Fair Value of Assets and Liabilities (Financial Instruments where Carrying Amounts and Fair Values May Differ) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets:
Commercial mortgage and other loans	$ 1,083,419	$ 1,150,381
Policy loans	1,161,101	1,166,456
Cash and cash equivalents	212,569	96,157	$ 370,286	$ 214,952
Accrued investment income	82,341	87,322
Receivables from parent and affiliates	300,116	213,952
Liabilities:
Securities sold under agreements to repurchase	0	68,904
Cash collateral for loaned securities	33,169	74,976
Fair Value
Assets:
Commercial mortgage and other loans	1,111,625	1,181,582
Policy loans	1,161,101	1,166,456
Cash and cash equivalents	184,562	88,515
Accrued investment income	82,341	87,322
Receivables from parent and affiliates	167,545	76,315
Other assets	50,407	37,969
Total assets	2,757,581	2,638,159
Liabilities:
Policyholders’ account balances - investment contracts	1,455,018	1,382,385
Securities sold under agreements to repurchase	0	68,904
Cash collateral for loaned securities	33,169	74,976
Payables to parent and affiliates	223,550	73,628
Other liabilities	362,592	305,969
Total liabilities	2,074,329	1,905,862
Carrying Amount
Assets:
Commercial mortgage and other loans	1,083,419	1,150,381
Policy loans	1,161,101	1,166,456
Cash and cash equivalents	184,562	88,515
Accrued investment income	82,341	87,322
Receivables from parent and affiliates	167,545	76,315
Other assets	50,407	37,969
Total assets	2,729,375	2,606,958
Liabilities:
Policyholders’ account balances - investment contracts	1,458,599	1,386,099
Securities sold under agreements to repurchase	0	68,904
Cash collateral for loaned securities	33,169	74,976
Payables to parent and affiliates	223,550	73,628
Other liabilities	362,592	305,969
Total liabilities	2,077,910	1,909,576
Level 1 | Fair Value
Assets:
Commercial mortgage and other loans	0	0
Policy loans	0	0
Cash and cash equivalents	36,562	30,149
Accrued investment income	0	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	36,562	30,149
Liabilities:
Policyholders’ account balances - investment contracts	0	0
Securities sold under agreements to repurchase	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	148,000	58,366
Accrued investment income	82,341	87,322
Receivables from parent and affiliates	167,545	76,315
Other assets	50,407	37,969
Total assets	448,293	259,972
Liabilities:
Policyholders’ account balances - investment contracts	1,178,583	1,129,378
Securities sold under agreements to repurchase	0	68,904
Cash collateral for loaned securities	33,169	74,976
Payables to parent and affiliates	223,550	73,628
Other liabilities	362,592	305,969
Total liabilities	1,797,894	1,652,855
Level 3 | Fair Value
Assets:
Commercial mortgage and other loans	1,111,625	1,181,582
Policy loans	1,161,101	1,166,456
Cash and cash equivalents	0	0
Accrued investment income	0	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	2,272,726	2,348,038
Liabilities:
Policyholders’ account balances - investment contracts	276,435	253,007
Securities sold under agreements to repurchase	0	0
Cash collateral for loaned securities	0	0
Payables to parent and affiliates	0	0
Other liabilities	0	0
Total liabilities	276,435	253,007
Other long-term investments | 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	35,000
Other long-term investments | 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	$ 32,000