Schedule III - Supplementary insurance information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Deferred acquisition costs	$ 213,816	$ 209,227	$ 181,002
Reserve for losses and loss expenses	4,831,390	2,995,195	2,996,567
Unearned premiums	1,147,186	1,076,049	966,210
Net premiums earned	2,581,088	2,249,165	2,246,889
Net investment income	177,873	150,385	127,824
Losses and loss expenses	2,300,178	1,065,097	977,833
Amortization of deferred acquisition costs	471,553	449,482	410,058
Other operating expenses	392,248	379,201	402,050
Net premiums written	2,481,305	2,359,000	2,228,825
Reinsurance Segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred acquisition costs	83,778	81,545	61,597
Reserve for losses and loss expenses	1,816,654	1,193,497	1,237,245
Unearned premiums	393,289	380,071	310,588
Net premiums earned	1,023,004	996,149	1,047,983
Net investment income	0	0	0
Losses and loss expenses	692,719	415,505	467,788
Amortization of deferred acquisition costs	199,430	189,797	173,574
Other operating expenses	90,939	96,668	105,668
Net premiums written	985,918	1,063,581	1,036,156
Insurance Segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred acquisition costs	125,167	125,239	118,218
Reserve for losses and loss expenses	2,300,437	1,753,164	1,748,309
Unearned premiums	735,606	669,865	637,003
Net premiums earned	1,256,085	1,002,944	1,038,971
Net investment income	0	0	0
Losses and loss expenses	934,199	604,741	509,388
Amortization of deferred acquisition costs	241,186	232,780	220,157
Other operating expenses	219,960	180,516	191,587
Net premiums written	1,192,078	1,031,468	1,021,081
Asset Management Segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred acquisition costs	5,578	4,060	2,526
Reserve for losses and loss expenses	714,299	48,534	11,013
Unearned premiums	39,087	38,961	24,643
Net premiums earned	301,999	250,072	159,935
Net investment income	24,918	9,264	6,658
Losses and loss expenses	673,260	44,851	657
Amortization of deferred acquisition costs	30,937	26,905	16,327
Other operating expenses	34,687	34,582	39,635
Net premiums written	303,309	263,951	171,588
Corporate & Eliminations
Supplementary Insurance Information, by Segment [Line Items]
Deferred acquisition costs	(707)	(1,617)	(1,339)
Reserve for losses and loss expenses	0	0	0
Unearned premiums	(20,796)	(12,848)	(6,024)
Net premiums earned	0	0	0
Net investment income	152,955	141,121	121,166
Losses and loss expenses	0	0	0
Amortization of deferred acquisition costs	0	0	0
Other operating expenses	46,662	67,435	65,160
Net premiums written	$ 0	$ 0	$ 0