Segment Information - Net Premiums Earned by Line of Business (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Net premiums earned by line of business:
P&C Operations		$ 2,063,795	$ 1,438,076	$ 3,383,289	$ 2,850,604
Life Operations		18,258	58,518	32,764	134,829
Total		2,082,053	1,496,594	3,416,053	2,985,433
Corporate and Other
Net premiums earned by line of business:
Life Operations		18,258	58,518	32,764	134,829
Total	[1]	18,258	58,518	32,764	134,829
Professional | Total P&C
Net premiums earned by line of business:
P&C Operations		329,573	311,748	630,992	637,279
Casualty | Total P&C
Net premiums earned by line of business:
P&C Operations		546,259	441,919	913,802	862,644
Property catastrophe | Total P&C
Net premiums earned by line of business:
P&C Operations		163,453	112,562	257,526	220,999
Property | Total P&C
Net premiums earned by line of business:
P&C Operations		447,025	285,211	706,490	560,183
Marine, energy, aviation and satellite | Total P&C
Net premiums earned by line of business:
P&C Operations		32,533	21,065	50,608	44,579
Specialty | Total P&C
Net premiums earned by line of business:
P&C Operations		411,438	179,948	596,162	356,857
Other | Total P&C
Net premiums earned by line of business:
P&C Operations	[2]	133,514	85,623	227,709	168,063
Annuity
Net premiums earned by line of business:
Life Operations		1	21,564	0	53,361
Annuity | Corporate and Other
Net premiums earned by line of business:
Life Operations		1	21,564	0	53,361
Other life
Net premiums earned by line of business:
Life Operations		18,257	36,954	32,764	81,468
Other life | Corporate and Other
Net premiums earned by line of business:
Life Operations		18,257	36,954	32,764	81,468
Operating Segments [Member] | Total P&C
Net premiums earned by line of business:
Total		2,063,795	1,438,076	3,383,289	2,850,604
Operating Segments [Member] | Insurance Segment
Net premiums earned by line of business:
P&C Operations		1,412,906	1,003,990	2,375,212	1,996,430
Total		1,412,906	1,003,990	2,375,212	1,996,430
Operating Segments [Member] | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		650,889	434,086	1,008,077	854,174
Total		650,889	434,086	1,008,077	854,174
Operating Segments [Member] | Professional | Insurance Segment
Net premiums earned by line of business:
P&C Operations		286,348	267,509	552,759	542,936
Operating Segments [Member] | Professional | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		43,225	44,239	78,233	94,343
Operating Segments [Member] | Casualty | Insurance Segment
Net premiums earned by line of business:
P&C Operations		420,472	358,990	729,807	702,994
Operating Segments [Member] | Casualty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		125,787	82,929	183,995	159,650
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		163,453	112,562	257,526	220,999
Operating Segments [Member] | Property | Insurance Segment
Net premiums earned by line of business:
P&C Operations		215,128	142,874	343,369	282,607
Operating Segments [Member] | Property | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		231,897	142,337	363,121	277,576
Operating Segments [Member] | Marine, energy, aviation and satellite | Insurance Segment
Net premiums earned by line of business:
P&C Operations		(16)	0	(16)	0
Operating Segments [Member] | Marine, energy, aviation and satellite | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		32,549	21,065	50,624	44,579
Operating Segments [Member] | Specialty | Insurance Segment
Net premiums earned by line of business:
P&C Operations		411,438	179,948	596,162	356,857
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]	79,536	54,669	153,131	111,036
Operating Segments [Member] | Other | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	[2]	$ 53,978	$ 30,954	$ 74,578	$ 57,027

[1] Corporate and Other includes other items of our revenue 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.