Segment Information - Summary of net premiums written and earned by major line of business and net premiums written by location (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Net premiums written	$ 895,481	$ 959,539	$ 971,928	$ 1,064,990	$ 748,921	$ 839,135	$ 810,535	$ 952,776	$ 3,891,938		$ 3,351,367		$ 3,052,235
Net premiums earned	923,165	903,651	907,152	859,780	839,366	795,000	758,816	752,770	3,593,748		3,145,952		2,935,140
Reinsurance
Segment Reporting Information [Line Items]
Net premiums written									1,265,991		1,313,001		1,158,790
Insurance
Segment Reporting Information [Line Items]
Net premiums written									2,146,654		1,948,796		1,825,334
Mortgage
Segment Reporting Information [Line Items]
Net premiums written									204,837		89,570		68,111
Other
Segment Reporting Information [Line Items]
Net premiums written									274,456		0		0
Net premiums earned									103,477		0		0
Other | Client Location [Member] | United States
Segment Reporting Information [Line Items]
Net premiums written									186,795	[1]	0	[1]	0	[1]
Other | Client Location [Member] | Europe
Segment Reporting Information [Line Items]
Net premiums written									54,131	[1]	0	[1]	0	[1]
Other | Client Location [Member] | Bermuda
Segment Reporting Information [Line Items]
Net premiums written									27,977	[1]	0	[1]	0	[1]
Other | Client Location [Member] | Other geographic location
Segment Reporting Information [Line Items]
Net premiums written									5,553	[1]	0	[1]	0	[1]
Other | Other specialty [Member]
Segment Reporting Information [Line Items]
Net premiums written									66,524	[1],[2]	0	[1],[2]	0	[1],[2]
Net premiums earned									24,157	[1],[2]	0	[1],[2]	0	[1],[2]
Other | Property excluding property catastrophe [Member]
Segment Reporting Information [Line Items]
Net premiums written									564	[1]	0	[1]	0	[1]
Net premiums earned									182	[1]	0	[1]	0	[1]
Other | Casualty [Member]
Segment Reporting Information [Line Items]
Net premiums written									179,054	[1],[3]	0	[1],[3]	0	[1],[3]
Net premiums earned									72,395	[1],[3]	0	[1],[3]	0	[1],[3]
Other | Property catastrophe [Member]
Segment Reporting Information [Line Items]
Net premiums written									9,280	[1]	0	[1]	0	[1]
Net premiums earned									5,115	[1]	0	[1]	0	[1]
Other | Marine and aviation [Member]
Segment Reporting Information [Line Items]
Net premiums written									1,251	[1]	0	[1]	0	[1]
Net premiums earned									789	[1]	0	[1]	0	[1]
Other | Other [Member]
Segment Reporting Information [Line Items]
Net premiums written									17,783	[1],[4]	0	[1],[4]	0	[1],[4]
Net premiums earned									839	[1],[4]	0	[1],[4]	0	[1],[4]
Underwriting segments
Segment Reporting Information [Line Items]
Net premiums written									3,617,482		3,351,367		3,052,235
Net premiums earned									3,490,271		3,145,952		2,935,140
Underwriting segments | Reinsurance
Segment Reporting Information [Line Items]
Net premiums written									1,265,991		1,313,001		1,158,790
Net premiums earned									1,279,328		1,218,672		1,118,127
Underwriting segments | Reinsurance | Pro Rata [Member]
Segment Reporting Information [Line Items]
Net premiums written									663,135	[5]	692,024	[5]	530,763	[5]
Net premiums earned									686,201	[5]	608,586	[5]	499,094	[5]
Underwriting segments | Reinsurance | Excess Of Loss [Member]
Segment Reporting Information [Line Items]
Net premiums written									602,856	[5]	620,977	[5]	628,027	[5]
Net premiums earned									593,127	[5]	610,086	[5]	619,033	[5]
Underwriting segments | Reinsurance | Client Location [Member] | United States
Segment Reporting Information [Line Items]
Net premiums written									589,255	[5]	706,388	[5]	568,043	[5]
Underwriting segments | Reinsurance | Client Location [Member] | Europe
Segment Reporting Information [Line Items]
Net premiums written									355,735	[5]	327,059	[5]	341,653	[5]
Underwriting segments | Reinsurance | Client Location [Member] | Asia and Pacific
Segment Reporting Information [Line Items]
Net premiums written									142,626	[5]	94,252	[5]	97,879	[5]
Underwriting segments | Reinsurance | Client Location [Member] | Bermuda
Segment Reporting Information [Line Items]
Net premiums written									77,620	[5]	87,047	[5]	72,864	[5]
Underwriting segments | Reinsurance | Client Location [Member] | Other geographic location
Segment Reporting Information [Line Items]
Net premiums written									100,755	[5]	98,255	[5]	78,351	[5]
Underwriting segments | Reinsurance | Underwriting Location [Member] | United States
Segment Reporting Information [Line Items]
Net premiums written									492,891	[5]	507,183	[5]	379,239	[5]
Underwriting segments | Reinsurance | Underwriting Location [Member] | Europe
Segment Reporting Information [Line Items]
Net premiums written									394,351	[5]	459,467	[5]	527,909	[5]
Underwriting segments | Reinsurance | Underwriting Location [Member] | Bermuda
Segment Reporting Information [Line Items]
Net premiums written									343,823	[5]	309,129	[5]	225,470	[5]
Underwriting segments | Reinsurance | Underwriting Location [Member] | Other geographic location
Segment Reporting Information [Line Items]
Net premiums written									34,926	[5]	37,222	[5]	26,172	[5]
Underwriting segments | Reinsurance | Other specialty [Member]
Segment Reporting Information [Line Items]
Net premiums written									405,126	[5],[6]	417,865	[5],[6]	308,104	[5],[6]
Net premiums earned									424,725	[5],[6]	387,630	[5],[6]	309,101	[5],[6]
Underwriting segments | Reinsurance | Property excluding property catastrophe [Member]
Segment Reporting Information [Line Items]
Net premiums written									343,043	[5],[7]	292,536	[5],[7]	265,783	[5],[7]
Net premiums earned									303,496	[5],[7]	274,719	[5],[7]	254,338	[5],[7]
Underwriting segments | Reinsurance | Casualty [Member]
Segment Reporting Information [Line Items]
Net premiums written									317,996	[5],[8]	306,304	[5],[8]	205,925	[5],[8]
Net premiums earned									327,518	[5],[8]	241,774	[5],[8]	188,963	[5],[8]
Underwriting segments | Reinsurance | Property catastrophe [Member]
Segment Reporting Information [Line Items]
Net premiums written									137,471	[5]	220,749	[5]	283,677	[5]
Net premiums earned									150,761	[5]	232,423	[5]	280,185	[5]
Underwriting segments | Reinsurance | Marine and aviation [Member]
Segment Reporting Information [Line Items]
Net premiums written									50,444	[5]	64,380	[5]	84,649	[5]
Net premiums earned									61,118	[5]	70,105	[5]	76,145	[5]
Underwriting segments | Reinsurance | Other [Member]
Segment Reporting Information [Line Items]
Net premiums written									11,911	[5],[9]	11,167	[5],[9]	10,652	[5],[9]
Net premiums earned									11,710	[5],[9]	12,021	[5],[9]	9,395	[5],[9]
Underwriting segments | Insurance
Segment Reporting Information [Line Items]
Net premiums written									2,146,654		1,948,796		1,825,334
Net premiums earned									2,017,370		1,876,014		1,800,343
Underwriting segments | Insurance | Client Location [Member] | United States
Segment Reporting Information [Line Items]
Net premiums written									1,726,181	[10]	1,526,156	[10]	1,314,577	[10]
Underwriting segments | Insurance | Client Location [Member] | Europe
Segment Reporting Information [Line Items]
Net premiums written									240,136	[10]	226,254	[10]	271,278	[10]
Underwriting segments | Insurance | Client Location [Member] | Asia and Pacific
Segment Reporting Information [Line Items]
Net premiums written									79,564	[10]	95,970	[10]	120,492	[10]
Underwriting segments | Insurance | Client Location [Member] | Other geographic location
Segment Reporting Information [Line Items]
Net premiums written									100,773	[10]	100,416	[10]	118,987	[10]
Underwriting segments | Insurance | Underwriting Location [Member] | United States
Segment Reporting Information [Line Items]
Net premiums written									1,688,887	[10]	1,478,930	[10]	1,254,623	[10]
Underwriting segments | Insurance | Underwriting Location [Member] | Europe
Segment Reporting Information [Line Items]
Net premiums written									394,430	[10]	389,763	[10]	472,132	[10]
Underwriting segments | Insurance | Underwriting Location [Member] | Other geographic location
Segment Reporting Information [Line Items]
Net premiums written									63,337	[10]	80,103	[10]	98,579	[10]
Underwriting segments | Insurance | Programs [Member]
Segment Reporting Information [Line Items]
Net premiums written									480,580	[10]	419,673	[10]	340,130	[10]
Net premiums earned									460,392	[10]	386,840	[10]	318,740	[10]
Underwriting segments | Insurance | Professional lines [Member]
Segment Reporting Information [Line Items]
Net premiums written									476,604	[10],[11]	476,193	[10],[11]	548,423	[10],[11]
Net premiums earned									456,508	[10],[11]	491,791	[10],[11]	536,971	[10],[11]
Underwriting segments | Insurance | Construction and national accounts [Member]
Segment Reporting Information [Line Items]
Net premiums written									286,994	[10]	271,110	[10]	211,130	[10]
Net premiums earned									277,811	[10]	250,729	[10]	209,217	[10]
Underwriting segments | Insurance | Property, energy, marine and aviation [Member]
Segment Reporting Information [Line Items]
Net premiums written									244,640	[10]	280,551	[10]	294,690	[10]
Net premiums earned									244,974	[10]	304,294	[10]	313,081	[10]
Underwriting segments | Insurance | Excess and surplus casualty [Member]
Segment Reporting Information [Line Items]
Net premiums written									212,519	[10],[12]	149,286	[10],[12]	112,307	[10],[12]
Net premiums earned									182,024	[10],[12]	118,395	[10],[12]	113,597	[10],[12]
Underwriting segments | Insurance | Travel, accident and health [Member]
Segment Reporting Information [Line Items]
Net premiums written									145,732	[10]	104,903	[10]	106,871	[10]
Net premiums earned									127,691	[10]	97,135	[10]	98,686	[10]
Underwriting segments | Insurance | Lenders products [Member]
Segment Reporting Information [Line Items]
Net premiums written									100,407	[10]	101,576	[10]	99,724	[10]
Net premiums earned									94,438	[10]	99,847	[10]	103,478	[10]
Underwriting segments | Insurance | Other [Member]
Segment Reporting Information [Line Items]
Net premiums written									199,178	[10],[13]	145,504	[10],[13]	112,059	[10],[13]
Net premiums earned									173,532	[10],[13]	126,983	[10],[13]	106,573	[10],[13]
Underwriting segments | Mortgage
Segment Reporting Information [Line Items]
Net premiums written									204,837		89,570		68,111
Net premiums earned									193,573		51,266		16,670
Underwriting segments | Mortgage | Client Location [Member] | United States
Segment Reporting Information [Line Items]
Net premiums written									184,333		63,692		61,571
Underwriting segments | Mortgage | Client Location [Member] | Other geographic location
Segment Reporting Information [Line Items]
Net premiums written									20,504		25,878		6,540
Underwriting segments | Mortgage | Underwriting Location [Member] | United States
Segment Reporting Information [Line Items]
Net premiums written									98,809		0		0
Underwriting segments | Mortgage | Underwriting Location [Member] | Other geographic location
Segment Reporting Information [Line Items]
Net premiums written									$ 106,028		$ 89,570		$ 68,111

[1] Other segment results include premiums written and earned assumed through intersegment transactions and exclude premiums written and earned ceded through intersegment transactions.

[2] Includes non-excess motor and other.

[3] Includes professional liability, excess motor, programs and other.

[4] Includes mortgage and other.

[5] Reinsurance segment results include premiums written and earned assumed through intersegment transactions and exclude premiums written and earned ceded through intersegment transactions.

[6] Includes non-excess motor, surety, accident and health, workers’ compensation catastrophe, agriculture, trade credit and other.

[7] Includes facultative business.

[8] Includes executive assurance, professional liability, workers’ compensation, excess motor, healthcare and other.

[9] Includes life, casualty clash and other.

[10] Insurance segment results include premiums written and earned assumed through intersegment transactions and exclude premiums written and earned ceded through intersegment transactions.

[11] Includes professional liability, executive assurance and healthcare business.

[12] Includes casualty and contract binding business.

[13] Includes alternative markets, excess workers' compensation and surety business.