Investments (Contractual Maturity) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Principal Balance by Contractual Maturity, Amount
Due in one year or less	$ 371,423
Due after one year through five years	2,885,587
Due after five years through ten years	2,486,014
Totals	7,285,254	$ 7,247,024
Commercial Mortgage Loans
Principal Balance by Contractual Maturity, Amount
Due in one year or less	23,839	22,966
Due after one year through five years	39,391	41,248
Due after five years through ten years	134,574	119,966
Due after ten years through fifteen years	6,642	25,429
Totals	$ 204,446	$ 209,609
Principal Balance by Contractual Maturity, Percentage
Due in one year or less (in percentage)	11.70%	11.00%
Due after one year through five years (in percentage)	19.30%	19.70%
Due after five years through ten years (in percentage)	65.80%	57.20%
Due after ten years through fifteen years (in percentage)	3.20%	12.10%
Totals (in percentage)	100.00%	100.00%