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	$ 12,656	$ 12,558
Short-term investments		711,071
Cash and cash equivalents	394,262	1,639,939	$ 2,774,478	$ 1,848,039
Accrued investment income	76,228	88,331
Reinsurance recoverables	487,748	563,428
Receivables from parent and affiliates	56,368	49,351
Liabilities:
Cash collateral for loaned securities	7,147	17,383
Short-term debt	184,303	43,734
Long-term debt	787,596	928,165
Reinsurance payables	214,420	262,588
Fair Value
Assets:
Commercial mortgage and other loans	1,324,292	1,396,167
Policy loans	12,656	12,558
Short-term investments	7,750
Cash and cash equivalents	347,403	493,473
Accrued investment income	76,228	88,331
Reinsurance recoverables	56,348	59,588
Receivables from parent and affiliates	18,649	11,206
Other assets	15,047	13,802
Total assets	1,858,373	2,075,125
Liabilities:
Policyholders’ account balances - investment contracts	431,293	281,582
Cash collateral for loaned securities	7,147	17,383
Short-term debt	183,673	43,734
Long-term debt	789,881	1,003,251
Reinsurance payables	56,348	59,588
Payables to parent and affiliates	46,010	36,026
Other liabilities	138,859	135,556
Separate account liabilities - investment contracts	86	102
Total liabilities	1,653,297	1,577,222
Carrying Amount
Assets:
Commercial mortgage and other loans	1,352,403	1,387,012
Policy loans	12,656	12,558
Short-term investments	7,750
Cash and cash equivalents	347,403	493,473
Accrued investment income	76,228	88,331
Reinsurance recoverables	56,348	59,588
Receivables from parent and affiliates	18,649	11,206
Other assets	15,047	13,802
Total assets	1,886,484	2,065,970
Liabilities:
Policyholders’ account balances - investment contracts	438,448	281,051
Cash collateral for loaned securities	7,147	17,383
Short-term debt	184,303	43,734
Long-term debt	787,596	928,165
Reinsurance payables	56,348	59,588
Payables to parent and affiliates	46,010	36,026
Other liabilities	138,859	135,556
Separate account liabilities - investment contracts	86	102
Total liabilities	1,658,797	1,501,605
Level 1 | Fair Value
Assets:
Commercial mortgage and other loans	0	0
Policy loans	0	0
Short-term investments	7,750
Cash and cash equivalents	20,403	493,473
Accrued investment income	0	0
Reinsurance recoverables	0	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	28,153	493,473
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
Level 2 | Fair Value
Assets:
Commercial mortgage and other loans	0	0
Policy loans	0	0
Short-term investments	0
Cash and cash equivalents	327,000	0
Accrued investment income	76,228	88,331
Reinsurance recoverables	0	0
Receivables from parent and affiliates	18,649	11,206
Other assets	15,047	13,802
Total assets	436,924	113,339
Liabilities:
Policyholders’ account balances - investment contracts	0	0
Cash collateral for loaned securities	7,147	17,383
Short-term debt	183,673	43,734
Long-term debt	789,881	1,003,251
Reinsurance payables	0	0
Payables to parent and affiliates	46,010	36,026
Other liabilities	138,859	135,556
Separate account liabilities - investment contracts	86	102
Total liabilities	1,165,656	1,236,052
Level 3 | Fair Value
Assets:
Commercial mortgage and other loans	1,324,292	1,396,167
Policy loans	12,656	12,558
Short-term investments	0
Cash and cash equivalents	0	0
Accrued investment income	0	0
Reinsurance recoverables	56,348	59,588
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	1,393,296	1,468,313
Liabilities:
Policyholders’ account balances - investment contracts	431,293	281,582
Cash collateral for loaned securities	0	0
Short-term debt	0	0
Long-term debt	0	0
Reinsurance payables	56,348	59,588
Payables to parent and affiliates	0	0
Other liabilities	0	0
Separate account liabilities - investment contracts	0	0
Total liabilities	$ 487,641	341,170
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		6,400
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		$ 6,000