Segment Information - Net Premiums Earned by Line of Business (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018		Mar. 31, 2017
Net premiums earned by line of business:
P&C Operations		$ 2,597,181		$ 2,519,481
Life Operations		3,107		3,310
Total		2,600,288		2,522,791
Life insurance product line
Net premiums earned by line of business:
Life Operations		3,107		3,310
Total P&C | Professional
Net premiums earned by line of business:
P&C Operations		385,325		358,408
Total P&C | Casualty
Net premiums earned by line of business:
P&C Operations		630,973		630,829
Total P&C | Property catastrophe
Net premiums earned by line of business:
P&C Operations		136,503		148,406
Total P&C | Property
Net premiums earned by line of business:
P&C Operations		664,867		722,849
Total P&C | Specialty
Net premiums earned by line of business:
P&C Operations		501,911		439,313
Total P&C | Other
Net premiums earned by line of business:
P&C Operations		277,602	[1]	219,676
Corporate and Other
Net premiums earned by line of business:
Life Operations		3,107		3,310
Total	[2]	3,107		3,310
Corporate and Other | Life insurance product line
Net premiums earned by line of business:
Life Operations		3,107		3,310
Operating segments | Total P&C
Net premiums earned by line of business:
Total		2,597,181		2,519,481
Operating segments | Insurance Segment
Net premiums earned by line of business:
P&C Operations		1,665,789		1,635,315
Total		1,665,789		1,635,315
Operating segments | Insurance Segment | Professional
Net premiums earned by line of business:
P&C Operations		320,717		305,482	[3]
Operating segments | Insurance Segment | Casualty
Net premiums earned by line of business:
P&C Operations		481,010		474,195	[3]
Operating segments | Insurance Segment | Property catastrophe
Net premiums earned by line of business:
P&C Operations		0		0	[3]
Operating segments | Insurance Segment | Property
Net premiums earned by line of business:
P&C Operations		406,778		450,503	[3]
Operating segments | Insurance Segment | Specialty
Net premiums earned by line of business:
P&C Operations		457,650		402,687	[3]
Operating segments | Insurance Segment | Other
Net premiums earned by line of business:
P&C Operations		(366)	[1]	2,448	[3]
Operating segments | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		931,392		884,166
Total		931,392		884,166
Operating segments | Reinsurance Segment | Professional
Net premiums earned by line of business:
P&C Operations		64,608		52,926
Operating segments | Reinsurance Segment | Casualty
Net premiums earned by line of business:
P&C Operations		149,963		156,634
Operating segments | Reinsurance Segment | Property catastrophe
Net premiums earned by line of business:
P&C Operations		136,503		148,406
Operating segments | Reinsurance Segment | Property
Net premiums earned by line of business:
P&C Operations		258,089		272,346
Operating segments | Reinsurance Segment | Specialty
Net premiums earned by line of business:
P&C Operations		44,261		36,626
Operating segments | Reinsurance Segment | Other
Net premiums earned by line of business:
P&C Operations		$ 277,968	[1]	$ 217,228

[1] Other within the Insurance segment includes: surety and certain discontinued lines. Other within the Reinsurance segment includes multiline, whole account contracts, credit and surety, accident and health 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 out of Other are in the Insurance segment, and include: livestock business into Specialty; and credit surety into Casualty.