Segment Information - Net Premiums Earned by Line of Business (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015		Sep. 30, 2016	Sep. 30, 2015
Net premiums earned by line of business:
P&C Operations		$ 2,431,677	$ 2,405,740		$ 7,311,827	$ 5,789,029
Life Operations		2,360	17,812		9,032	50,576
Total		2,434,037	2,423,552		7,320,859	5,839,605
Corporate and Other
Net premiums earned by line of business:
Life Operations		2,360	17,812		9,032	50,576
Total	[1]	2,360	17,812		9,032	50,576
Life insurance annuity
Net premiums earned by line of business:
Life Operations		0	0		0	0
Life insurance annuity | Corporate and Other
Net premiums earned by line of business:
Life Operations		0	0		0	0
Life insurance product line
Net premiums earned by line of business:
Life Operations		2,360	17,812		9,032	50,576
Life insurance product line | Corporate and Other
Net premiums earned by line of business:
Life Operations		2,360	17,812		9,032	50,576
Professional | Total P&C
Net premiums earned by line of business:
P&C Operations		357,002	347,652		1,092,253	978,657
Casualty | Total P&C
Net premiums earned by line of business:
P&C Operations		649,691	644,325		1,975,900	1,623,163
Property catastrophe | Total P&C
Net premiums earned by line of business:
P&C Operations		183,381	205,337		602,995	462,862
Property | Total P&C
Net premiums earned by line of business:
P&C Operations		714,331	640,304		1,994,257	1,481,873
Specialty | Total P&C
Net premiums earned by line of business:
P&C Operations		451,889	483,987		1,412,455	1,061,164
Other | Total P&C
Net premiums earned by line of business:
P&C Operations	[2]	75,383	84,135		233,967	181,310
Operating Segments [Member] | Total P&C
Net premiums earned by line of business:
Total		2,431,677	2,405,740		7,311,827	5,789,029
Operating Segments [Member] | Insurance Segment
Net premiums earned by line of business:
P&C Operations		1,653,461	1,632,988		4,944,055	4,008,200
Total		1,653,461	1,632,988		4,944,055	4,008,200
Operating Segments [Member] | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		778,216	772,752		2,367,772	1,780,829
Total		778,216	772,752		2,367,772	1,780,829
Operating Segments [Member] | Professional | Insurance Segment
Net premiums earned by line of business:
P&C Operations		317,973	300,775	[3]	970,271	853,547	[3]
Operating Segments [Member] | Professional | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		39,029	46,877		121,982	125,110
Operating Segments [Member] | Casualty | Insurance Segment
Net premiums earned by line of business:
P&C Operations		485,846	503,190	[3]	1,465,869	1,298,033	[3]
Operating Segments [Member] | Casualty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		163,845	141,135		510,031	325,130
Operating Segments [Member] | Property catastrophe | Insurance Segment
Net premiums earned by line of business:
P&C Operations		0	0	[3]	0	0	[3]
Operating Segments [Member] | Property catastrophe | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		183,381	205,337		602,995	462,862
Operating Segments [Member] | Property | Insurance Segment
Net premiums earned by line of business:
P&C Operations		451,007	369,007	[3]	1,216,264	847,454	[3]
Operating Segments [Member] | Property | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		263,324	271,297		777,993	634,419
Operating Segments [Member] | Specialty | Insurance Segment
Net premiums earned by line of business:
P&C Operations		408,177	447,642	[3]	1,286,738	974,195	[3]
Operating Segments [Member] | Specialty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		43,712	36,345		125,717	86,969
Operating Segments [Member] | Other | Insurance Segment
Net premiums earned by line of business:
P&C Operations	[2]	(9,542)	12,374	[3]	4,913	34,971	[3]
Operating Segments [Member] | Other | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	[2]	$ 84,925	$ 71,761		$ 229,054	$ 146,339

[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: surety, structured indemnity and certain discontinued lines. Other within the Reinsurance segment includes: whole account contracts, structured indemnity and other lines.(2)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.

[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.