Investments (Fixed Maturity Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
AFS due in one year or less fair value	$ 433,241
AFS due after one year through five years fair value	3,639,658
AFS due after five years through 10 years fair value	1,910,480
AFS due after 10 years fair value	472,549
Fixed income securities, available-for-sale securities	6,455,928	$ 6,095,620
HTM due in one year or less carrying value	1,132
HTM due after one year through five years carrying value	15,692
HTM due after five years through 10 years carrying value	0
HTM due after 10 years carrying value	0
Total HTM fixed income securities carrying value	16,824	20,800
HTM due in one year or less fair value	1,149
HTM due after one year through five years fair value	16,852
HTM due after five years through 10 years fair value	0
HTM due after 10 years fair value	0
Total HTM fixed income securities fair value	$ 18,001	$ 21,975