Fair Value of Assets and Liabilities (Financial Instruments where Carrying Amounts and Fair Values May Differ) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets:
Policy loans	$ 12,664	$ 12,805
Cash and cash equivalents	2,396,595	4,503,534	$ 2,272,907	$ 1,639,939
Accrued investment income	96,245	90,895
Reinsurance recoverables	592,751	572,102
Receivables from parent and affiliates	15,676	46,381
Liabilities:
Cash collateral for loaned securities	0	384
Short-term debt	140,569	140,569
Long-term debt	653,596	787,596
Reinsurance payables	268,835	232,937
Fair Value
Assets:
Commercial mortgage and other loans	1,280,080	1,339,707
Policy loans	12,664	12,805
Short-term investments	7,750	7,750
Cash and cash equivalents	1,065,067	811,175
Accrued investment income	96,245	90,895
Reinsurance recoverables	54,113	55,236
Receivables from parent and affiliates	12,505	9,188
Other assets	5,823	3,735
Total assets	2,534,247	2,330,491
Liabilities:
Policyholders’ account balances - investment contracts	752,629	560,548
Cash collateral for loaned securities	0	384
Short-term debt	140,329	139,843
Long-term debt	675,041	791,670
Reinsurance payables	54,113	55,236
Payables to parent and affiliates	45,018	30,846
Other liabilities	142,407	554,162
Separate account liabilities - investment contracts	70	71
Total liabilities	1,809,607	2,132,760
Carrying Amount
Assets:
Commercial mortgage and other loans	1,268,377	1,353,478
Policy loans	12,664	12,805
Short-term investments	7,750	7,750
Cash and cash equivalents	1,065,067	811,175
Accrued investment income	96,245	90,895
Reinsurance recoverables	54,113	55,236
Receivables from parent and affiliates	12,505	9,188
Other assets	5,823	3,735
Total assets	2,522,544	2,344,262
Liabilities:
Policyholders’ account balances - investment contracts	753,178	565,903
Cash collateral for loaned securities	0	384
Short-term debt	140,569	140,569
Long-term debt	653,596	787,596
Reinsurance payables	54,113	55,236
Payables to parent and affiliates	45,018	30,846
Other liabilities	142,407	554,162
Separate account liabilities - investment contracts	70	71
Total liabilities	1,788,951	2,134,767
Level 1 | Fair Value
Assets:
Commercial mortgage and other loans	0	0
Policy loans	0	0
Short-term investments	7,750	7,750
Cash and cash equivalents	62,067	136,175
Accrued investment income	0	0
Reinsurance recoverables	0	0
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	69,817	143,925
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	0
Cash and cash equivalents	1,003,000	675,000
Accrued investment income	96,245	90,895
Reinsurance recoverables	0	0
Receivables from parent and affiliates	12,505	9,188
Other assets	5,823	3,735
Total assets	1,117,573	778,818
Liabilities:
Policyholders’ account balances - investment contracts	0	0
Cash collateral for loaned securities	0	384
Short-term debt	140,329	139,843
Long-term debt	675,041	791,670
Reinsurance payables	0	0
Payables to parent and affiliates	45,018	30,846
Other liabilities	142,407	554,162
Separate account liabilities - investment contracts	70	71
Total liabilities	1,002,865	1,516,976
Level 3 | Fair Value
Assets:
Commercial mortgage and other loans	1,280,080	1,339,707
Policy loans	12,664	12,805
Short-term investments	0	0
Cash and cash equivalents	0	0
Accrued investment income	0	0
Reinsurance recoverables	54,113	55,236
Receivables from parent and affiliates	0	0
Other assets	0	0
Total assets	1,346,857	1,407,748
Liabilities:
Policyholders’ account balances - investment contracts	752,629	560,548
Cash collateral for loaned securities	0	0
Short-term debt	0	0
Long-term debt	0	0
Reinsurance payables	54,113	55,236
Payables to parent and affiliates	0	0
Other liabilities	0	0
Separate account liabilities - investment contracts	0	0
Total liabilities	$ 806,742	$ 615,784