Investments - Asset Types (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale	$ 2,041,724	$ 151,609
Total investments	9,612,888	8,697,892
U.S. government and agency
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale	410,312	573
Total investments	736,043	510,245
U.K. government
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale	15,349
Total investments	161,772	300,631
Other government
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale	85,147	73,185
Total investments	702,857	793,810
Corporate
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale	1,081,171	75,359
Total investments	5,697,067	4,839,840
Municipal
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale	15,872	2,480
Total investments	167,882	130,265
Residential mortgage-backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale	127,219	12
Total investments	471,836	773,557
Commercial mortgage-backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale	98,557
Total investments	900,029	713,063
Asset-backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale	208,097
Total investments	775,402	636,481
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	356,900	656,600
Senior Secured Debt Securities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	31,400	20,400
Short-term Investments
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	51,490	114,116
Debt securities, available-for-sale	128,335	0
Short-term Investments | U.S. government and agency
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	0	45,885
Debt securities, available-for-sale	111,583
Short-term Investments | U.K. government
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	24,411	2,275
Debt securities, available-for-sale	1,069
Short-term Investments | Other government
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	21,958	19,064
Debt securities, available-for-sale	387
Short-term Investments | Corporate
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	5,121	44,900
Debt securities, available-for-sale	13,915
Short-term Investments | Municipal
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	0	0
Debt securities, available-for-sale	1,381
Short-term Investments | Residential mortgage-backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	0	0
Debt securities, available-for-sale	0
Short-term Investments | Commercial mortgage-backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	0	0
Debt securities, available-for-sale	0
Short-term Investments | Asset-backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	0	1,992
Debt securities, available-for-sale	0
Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	6,346,329	7,248,793
Debt securities, available-for-sale	1,913,389	151,609
Fixed Maturities | U.S. government and agency
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	219,194	389,735
Debt securities, available-for-sale	298,729	573
Fixed Maturities | U.K. government
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	122,012	298,356
Debt securities, available-for-sale	14,280	0
Fixed Maturities | Other government
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	575,018	679,525
Debt securities, available-for-sale	84,760	73,185
Fixed Maturities | Corporate
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	4,007,386	4,081,793
Debt securities, available-for-sale	1,067,256	75,359
Fixed Maturities | Municipal
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	102,554	73,856
Debt securities, available-for-sale	14,491	2,480
Fixed Maturities | Residential mortgage-backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	258,412	682,962
Debt securities, available-for-sale	127,219	12
Fixed Maturities | Commercial mortgage-backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	571,129	488,598
Debt securities, available-for-sale	98,557	0
Fixed Maturities | Asset-backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	490,624	553,968
Debt securities, available-for-sale	208,097	0
Funds Held - Directly Managed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	1,173,345	1,183,374
Funds Held - Directly Managed | U.S. government and agency
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	106,537	74,052
Funds Held - Directly Managed | U.K. government
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	0	0
Funds Held - Directly Managed | Other government
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	20,734	22,036
Funds Held - Directly Managed | Corporate
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	603,389	637,788
Funds Held - Directly Managed | Municipal
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	49,456	53,929
Funds Held - Directly Managed | Residential mortgage-backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	86,205	90,583
Funds Held - Directly Managed | Commercial mortgage-backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	230,343	224,465
Funds Held - Directly Managed | Asset-backed
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading	$ 76,681	$ 80,521