Segment Information - Net Premiums Earned by Line of Business (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016		Mar. 31, 2015
Net premiums earned by line of business:
P&C Operations		$ 2,351,446		$ 1,319,494
Life Operations		3,164		14,506
Total		2,354,610		1,334,000
Corporate and Other
Net premiums earned by line of business:
Life Operations		3,164		14,506
Total		3,164	[1]	14,506
Life insurance product line
Net premiums earned by line of business:
Life Operations		3,164		14,507
Life insurance product line | Corporate and Other
Net premiums earned by line of business:
Life Operations		3,164		14,507
Professional | Total P&C
Net premiums earned by line of business:
P&C Operations		376,109		301,418
Casualty | Total P&C
Net premiums earned by line of business:
P&C Operations		574,951		399,525
Property catastrophe | Total P&C
Net premiums earned by line of business:
P&C Operations		205,301		94,073
Property | Total P&C
Net premiums earned by line of business:
P&C Operations		614,032		303,763
Specialty | Total P&C
Net premiums earned by line of business:
P&C Operations		403,545		189,953
Other | Total P&C
Net premiums earned by line of business:
P&C Operations	[2]	177,508		30,762
Investments Product Line [Member]
Net premiums earned by line of business:
Life Operations		0		(1)
Investments Product Line [Member] | Corporate and Other
Net premiums earned by line of business:
Life Operations		0		(1)
Operating Segments [Member] | Total P&C
Net premiums earned by line of business:
Total		2,351,446		1,319,494
Operating Segments [Member] | Insurance Segment
Net premiums earned by line of business:
P&C Operations		1,593,874		962,306
Total		1,593,874		962,306
Operating Segments [Member] | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		757,572		357,188
Total		757,572		357,188
Operating Segments [Member] | Professional | Insurance Segment
Net premiums earned by line of business:
P&C Operations		332,954		266,410
Operating Segments [Member] | Professional | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		43,155		35,008
Operating Segments [Member] | Casualty | Insurance Segment
Net premiums earned by line of business:
P&C Operations		420,212		341,317
Operating Segments [Member] | Casualty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		154,739		58,208
Operating Segments [Member] | Property catastrophe | Insurance Segment
Net premiums earned by line of business:
P&C Operations		0		0
Operating Segments [Member] | Property catastrophe | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		205,301		94,073
Operating Segments [Member] | Property | Insurance Segment
Net premiums earned by line of business:
P&C Operations		359,633		172,539
Operating Segments [Member] | Property | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		254,399		131,224
Operating Segments [Member] | Specialty | Insurance Segment
Net premiums earned by line of business:
P&C Operations		367,352		171,878
Operating Segments [Member] | Specialty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		36,193		18,075
Operating Segments [Member] | Other | Insurance Segment
Net premiums earned by line of business:
P&C Operations	[2]	113,723		10,162
Operating Segments [Member] | Other | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	[2]	$ 63,785		$ 20,600

[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, structured indemnity and other lines.