Fair Value of Assets and Liabilities (Financial Instruments where Carrying Amounts and Fair Values May Differ) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Assets:
Policy loans	$ 12,582	$ 12,558
Short-term investments		711,071
Cash and cash equivalents	1,380,913	1,639,939	$ 3,378,271	$ 1,848,039
Accrued investment income	83,912	88,331
Reinsurance recoverables	520,111	563,428
Receivables from parent and affiliates	51,809	49,351
Liabilities:
Cash collateral for loaned securities	11,539	17,383
Short-term debt	187,800	43,734
Long-term debt	787,595	928,165
Reinsurance payables	247,971	262,588
Fair Value
Assets:
Commercial mortgage and other loans	1,357,670	1,396,167
Policy loans	12,582	12,558
Short-term investments	7,750
Cash and cash equivalents	780,413	493,473
Accrued investment income	83,912	88,331
Reinsurance recoverables	58,304	59,588
Receivables from parent and affiliates	13,948	11,206
Other assets	12,306	13,802
Total assets	2,326,885	2,075,125
Liabilities:
Policyholders’ account balances - investment contracts	343,540	281,582
Cash collateral for loaned securities	11,539	17,383
Short-term debt	187,800	43,734
Long-term debt	788,042	1,003,251
Reinsurance payables	58,304	59,588
Payables to parent and affiliates	34,943	36,026
Other liabilities	151,108	135,556
Separate account liabilities - investment contracts	90	102
Total liabilities	1,575,366	1,577,222
Carrying Amount
Assets:
Commercial mortgage and other loans	1,376,710	1,387,012
Policy loans	12,582	12,558
Short-term investments	7,750
Cash and cash equivalents	780,413	493,473
Accrued investment income	83,912	88,331
Reinsurance recoverables	58,304	59,588
Receivables from parent and affiliates	13,948	11,206
Other assets	12,306	13,802
Total assets	2,345,925	2,065,970
Liabilities:
Policyholders’ account balances - investment contracts	349,796	281,051
Cash collateral for loaned securities	11,539	17,383
Short-term debt	187,800	43,734
Long-term debt	787,595	928,165
Reinsurance payables	58,304	59,588
Payables to parent and affiliates	34,943	36,026
Other liabilities	151,108	135,556
Separate account liabilities - investment contracts	90	102
Total liabilities	1,581,175	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	176,105	493,473
Accrued investment income	0	0
Reinsurance recoverables	0	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	183,855	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	604,308	0
Accrued investment income	83,912	88,331
Reinsurance recoverables	0	0
Receivables from parent and affiliates	13,948	11,206
Other assets	12,306	13,802
Total assets	714,474	113,339
Liabilities:
Policyholders’ account balances - investment contracts	0	0
Cash collateral for loaned securities	11,539	17,383
Short-term debt	187,800	43,734
Long-term debt	788,042	1,003,251
Reinsurance payables	0	0
Payables to parent and affiliates	34,943	36,026
Other liabilities	151,108	135,556
Separate account liabilities - investment contracts	90	102
Total liabilities	1,173,522	1,236,052
Level 3 | Fair Value
Assets:
Commercial mortgage and other loans	1,357,670	1,396,167
Policy loans	12,582	12,558
Short-term investments	0
Cash and cash equivalents	0	0
Accrued investment income	0	0
Reinsurance recoverables	58,304	59,588
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	1,428,556	1,468,313
Liabilities:
Policyholders’ account balances - investment contracts	343,540	281,582
Cash collateral for loaned securities	0	0
Short-term debt	0	0
Long-term debt	0	0
Reinsurance payables	58,304	59,588
Payables to parent and affiliates	0	0
Other liabilities	0	0
Separate account liabilities - investment contracts	0	0
Total liabilities	$ 401,844	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