Segment Information - Long Lived Assets and Total Assets of Business Segments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Segment Reporting Information
Premiums and other receivables, net	$ 1,598,316	$ 1,487,734	$ 1,428,948
Deferred acquisition costs	284,963		263,523
Reinsurance recoverable, net	1,286,852	$ 1,292,636	1,394,308
Prepaid reinsurance premiums	458,960		575,747
Goodwill and Intangible assets, net	525,302		545,151
Prepaid and other assets	66,890		94,642
Corporate and other assets	6,002,973		5,454,215
Total assets	10,224,256		9,756,534
Property and Casualty
Segment Reporting Information
Premiums and other receivables, net	1,379,512		1,292,813
Deferred acquisition costs	257,294		239,293
Reinsurance recoverable, net	1,260,791		1,377,284
Prepaid reinsurance premiums	458,578		575,712
Goodwill and Intangible assets, net	420,779		436,724
Prepaid and other assets	49,533		56,960
Corporate and other assets	0		0
Total assets	3,826,487		3,978,786
Accident and Health
Segment Reporting Information
Premiums and other receivables, net	197,487		131,877
Deferred acquisition costs	27,669		24,230
Reinsurance recoverable, net	26,061		17,024
Prepaid reinsurance premiums	382		35
Goodwill and Intangible assets, net	104,523		108,427
Prepaid and other assets	12,257		32,852
Corporate and other assets	0		0
Total assets	368,379		314,445
Corporate and Other
Segment Reporting Information
Premiums and other receivables, net	21,317		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	5,100		4,830
Corporate and other assets	6,002,973		5,454,215
Total assets	$ 6,029,390		$ 5,463,303