Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred acquisition costs, net	$ 154,717	$ 203,842	$ 258,793
Loss and loss adjustment expense reserves	1,111,692	937,157	720,570
Unearned premium	524,768	602,936	649,518
Net premiums earned	700,142	621,442	547,058
Net investment income	282,560	(251,433)	391,953
Other expenses	16,619	9,610	12,674
Loss and loss adjustment expenses incurred, net	403,499	438,414	370,058
Amortization of deferred acquisition costs, net	295,626	206,498	188,904
Other operating expenses	53,763	36,241	53,103
Net premiums written	622,581	558,357	639,145
Property and Casualty Reinsurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred acquisition costs, net	154,717	203,842	258,793
Loss and loss adjustment expense reserves	1,111,692	937,157	720,570
Unearned premium	524,768	602,936	649,518
Net premiums earned	700,142	621,442	547,058
Net investment income	0	0	0
Other expenses	0	0	0
Loss and loss adjustment expenses incurred, net	403,499	438,414	370,058
Amortization of deferred acquisition costs, net	295,626	206,498	188,904
Other operating expenses	23,366	18,635	30,656
Net premiums written	622,581	558,357	639,145
Corporate (1)
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred acquisition costs, net	0	0	0
Loss and loss adjustment expense reserves	0	0	0
Unearned premium	0	0	0
Net premiums earned	0	0	0
Net investment income	282,560	(251,433)	391,953
Other expenses	16,619	9,610	12,674
Loss and loss adjustment expenses incurred, net	0	0	0
Amortization of deferred acquisition costs, net	0	0	0
Other operating expenses	30,397	17,606	22,447
Net premiums written	$ 0	$ 0	$ 0