Investments - Contractual Maturities (Details) - Short-term investments and fixed maturities $ in Thousands	Dec. 31, 2019 USD ($)
Amortized Cost
One year or less	$ 537,944
More than one year through two years	801,473
More than two years through five years	2,175,547
More than five years through ten years	2,108,172
More than ten years	1,584,345
Fixed maturity investments, at amortized cost	9,336,295
Fair Value
One year or less	536,156
More than one year through two years	808,071
More than two years through five years	2,212,834
More than five years through ten years	2,198,505
More than ten years	1,710,055
Trading securities, debt	$ 9,612,888
% of Total Fair Value
One year or less	5.60%
More than one year through two years	8.40%
More than two years through five years	23.00%
More than five years through ten years	22.90%
More than ten years	17.80%
Trading securities, percentage of fair value	100.00%
Residential mortgage-backed
Amortized Cost
Securities without single maturity date	$ 466,932
Fair Value
Securities without single maturity date	$ 471,836
% of Total Fair Value
Securities without single maturity date	4.90%
Commercial mortgage-backed
Amortized Cost
Securities without single maturity date	$ 882,603
Fair Value
Securities without single maturity date	$ 900,029
% of Total Fair Value
Securities without single maturity date	9.40%
Asset-backed
Amortized Cost
Securities without single maturity date	$ 779,279
Fair Value
Securities without single maturity date	$ 775,402
% of Total Fair Value
Securities without single maturity date	8.00%