Segment Information - Net Premiums Earned by Line of Business (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Net premiums earned by line of business:
P&C Operations		$ 2,405,740	$ 1,453,673	$ 5,789,029	$ 4,304,277
Life Operations		17,812	19,739	50,576	154,568
Total		2,423,552	1,473,412	5,839,605	4,458,845
Corporate and Other
Net premiums earned by line of business:
Life Operations		17,812	19,739	50,576	154,568
Total	[1]	17,812	19,739	50,576	154,568
Professional | Total P&C
Net premiums earned by line of business:
P&C Operations		346,516	314,496	977,508	951,775
Casualty | Total P&C
Net premiums earned by line of business:
P&C Operations		614,912	425,143	1,528,714	1,287,787
Property catastrophe | Total P&C
Net premiums earned by line of business:
P&C Operations		205,337	103,813	462,862	324,812
Property | Total P&C
Net premiums earned by line of business:
P&C Operations		592,175	295,078	1,342,694	855,261
Marine, energy, aviation and satellite | Total P&C
Net premiums earned by line of business:
P&C Operations		36,345	30,511	86,969	75,090
Specialty | Total P&C
Net premiums earned by line of business:
P&C Operations		462,029	186,360	1,014,147	543,217
Other | Total P&C
Net premiums earned by line of business:
P&C Operations	[2]	148,426	98,272	376,135	266,335
Annuity
Net premiums earned by line of business:
Life Operations		0	2	0	53,363
Annuity | Corporate and Other
Net premiums earned by line of business:
Life Operations		0	2	0	53,363
Other life
Net premiums earned by line of business:
Life Operations		17,812	19,737	50,576	101,205
Other life | Corporate and Other
Net premiums earned by line of business:
Life Operations		17,812	19,737	50,576	101,205
Operating Segments [Member] | Total P&C
Net premiums earned by line of business:
Total		2,405,740	1,453,673	5,789,029	4,304,277
Operating Segments [Member] | Insurance Segment
Net premiums earned by line of business:
P&C Operations		1,632,988	1,018,416	4,008,200	3,014,846
Total		1,632,988	1,018,416	4,008,200	3,014,846
Operating Segments [Member] | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		772,752	435,257	1,780,829	1,289,431
Total		772,752	435,257	1,780,829	1,289,431
Operating Segments [Member] | Professional | Insurance Segment
Net premiums earned by line of business:
P&C Operations		299,639	269,275	852,398	812,211
Operating Segments [Member] | Professional | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		46,877	45,221	125,110	139,564
Operating Segments [Member] | Casualty | Insurance Segment
Net premiums earned by line of business:
P&C Operations		473,777	358,505	1,203,584	1,061,499
Operating Segments [Member] | Casualty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		141,135	66,638	325,130	226,288
Operating Segments [Member] | Property catastrophe | Insurance Segment
Net premiums earned by line of business:
P&C Operations		0	0	0	0
Operating Segments [Member] | Property catastrophe | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		205,337	103,813	462,862	324,812
Operating Segments [Member] | Property | Insurance Segment
Net premiums earned by line of business:
P&C Operations		320,878	139,663	708,275	422,270
Operating Segments [Member] | Property | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		271,297	155,415	634,419	432,991
Operating Segments [Member] | Marine, energy, aviation and satellite | Insurance Segment
Net premiums earned by line of business:
P&C Operations		0	0	0	0
Operating Segments [Member] | Marine, energy, aviation and satellite | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		36,345	30,511	86,969	75,090
Operating Segments [Member] | Specialty | Insurance Segment
Net premiums earned by line of business:
P&C Operations		462,029	186,360	1,014,147	543,217
Operating Segments [Member] | Specialty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		0	0	0	0
Operating Segments [Member] | Other | Insurance Segment
Net premiums earned by line of business:
P&C Operations	[2]	76,665	64,613	229,796	175,649
Operating Segments [Member] | Other | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	[2]	$ 71,761	$ 33,659	$ 146,339	$ 90,686

[1] Corporate and Other includes other items of our revenues and expenditures that are not evaluated at the segment level for reporting purposes, as well as the Company's Run-Off Life Operations.

[2] Other within the Insurance segment includes: excess and surplus, programs, surety, structured indemnity and certain discontinued lines. Other within the Reinsurance segment includes: whole account contracts, accident and health and other lines.