280,118	300,116
Liabilities:
Cash collateral for loaned securities	13,576	33,169
Fair Value
Assets:
Commercial mortgage and other loans	1,150,210	1,111,625
Policy loans	1,169,847	1,161,101
Cash and cash equivalents	81,186	184,562
Accrued investment income	83,878	82,341
Receivables from parent and affiliates	150,760	167,545
Other assets	43,638	50,407
Total assets	2,679,519	2,757,581
Liabilities:
Policyholders’ account balances - investment contracts	1,473,498	1,455,018
Cash collateral for loaned securities	13,576	33,169
Debt, Current	80
Payables to parent and affiliates	236,760	223,550
Other liabilities	353,291	362,592
Total liabilities	2,077,205	2,074,329
Carrying Amount
Assets:
Commercial mortgage and other loans	1,137,366	1,083,419
Policy loans	1,169,847	1,161,101
Cash and cash equivalents	81,186	184,562
Accrued investment income	83,878	82,341
Receivables from parent and affiliates	150,760	167,545
Other assets	43,638	50,407
Total assets	2,666,675	2,729,375
Liabilities:
Policyholders’ account balances - investment contracts	1,480,611	1,458,599
Cash collateral for loaned securities	13,576	33,169
Debt, Current	80
Payables to parent and affiliates	236,760	223,550
Other liabilities	353,291	362,592
Total liabilities	2,084,318	2,077,910
Level 1 | Fair Value
Assets:
Commercial mortgage and other loans	0	0
Policy loans	0	0
Cash and cash equivalents	52,686	36,562
Accrued investment income	0	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	52,686	36,562
Liabilities:
Policyholders’ account balances - investment contracts	0	0
Cash collateral for loaned securities	0	0
Debt, Current	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	28,500	148,000
Accrued investment income	83,878	82,341
Receivables from parent and affiliates	150,760	167,545
Other assets	43,638	50,407
Total assets	306,776	448,293
Liabilities:
Policyholders’ account balances - investment contracts	1,198,114	1,178,583
Cash collateral for loaned securities	13,576	33,169
Debt, Current	80
Payables to parent and affiliates	236,760	223,550
Other liabilities	353,291	362,592
Total liabilities	1,801,821	1,797,894
Level 3 | Fair Value
Assets:
Commercial mortgage and other loans	1,150,210	1,111,625
Policy loans	1,169,847	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	2,320,057	2,272,726
Liabilities:
Policyholders’ account balances - investment contracts	275,384	276,435
Cash collateral for loaned securities	0	0
Debt, Current	0
Payables to parent and affiliates	0	0
Other liabilities	0	0
Total liabilities	$ 275,384	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