Investments (Fixed Income Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Fixed Income Securities [Line Items]
Due in one year or less fair value, AFS	$ 453,772
Due after one year through five years fair value, AFS	3,400,223
Due after five years through 10 years fair value, AFS	2,043,563
Due after 10 years fair value, AFS	624,083
Total AFS fixed income securities, Fair Value	6,521,641	$ 6,455,928
Due in one year or less carrying value, HTM	1,057
Due after one year through five years carrying value, HTM	14,433
Due after five years through 10 years carrying value, HTM	8,854
Due after 10 years carrying value, HTM	0
Total HTM fixed income securities carrying value	24,344	16,824
Due in one year or less fair value, HTM	1,065
Due after one year through five years fair value, HTM	15,325
Due after five years through 10 years fair value, HTM	8,814
Due after 10 years fair value, HTM	0
Total HTM fixed income securities, Fair Value	$ 25,204	$ 18,001