Segment Information - Net Premiums Earned by Line of Business (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017	Sep. 30, 2016
Net premiums earned by line of business:
P&C Operations		$ 2,617,756	$ 2,431,677		$ 7,651,330	$ 7,311,827
Life Operations		3,146	2,360		9,280	9,032
Total		2,620,902	2,434,037		7,660,610	7,320,859
Life insurance product line
Net premiums earned by line of business:
Life Operations		3,146	2,360		9,280	9,032
Run-off annuity
Net premiums earned by line of business:
Life Operations		0	0		0	0
Total P&C | Professional
Net premiums earned by line of business:
P&C Operations		386,557	326,068		1,105,343	1,006,063
Total P&C | Casualty
Net premiums earned by line of business:
P&C Operations		659,096	717,068		2,038,149	2,146,839
Total P&C | Property catastrophe
Net premiums earned by line of business:
P&C Operations		221,023	183,381		533,486	602,995
Total P&C | Property
Net premiums earned by line of business:
P&C Operations		668,807	678,212		2,023,436	1,915,602
Total P&C | Specialty
Net premiums earned by line of business:
P&C Operations		490,961	452,569		1,381,050	1,412,938
Total P&C | Other
Net premiums earned by line of business:
P&C Operations	[1]	191,312	74,379		569,866	227,390
Corporate and Other
Net premiums earned by line of business:
Life Operations		3,146	2,360		9,280	9,032
Total	[2]	3,146	2,360		9,280	9,032
Corporate and Other | Life insurance product line
Net premiums earned by line of business:
Life Operations		3,146	2,360		9,280	9,032
Corporate and Other | Run-off annuity
Net premiums earned by line of business:
Life Operations		0	0		0	0
Operating segments | Total P&C
Net premiums earned by line of business:
Total		2,617,756	2,431,677		7,651,330	7,311,827
Operating segments | Insurance Segment
Net premiums earned by line of business:
P&C Operations		1,677,189	1,653,461		4,964,808	4,944,055
Total		1,677,189	1,653,461		4,964,808	4,944,055
Operating segments | Insurance Segment | Professional
Net premiums earned by line of business:
P&C Operations		328,253	287,039	[3]	939,809	884,081	[3]
Operating segments | Insurance Segment | Casualty
Net premiums earned by line of business:
P&C Operations		522,564	553,223	[3]	1,595,356	1,636,808	[3]
Operating segments | Insurance Segment | Property catastrophe
Net premiums earned by line of business:
P&C Operations		0	0	[3]	0	0	[3]
Operating segments | Insurance Segment | Property
Net premiums earned by line of business:
P&C Operations		390,156	414,888	[3]	1,185,946	1,137,609	[3]
Operating segments | Insurance Segment | Specialty
Net premiums earned by line of business:
P&C Operations		437,350	408,857	[3]	1,243,137	1,287,221	[3]
Operating segments | Insurance Segment | Other
Net premiums earned by line of business:
P&C Operations	[1]	(1,134)	(10,546)	[3]	560	(1,664)	[3]
Operating segments | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		940,567	778,216		2,686,522	2,367,772
Total		940,567	778,216		2,686,522	2,367,772
Operating segments | Reinsurance Segment | Professional
Net premiums earned by line of business:
P&C Operations		58,304	39,029		165,534	121,982
Operating segments | Reinsurance Segment | Casualty
Net premiums earned by line of business:
P&C Operations		136,532	163,845		442,793	510,031
Operating segments | Reinsurance Segment | Property catastrophe
Net premiums earned by line of business:
P&C Operations		221,023	183,381		533,486	602,995
Operating segments | Reinsurance Segment | Property
Net premiums earned by line of business:
P&C Operations		278,651	263,324		837,490	777,993
Operating segments | Reinsurance Segment | Specialty
Net premiums earned by line of business:
P&C Operations		53,611	43,712		137,913	125,717
Operating segments | Reinsurance Segment | Other
Net premiums earned by line of business:
P&C Operations	[1]	$ 192,446	$ 84,925		$ 569,306	$ 229,054

[1] 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] 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 (loss) and its components within this section are predominantly driven from the Company's run-off life operations.

[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 movements 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.