Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Gross Amount		$ 7,706,832	$ 6,553,910	$ 5,681,523
Ceded to other Companies		(3,734,150)	(2,650,352)	(2,099,893)
Assumed From Other Companies		5,045,655	3,534,158	2,457,437
Net Amount		$ 9,018,337	$ 7,437,716	$ 6,039,067
Percentage of Amount Assumed to Net		55.90%	47.50%	40.70%
Operating segments | Insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Gross Amount		$ 5,833,873	$ 4,659,416	$ 3,879,752
Ceded to other Companies	[1]	(1,719,541)	(1,525,655)	(1,266,267)
Assumed From Other Companies	[1]	33,861	29,146	28,241
Net Amount		$ 4,148,193	$ 3,162,907	$ 2,641,726
Percentage of Amount Assumed to Net		0.80%	0.90%	1.10%
Operating segments | Reinsurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Gross Amount		$ 408,520	$ 305,435	$ 238,229
Ceded to other Companies	[1]	(1,839,556)	(1,014,716)	(720,500)
Assumed From Other Companies	[1]	4,685,410	3,166,651	2,084,994
Net Amount		$ 3,254,374	$ 2,457,370	$ 1,602,723
Percentage of Amount Assumed to Net		144.00%	128.90%	130.10%
Operating segments | Mortgage
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Gross Amount		$ 1,213,333	$ 1,192,316	$ 1,224,373
Ceded to other Companies	[1]	(246,757)	(194,149)	(204,509)
Assumed From Other Companies	[1]	294,492	281,683	241,892
Net Amount		$ 1,261,068	$ 1,279,850	$ 1,261,756
Percentage of Amount Assumed to Net		23.40%	22.00%	19.20%
Operating segments | Other
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Gross Amount		$ 251,106	$ 396,743	$ 339,169
Ceded to other Companies	[1]	(102,763)	(190,957)	(222,019)
Assumed From Other Companies	[1]	206,359	331,803	415,712
Net Amount		$ 354,702	$ 537,589	$ 532,862
Percentage of Amount Assumed to Net		58.20%	61.70%	78.00%

[1] Certain amounts included in the gross premiums written of each segment are related to intersegment transactions and are included in the gross premiums written of each segment. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.