Segment Information - Net Premiums Earned by Line of Business (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Net premiums earned by line of business:
P&C Operations		$ 2,528,704	$ 2,063,795	$ 4,880,150	$ 3,383,289
Life Operations		3,508	18,258	6,672	32,764
Total		2,532,212	2,082,053	4,886,822	3,416,053
Corporate and Other
Net premiums earned by line of business:
Life Operations		3,508	18,258	6,672	32,764
Total	[1]	3,508	18,258	6,672	32,764
Life insurance annuity
Net premiums earned by line of business:
Life Operations		1	1	1	0
Life insurance annuity | Corporate and Other
Net premiums earned by line of business:
Life Operations		1	1	1	0
Life insurance product line
Net premiums earned by line of business:
Life Operations		3,507	18,257	6,671	32,764
Life insurance product line | Corporate and Other
Net premiums earned by line of business:
Life Operations		3,507	18,257	6,671	32,764
Professional | Total P&C
Net premiums earned by line of business:
P&C Operations		359,142	329,584	735,251	631,005
Casualty | Total P&C
Net premiums earned by line of business:
P&C Operations		639,447	579,314	1,214,398	978,838
Property catastrophe | Total P&C
Net premiums earned by line of business:
P&C Operations		214,313	163,452	419,614	257,525
Property | Total P&C
Net premiums earned by line of business:
P&C Operations		640,911	537,926	1,254,943	841,569
Specialty | Total P&C
Net premiums earned by line of business:
P&C Operations		441,774	387,106	845,319	577,177
Other | Total P&C
Net premiums earned by line of business:
P&C Operations	[2]	233,117	66,413	410,625	97,175
Operating Segments [Member] | Total P&C
Net premiums earned by line of business:
Total		2,528,704	2,063,795	4,880,150	3,383,289
Operating Segments [Member] | Insurance Segment
Net premiums earned by line of business:
P&C Operations		1,696,720	1,412,906	3,290,594	2,375,212
Total		1,696,720	1,412,906	3,290,594	2,375,212
Operating Segments [Member] | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		831,984	650,889	1,589,556	1,008,077
Total		831,984	650,889	1,589,556	1,008,077
Operating Segments [Member] | Professional | Insurance Segment
Net premiums earned by line of business:
P&C Operations		319,344	286,359	652,298	552,772
Operating Segments [Member] | Professional | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		39,798	43,225	82,953	78,233
Operating Segments [Member] | Casualty | Insurance Segment
Net premiums earned by line of business:
P&C Operations		448,000	453,527	868,212	794,843
Operating Segments [Member] | Casualty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		191,447	125,787	346,186	183,995
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		214,313	163,452	419,614	257,525
Operating Segments [Member] | Property | Insurance Segment
Net premiums earned by line of business:
P&C Operations		380,641	306,028	740,274	478,447
Operating Segments [Member] | Property | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		260,270	231,898	514,669	363,122
Operating Segments [Member] | Specialty | Insurance Segment
Net premiums earned by line of business:
P&C Operations		395,962	354,557	763,314	526,553
Operating Segments [Member] | Specialty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		45,812	32,549	82,005	50,624
Operating Segments [Member] | Other | Insurance Segment
Net premiums earned by line of business:
P&C Operations	[2]	152,773	12,435	266,496	22,597
Operating Segments [Member] | Other | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	[2]	$ 80,344	$ 53,978	$ 144,129	$ 74,578

[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.