Segment Information - Net Premiums Earned by Line of Business (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016		Jun. 30, 2017	Jun. 30, 2016
Net premiums earned by line of business:
P&C Operations		$ 2,514,093	$ 2,528,704		$ 5,033,574	$ 4,880,150
Life Operations		2,824	3,508		6,134	6,672
Total		2,516,917	2,532,212		5,039,708	4,886,822
Corporate and Other
Net premiums earned by line of business:
Life Operations		2,824	3,508		6,134	6,672
Total	[1]	2,824	3,508		6,134	6,672
Life insurance product line
Net premiums earned by line of business:
Life Operations		2,824	3,507		6,134	6,671
Life insurance product line | Corporate and Other
Net premiums earned by line of business:
Life Operations		2,824	3,507		6,134	6,671
Professional | Total P&C
Net premiums earned by line of business:
P&C Operations		360,376	332,562		718,786	679,998
Casualty | Total P&C
Net premiums earned by line of business:
P&C Operations		692,066	754,606		1,379,053	1,429,772
Property catastrophe | Total P&C
Net premiums earned by line of business:
P&C Operations		164,057	214,313		312,463	419,614
Property | Total P&C
Net premiums earned by line of business:
P&C Operations		687,939	636,799		1,354,629	1,237,386
Specialty | Total P&C
Net premiums earned by line of business:
P&C Operations		450,777	500,500		890,089	960,369
Other | Total P&C
Net premiums earned by line of business:
P&C Operations	[2]	158,878	89,924		378,554	153,011
Investments Product Line [Member]
Net premiums earned by line of business:
Life Operations		0	1		0	1
Investments Product Line [Member] | Corporate and Other
Net premiums earned by line of business:
Life Operations		0	1		0	1
Operating Segments [Member] | Total P&C
Net premiums earned by line of business:
Total		2,514,093	2,528,704		5,033,574	4,880,150
Operating Segments [Member] | Insurance Segment
Net premiums earned by line of business:
P&C Operations		1,652,304	1,696,720		3,287,619	3,290,594
Total		1,652,304	1,696,720		3,287,619	3,290,594
Operating Segments [Member] | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		861,789	831,984		1,745,955	1,589,556
Total		861,789	831,984		1,745,955	1,589,556
Operating Segments [Member] | Professional | Insurance Segment
Net premiums earned by line of business:
P&C Operations		306,072	292,764	[3]	611,556	597,045	[3]
Operating Segments [Member] | Professional | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		54,304	39,798		107,230	82,953
Operating Segments [Member] | Casualty | Insurance Segment
Net premiums earned by line of business:
P&C Operations		542,439	563,159	[3]	1,072,792	1,083,586	[3]
Operating Segments [Member] | Casualty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		149,627	191,447		306,261	346,186
Operating Segments [Member] | Property catastrophe | Insurance Segment
Net premiums earned by line of business:
P&C Operations				[3]			[3]
Operating Segments [Member] | Property catastrophe | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		164,057	214,313		312,463	419,614
Operating Segments [Member] | Property | Insurance Segment
Net premiums earned by line of business:
P&C Operations		401,446	376,529	[3]	795,790	722,717	[3]
Operating Segments [Member] | Property | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		286,493	260,270		558,839	514,669
Operating Segments [Member] | Specialty | Insurance Segment
Net premiums earned by line of business:
P&C Operations		403,101	454,688	[3]	805,787	878,364	[3]
Operating Segments [Member] | Specialty | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		47,676	45,812		84,302	82,005
Operating Segments [Member] | Other | Insurance Segment
Net premiums earned by line of business:
P&C Operations	[2]	(754)	9,580	[3]	1,694	8,882	[3]
Operating Segments [Member] | Other | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations	[2]	$ 159,632	$ 80,344		$ 376,860	$ 144,129

[1] Corporate and Other includes other items of our revenue and expenditures that are not evaluated at the segment level for reporting purposes. The underwriting profit and its components within this section are predominantly driven from 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.

[3] Amounts from the prior year have been re-presented 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 movement of certain items out of Other as follows: Programs business is now reflected in Property or Casualty, depending upon the predominant line for each program; Surplus lines is now reflected primarily in Casualty; and Accident & Health is now reflected in Specialty.