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:
Policy loans	$ 12,558	$ 12,719
Cash and cash equivalents	1,639,939	1,848,039	$ 536	$ 594
Accrued investment income	88,331	86,004
Reinsurance recoverables	563,428	588,608
Receivables from parent and affiliates	49,351	111,703
Liabilities:
Cash collateral for loaned securities	17,383	23,350
Short-term debt	43,734	28,101
Reinsurance payables	262,588	275,822
Fair Value
Assets:
Commercial mortgage and other loans	1,396,167	1,235,842
Policy loans	12,558	12,719
Short-term investments	0	35,000
Cash and cash equivalents	493,473	261,886
Accrued investment income	88,331	86,004
Reinsurance recoverables	59,588	63,775
Receivables from parent and affiliates	11,206	70,779
Other assets	13,802	53,858
Total assets	2,075,125	1,819,863
Liabilities:
Policyholders’ account balances - investment contracts	281,582	247,986
Cash collateral for loaned securities	17,383	23,350
Short-term debt	43,734	28,146
Long-term debt	1,003,251	994,198
Reinsurance payables	59,588	63,775
Payables to parent and affiliates	36,026	91,432
Other liabilities	135,556	189,366
Separate account liabilities - investment contracts	102	187
Total liabilities	1,577,222	1,638,440
Fair Value | Level 1
Assets:
Commercial mortgage and other loans	0	0
Policy loans	0	0
Short-term investments	0	0
Cash and cash equivalents	493,473	6,886
Accrued investment income	0	0
Reinsurance recoverables	0	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	493,473	6,886
Liabilities:
Policyholders’ account balances - investment contracts	0	0
Cash collateral for loaned securities	0	0
Short-term debt	0	0
Long-term debt	0	0
Reinsurance payables	0	0
Payables to parent and affiliates	0	0
Other liabilities	0	0
Separate account liabilities - investment contracts	0	0
Total liabilities	0	0
Fair Value | Level 2
Assets:
Commercial mortgage and other loans	0	0
Policy loans	0	0
Short-term investments	0	35,000
Cash and cash equivalents	0	255,000
Accrued investment income	88,331	86,004
Reinsurance recoverables	0	0
Receivables from parent and affiliates	11,206	70,779
Other assets	13,802	53,858
Total assets	113,339	500,641
Liabilities:
Policyholders’ account balances - investment contracts	0	0
Cash collateral for loaned securities	17,383	23,350
Short-term debt	43,734	28,146
Long-term debt	1,003,251	994,198
Reinsurance payables	0	0
Payables to parent and affiliates	36,026	91,432
Other liabilities	135,556	189,366
Separate account liabilities - investment contracts	102	187
Total liabilities	1,236,052	1,326,679
Fair Value | Level 3
Assets:
Commercial mortgage and other loans	1,396,167	1,235,842
Policy loans	12,558	12,719
Short-term investments	0	0
Cash and cash equivalents	0	0
Accrued investment income	0	0
Reinsurance recoverables	59,588	63,775
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	1,468,313	1,312,336
Liabilities:
Policyholders’ account balances - investment contracts	281,582	247,986
Cash collateral for loaned securities	0	0
Short-term debt	0	0
Long-term debt	0	0
Reinsurance payables	59,588	63,775
Payables to parent and affiliates	0	0
Other liabilities	0	0
Separate account liabilities - investment contracts	0	0
Total liabilities	341,170	311,761
Carrying Amount
Assets:
Commercial mortgage and other loans	1,387,012	1,231,893
Policy loans	12,558	12,719
Short-term investments	0	35,000
Cash and cash equivalents	493,473	261,886
Accrued investment income	88,331	86,004
Reinsurance recoverables	59,588	63,775
Receivables from parent and affiliates	11,206	70,779
Other assets	13,802	53,858
Total assets	2,065,970	1,815,914
Liabilities:
Policyholders’ account balances - investment contracts	281,051	250,493
Cash collateral for loaned securities	17,383	23,350
Short-term debt	43,734	28,101
Long-term debt	928,165	971,899
Reinsurance payables	59,588	63,775
Payables to parent and affiliates	36,026	91,432
Other liabilities	135,556	189,366
Separate account liabilities - investment contracts	102	187
Total liabilities	1,501,605	1,618,603
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	6,400	3,400
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	$ 6,000	$ 3,100