Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Gross Amount										$ 5,681,523	$ 4,838,902	$ 4,447,457
Ceded to other Companies										(2,099,893)	(1,614,257)	(1,407,052)
Assumed From Other Companies										2,457,437	2,122,102	1,920,968
Net Amount		$ 1,455,453	$ 1,613,457	$ 1,444,898	$ 1,525,259	$ 1,301,754	$ 1,333,553	$ 1,298,896	$ 1,412,544	$ 6,039,067	$ 5,346,747	$ 4,961,373
Percentage of Amount Assumed to Net										40.70%	39.70%	38.70%
Other
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Gross Amount										$ 339,169	$ 253,760	$ 133,858
Ceded to other Companies	[1]									(222,019)	(130,840)	(47,187)
Assumed From Other Companies	[1]									415,712	481,255	466,446
Net Amount										$ 532,862	$ 604,175	$ 553,117
Percentage of Amount Assumed to Net										78.00%	79.70%	84.30%
Underwriting segments
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Ceded to other Companies										$ (2,189,440)	$ (1,791,851)	$ (1,679,998)
Net Amount										5,506,205	4,742,572	4,408,256
Underwriting segments | Insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Gross Amount										3,879,752	3,232,234	3,050,876
Ceded to other Companies	[1]									(1,266,267)	(1,050,207)	(958,646)
Assumed From Other Companies	[1]									28,241	30,098	30,210
Net Amount										$ 2,641,726	$ 2,212,125	$ 2,122,440
Percentage of Amount Assumed to Net										1.10%	1.40%	1.40%
Underwriting segments | Reinsurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Gross Amount										$ 238,229	$ 213,809	$ 152,404
Ceded to other Companies	[1]									(720,500)	(539,950)	(465,925)
Assumed From Other Companies	[1]									2,084,994	1,698,713	1,487,995
Net Amount										$ 1,602,723	$ 1,372,572	$ 1,174,474
Percentage of Amount Assumed to Net										130.10%	123.80%	126.70%
Underwriting segments | Mortgage
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Gross Amount										$ 1,224,373	$ 1,139,099	$ 1,110,319
Ceded to other Companies	[1]									(204,509)	(202,833)	(256,796)
Assumed From Other Companies	[1]									241,892	221,609	257,819
Net Amount										$ 1,261,756	$ 1,157,875	$ 1,111,342
Percentage of Amount Assumed to Net										19.20%	19.10%	23.20%

[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.