Segment Information - Net Premiums Earned by Line of Business (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017
Net premiums earned by line of business:
P&C Operations		$ 2,692,664		$ 2,514,093		$ 5,289,845		$ 5,033,574
Life Operations		2,505		2,824		5,612		6,134
Total		2,695,169		2,516,917		5,295,457		5,039,708
Other life
Net premiums earned by line of business:
Life Operations		2,505		2,824		5,612		6,134
Total P&C | Professional
Net premiums earned by line of business:
P&C Operations		399,223		360,374		784,548		718,782
Total P&C | Casualty
Net premiums earned by line of business:
P&C Operations		667,376		687,282		1,298,349		1,318,111
Total P&C | Property catastrophe
Net premiums earned by line of business:
P&C Operations		129,727		164,057		266,230		312,463
Total P&C | Property
Net premiums earned by line of business:
P&C Operations		683,268		692,723		1,348,135		1,415,572
Total P&C | Specialty
Net premiums earned by line of business:
P&C Operations		518,058		450,779		1,019,969		890,092
Total P&C | Other
Net premiums earned by line of business:
P&C Operations	[1]	295,012		158,878		572,614		378,554
Corporate and Other
Net premiums earned by line of business:
Life Operations		2,505		2,824		5,612		6,134
Total	[2]	2,505		2,824		5,612		6,134
Corporate and Other | Other life
Net premiums earned by line of business:
Life Operations		2,505		2,824		5,612		6,134
Operating segments | Total P&C
Net premiums earned by line of business:
Total		2,692,664		2,514,093		5,289,845		5,033,574
Operating segments | Insurance Segment
Net premiums earned by line of business:
P&C Operations		1,748,355		1,652,304		3,414,144		3,287,619	[3]
Total		1,748,355		1,652,304		3,414,144		3,287,619
Operating segments | Insurance Segment | Professional
Net premiums earned by line of business:
P&C Operations		335,442		306,070	[3]	656,159		611,552	[3]
Operating segments | Insurance Segment | Casualty
Net premiums earned by line of business:
P&C Operations		512,307		537,655	[3]	993,317		1,011,850	[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		429,472		406,230	[3]	836,250		856,733	[3]
Operating segments | Insurance Segment | Specialty
Net premiums earned by line of business:
P&C Operations		470,765		403,103	[3]	928,415		805,790	[3]
Operating segments | Insurance Segment | Other
Net premiums earned by line of business:
P&C Operations		369	[1]	(754)	[3]	3	[1]	1,694	[1],[3]
Operating segments | Reinsurance Segment
Net premiums earned by line of business:
P&C Operations		944,309		861,789		1,875,701		1,745,955
Total		944,309		861,789		1,875,701		1,745,955
Operating segments | Reinsurance Segment | Professional
Net premiums earned by line of business:
P&C Operations		63,781		54,304		128,389		107,230
Operating segments | Reinsurance Segment | Casualty
Net premiums earned by line of business:
P&C Operations		155,069		149,627		305,032		306,261
Operating segments | Reinsurance Segment | Property catastrophe
Net premiums earned by line of business:
P&C Operations		129,727		164,057		266,230		312,463
Operating segments | Reinsurance Segment | Property
Net premiums earned by line of business:
P&C Operations		253,796		286,493		511,885		558,839
Operating segments | Reinsurance Segment | Specialty
Net premiums earned by line of business:
P&C Operations		47,293		47,676		91,554		84,302
Operating segments | Reinsurance Segment | Other
Net premiums earned by line of business:
P&C Operations	[1]	$ 294,643		$ 159,632		$ 572,611		$ 376,860

[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 & 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 the following movements out of Other in the Insurance segment: livestock business into Specialty; and credit surety into Casualty.