Investments (Fixed Income Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Fixed Income Securities [Line Items]
Due in one year or less fair value, AFS	$ 336,142
Due after one year through five years fair value, AFS	3,714,414
Due after five years through 10 years fair value, AFS	1,960,132
Due after 10 years fair value, AFS	426,849
AFS, Fair Value	6,437,537	$ 6,095,620
Due in one year or less carrying value, HTM	1,456
Due after one year through five years carrying value, HTM	15,131
Due after five years through 10 years carrying value, HTM	2,500
Due after 10 years carrying value, HTM	0
HTM, Carrying Value	19,087	20,800
Due in one year or less fair value, HTM	1,490
Due after one year through five years fair value, HTM	16,293
Due after five years through 10 years fair value, HTM	2,430
Due after 10 years fair value, HTM	0
HTM, Fair Value	$ 20,213	$ 21,975