Supplemental Guarantor Information - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Assets
Equity securities	$ 1,289,840	$ 1,177,796
Debt securities	737,039	569,648
Other investments, at fair value	52,882	83,394
Total investments in securities and commodities	2,079,761	1,830,838
Cash and cash equivalents	10,819	28,734	$ 32,693	$ 31,625
Restricted cash and cash equivalents	604,428	417,307
Investment in subsidiaries	0	0
Due from brokers	303,597	58,241
Securities purchased under an agreement to sell	0	29,852
Derivative assets, at fair value	27,337	21,130
Interest and dividends receivable	10,030	2,602
Reinsurance balances receivable	314,693	303,649
Deferred acquisition costs, net	192,451	155,901
Unearned premiums ceded	808	0
Loss and loss adjustment expenses recoverable	184	814	1,412	9,277
Other assets	14,231	3,512
Amounts due from (to) affiliates	0	0
Total assets	3,558,339	2,852,580
Liabilities
Accounts payable and accrued expenses	12,298	10,085
Reinsurance balances payable	34,833	27,040
Deposit liabilities	167,210	145,430		120,946
Unearned premium reserves	567,565	433,809
Loss and loss adjustment expense reserves	420,649	277,362
Securities sold, not yet purchased, at fair value	172,074	82,485
Due to brokers	695,019	312,609
Derivative liabilities, at fair value	22,495	11,015
Interest and dividends payable	1,673	697
Senior notes payable, net of deferred costs	113,332	0
Total liabilities	2,207,148	1,300,532
Shareholders’ equity
Common shares	10,548	10,447
Additional paid-in capital	1,078,327	1,065,489
Retained earnings	246,394	375,977
Shareholders’ equity attributable to shareholders	1,335,269	1,451,913
Non-controlling interests	15,922	100,135
Total shareholders’ equity	1,351,191	1,552,048	1,543,638
Total liabilities and shareholders’ equity	3,558,339	2,852,580
Eliminations
Assets
Equity securities	0	0
Debt securities	0	0
Other investments, at fair value	0	0
Total investments in securities and commodities	0	0
Cash and cash equivalents	0	0	0	0
Restricted cash and cash equivalents	0	0
Investment in subsidiaries	(1,751,521)	(1,451,060)
Due from brokers	0	0
Securities purchased under an agreement to sell		0
Derivative assets, at fair value	0	0
Interest and dividends receivable	0	0
Reinsurance balances receivable	0	0
Deferred acquisition costs, net	0	0
Unearned premiums ceded	0
Loss and loss adjustment expenses recoverable	0	0
Other assets	0	0
Amounts due from (to) affiliates	0	0
Total assets	(1,751,521)	(1,451,060)
Liabilities
Accounts payable and accrued expenses	0	0
Reinsurance balances payable	0	0
Deposit liabilities	0	0
Unearned premium reserves	0	0
Loss and loss adjustment expense reserves	0	0
Securities sold, not yet purchased, at fair value	0	0
Due to brokers	0	0
Derivative liabilities, at fair value	0	0
Interest and dividends payable	0	0
Senior notes payable, net of deferred costs	0
Total liabilities	0	0
Shareholders’ equity
Common shares	(1,250)	(1,251)
Additional paid-in capital	(1,666,405)	(1,072,671)
Retained earnings	(83,866)	(377,138)
Shareholders’ equity attributable to shareholders	(1,751,521)	(1,451,060)
Non-controlling interests	0	0
Total shareholders’ equity	(1,751,521)	(1,451,060)
Total liabilities and shareholders’ equity	(1,751,521)	(1,451,060)
Parent Guarantor | Reportable Legal Entities
Assets
Equity securities	0	0
Debt securities	0	0
Other investments, at fair value	0	0
Total investments in securities and commodities	0	0
Cash and cash equivalents	2,517	140	492	294
Restricted cash and cash equivalents	0	0
Investment in subsidiaries	1,336,802	1,451,060
Due from brokers	0	0
Securities purchased under an agreement to sell		0
Derivative assets, at fair value	0	0
Interest and dividends receivable	0	0
Reinsurance balances receivable	0	0
Deferred acquisition costs, net	0	0
Unearned premiums ceded	0
Loss and loss adjustment expenses recoverable	0	0
Other assets	792	600
Amounts due from (to) affiliates	(41)	1,339
Total assets	1,340,070	1,453,139
Liabilities
Accounts payable and accrued expenses	4,801	1,226
Reinsurance balances payable	0	0
Deposit liabilities	0	0
Unearned premium reserves	0	0
Loss and loss adjustment expense reserves	0	0
Securities sold, not yet purchased, at fair value	0	0
Due to brokers	0	0
Derivative liabilities, at fair value	0	0
Interest and dividends payable	0	0
Senior notes payable, net of deferred costs	0
Total liabilities	4,801	1,226
Shareholders’ equity
Common shares	10,548	10,447
Additional paid-in capital	1,078,327	1,065,489
Retained earnings	246,394	375,977
Shareholders’ equity attributable to shareholders	1,335,269	1,451,913
Non-controlling interests	0	0
Total shareholders’ equity	1,335,269	1,451,913
Total liabilities and shareholders’ equity	1,340,070	1,453,139
Issuer of Debt | Reportable Legal Entities
Assets
Equity securities	0	0
Debt securities	0	0
Other investments, at fair value	0	0
Total investments in securities and commodities	0	0
Cash and cash equivalents	28	0	0	0
Restricted cash and cash equivalents	0	0
Investment in subsidiaries	255,462	0
Due from brokers	0	0
Securities purchased under an agreement to sell		0
Derivative assets, at fair value	0	0
Interest and dividends receivable	0	0
Reinsurance balances receivable	0	0
Deferred acquisition costs, net	0	0
Unearned premiums ceded	0
Loss and loss adjustment expenses recoverable	0	0
Other assets	1,884	666
Amounts due from (to) affiliates	(230)	(403)
Total assets	257,144	263
Liabilities
Accounts payable and accrued expenses	55	518
Reinsurance balances payable	0	0
Deposit liabilities	0	0
Unearned premium reserves	0	0
Loss and loss adjustment expense reserves	0	0
Securities sold, not yet purchased, at fair value	0	0
Due to brokers	0	0
Derivative liabilities, at fair value	0	0
Interest and dividends payable	1,026	0
Senior notes payable, net of deferred costs	113,332
Total liabilities	114,413	518
Shareholders’ equity
Common shares	0	0
Additional paid-in capital	159,257	0
Retained earnings	(16,526)	(255)
Shareholders’ equity attributable to shareholders	142,731	(255)
Non-controlling interests	0	0
Total shareholders’ equity	142,731	(255)
Total liabilities and shareholders’ equity	257,144	263
Non-Guarantor Subsidiaries | Reportable Legal Entities
Assets
Equity securities	1,289,840	1,177,796
Debt securities	737,039	569,648
Other investments, at fair value	52,882	83,394
Total investments in securities and commodities	2,079,761	1,830,838
Cash and cash equivalents	8,274	28,594	$ 32,201	$ 31,331
Restricted cash and cash equivalents	604,428	417,307
Investment in subsidiaries	159,257	0
Due from brokers	303,597	58,241
Securities purchased under an agreement to sell		29,852
Derivative assets, at fair value	27,337	21,130
Interest and dividends receivable	10,030	2,602
Reinsurance balances receivable	314,693	303,649
Deferred acquisition costs, net	192,451	155,901
Unearned premiums ceded	808
Loss and loss adjustment expenses recoverable	184	814
Other assets	11,555	2,246
Amounts due from (to) affiliates	271	(936)
Total assets	3,712,646	2,850,238
Liabilities
Accounts payable and accrued expenses	7,442	8,341
Reinsurance balances payable	34,833	27,040
Deposit liabilities	167,210	145,430
Unearned premium reserves	567,565	433,809
Loss and loss adjustment expense reserves	420,649	277,362
Securities sold, not yet purchased, at fair value	172,074	82,485
Due to brokers	695,019	312,609
Derivative liabilities, at fair value	22,495	11,015
Interest and dividends payable	647	697
Senior notes payable, net of deferred costs	0
Total liabilities	2,087,934	1,298,788
Shareholders’ equity
Common shares	1,250	1,251
Additional paid-in capital	1,507,148	1,072,671
Retained earnings	100,392	377,393
Shareholders’ equity attributable to shareholders	1,608,790	1,451,315
Non-controlling interests	15,922	100,135
Total shareholders’ equity	1,624,712	1,551,450
Total liabilities and shareholders’ equity	3,712,646	$ 2,850,238
Financial Guarantee | Issuer of Debt
Condensed Balance Sheet Statements, Captions [Line Items]
Debt obligation	$ 115,000