Segment Information - Long Lived Assets and Total Assets of Business Segments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Segment Reporting Information
Premiums and other receivables, net	$ 1,541,760	$ 1,428,948
Deferred acquisition costs	277,368	263,523
Reinsurance recoverable, net	1,401,681	1,394,308
Prepaid reinsurance premiums	511,648	575,747
Goodwill and Intangible assets, net	536,896	545,151
Prepaid and other assets	86,359	94,642
Corporate and other assets	5,504,862	5,454,215
Total assets	9,860,574	9,756,534
Property and Casualty
Segment Reporting Information
Premiums and other receivables, net	1,373,437	1,292,813
Deferred acquisition costs	253,028	239,293
Reinsurance recoverable, net	1,380,374	1,377,284
Prepaid reinsurance premiums	511,613	575,712
Goodwill and Intangible assets, net	429,784	436,724
Prepaid and other assets	53,545	56,960
Corporate and other assets	0	0
Total assets	4,001,781	3,978,786
Accident and Health
Segment Reporting Information
Premiums and other receivables, net	167,928	131,877
Deferred acquisition costs	24,340	24,230
Reinsurance recoverable, net	21,307	17,024
Prepaid reinsurance premiums	35	35
Goodwill and Intangible assets, net	107,112	108,427
Prepaid and other assets	27,577	32,852
Corporate and other assets	0	0
Total assets	348,299	314,445
Corporate and Other
Segment Reporting Information
Premiums and other receivables, net	395	4,258
Deferred acquisition costs	0	0
Reinsurance recoverable, net	0	0
Prepaid reinsurance premiums	0	0
Goodwill and Intangible assets, net	0	0
Prepaid and other assets	5,237	4,830
Corporate and other assets	5,504,862	5,454,215
Total assets	$ 5,510,494	$ 5,463,303