Segment Information - Net premiums earned by line of business (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
P&C Operations:
P&C Operations										$ 9,765,887	$ 8,164,184	$ 5,717,438
Corporate and Other:
Life Operations										12,047	62,241	177,632
Net amount, premiums earned										9,777,934	8,226,425	5,895,070
Annuity
Corporate and Other:
Life Operations										1	1	53,363
Life operations
Corporate and Other:
Life Operations										12,046	62,240	124,269
Property and casualty operations | Professional
P&C Operations:
P&C Operations										1,494,066	1,332,813	1,256,643
Property and casualty operations | Casualty
P&C Operations:
P&C Operations										2,631,262	2,273,849	1,969,320
Property and casualty operations | Property catastrophe
P&C Operations:
P&C Operations										792,445	663,958	433,602
Property and casualty operations | Property Other Than Property Catastrophe [Member]
P&C Operations:
P&C Operations										2,646,759	2,126,669	1,100,439
Property and casualty operations | Specialty
P&C Operations:
P&C Operations										1,885,821	1,548,921	833,026
Property and casualty operations | Other
P&C Operations:
P&C Operations	[1]									315,534	217,974	124,408
Corporate and other
Corporate and Other:
Life Operations										12,047	62,241	177,632
Net amount, premiums earned	[2]									12,047	62,241	177,632
Corporate and other | Annuity
Corporate and Other:
Life Operations										1	1	53,363
Corporate and other | Life operations
Corporate and Other:
Life Operations										12,046	62,240	124,269
Operating Segments [Member] | Property and casualty operations
Corporate and Other:
Net amount, premiums earned		$ 2,454,060	$ 2,431,677	$ 2,528,704	$ 2,351,446	$ 2,375,155	$ 2,405,740	$ 2,063,795	$ 1,319,494	9,765,887	8,164,184	5,717,438
Operating Segments [Member] | Corporate and other
Corporate and Other:
Net amount, premiums earned		$ 3,015	$ 2,360	$ 3,508	$ 3,164	$ 11,665	$ 17,812	$ 18,258	$ 14,506
Operating Segments [Member] | Insurance Segment
P&C Operations:
P&C Operations										6,651,495	5,648,482	4,026,713
Corporate and Other:
Net amount, premiums earned										6,651,495	5,648,482	4,026,713
Operating Segments [Member] | Insurance Segment | Professional
P&C Operations:
P&C Operations	[3]									1,330,909	1,164,446	1,075,420
Operating Segments [Member] | Insurance Segment | Casualty
P&C Operations:
P&C Operations	[3]									1,973,672	1,805,563	1,669,097
Operating Segments [Member] | Insurance Segment | Property catastrophe
P&C Operations:
P&C Operations	[3]									0	0	0
Operating Segments [Member] | Insurance Segment | Property Other Than Property Catastrophe [Member]
P&C Operations:
P&C Operations	[3]									1,625,133	1,257,383	544,856
Operating Segments [Member] | Insurance Segment | Specialty
P&C Operations:
P&C Operations	[3]									1,711,344	1,421,124	737,281
Operating Segments [Member] | Insurance Segment | Other
P&C Operations:
P&C Operations	[1],[3]									10,437	(34)	59
Operating Segments [Member] | Reinsurance Segment
P&C Operations:
P&C Operations										3,114,392	2,515,702	1,690,725
Corporate and Other:
Net amount, premiums earned										3,114,392	2,515,702	1,690,725
Operating Segments [Member] | Reinsurance Segment | Professional
P&C Operations:
P&C Operations										163,157	168,367	181,223
Operating Segments [Member] | Reinsurance Segment | Casualty
P&C Operations:
P&C Operations										657,590	468,286	300,223
Operating Segments [Member] | Reinsurance Segment | Property catastrophe
P&C Operations:
P&C Operations										792,445	663,958	433,602
Operating Segments [Member] | Reinsurance Segment | Property Other Than Property Catastrophe [Member]
P&C Operations:
P&C Operations										1,021,626	869,286	555,583
Operating Segments [Member] | Reinsurance Segment | Specialty
P&C Operations:
P&C Operations										174,477	127,797	95,745
Operating Segments [Member] | Reinsurance Segment | Other
P&C Operations:
P&C Operations	[1]									$ 305,097	$ 218,008	$ 124,349

[1] . Other within the Reinsurance segment includes: whole account contracts, accident and health and other lines.

[2] 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.

[3] Amounts from the prior year have been represented to reflect current mapping of underlying lines of business to be consistent with the manner in which they are reflected in the current period. The most significant movements include the following: Energy business previously shown under Specialty is now reflected in Property; Programs business formerly in Other is now reflected in Property or Casualty, depending upon the predominant line for each program; and Excess and Surplus is now reflected primarily in Casualty.