Segment Information (Net Premiums Earned by Line of Business) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net premiums earned	$ 573,489	$ 541,737	$ 537,247	$ 530,285	$ 524,561	$ 510,773	$ 507,271	$ 463,228	$ 2,182,758	$ 2,005,833	$ 1,748,898
Operating segments | North American Insurance
Segment Reporting Information [Line Items]
Net premiums earned									1,111,164	1,022,979	892,721
Operating segments | North American Insurance | General casualty
Segment Reporting Information [Line Items]
Net premiums earned									334,603	269,114	211,238
Operating segments | North American Insurance | Professional liability
Segment Reporting Information [Line Items]
Net premiums earned									276,105	259,215	245,583
Operating segments | North American Insurance | Healthcare
Segment Reporting Information [Line Items]
Net premiums earned									168,476	194,344	187,053
Operating segments | North American Insurance | General property
Segment Reporting Information [Line Items]
Net premiums earned									126,646	141,896	137,002
Operating segments | North American Insurance | Programs
Segment Reporting Information [Line Items]
Net premiums earned									119,756	107,870	88,180
Operating segments | North American Insurance | Inland marine
Segment Reporting Information [Line Items]
Net premiums earned									43,666	25,939	10,612
Operating segments | North American Insurance | Environmental
Segment Reporting Information [Line Items]
Net premiums earned									19,507	17,017	12,627
Operating segments | North American Insurance | Other
Segment Reporting Information [Line Items]
Net premiums earned									22,405	7,584	426
Operating segments | Global Markets Insurance
Segment Reporting Information [Line Items]
Net premiums earned									162,566	126,008	115,660
Operating segments | Global Markets Insurance | General casualty
Segment Reporting Information [Line Items]
Net premiums earned									15,266	13,903	16,762
Operating segments | Global Markets Insurance | Professional liability
Segment Reporting Information [Line Items]
Net premiums earned									57,176	45,967	43,621
Operating segments | Global Markets Insurance | Healthcare
Segment Reporting Information [Line Items]
Net premiums earned									11,963	11,154	9,055
Operating segments | Global Markets Insurance | General property
Segment Reporting Information [Line Items]
Net premiums earned									33,957	31,364	36,751
Operating segments | Global Markets Insurance | Other
Segment Reporting Information [Line Items]
Net premiums earned									4,239	0	0
Operating segments | Global Markets Insurance | Aviation
Segment Reporting Information [Line Items]
Net premiums earned									22,342	9,521	0
Operating segments | Global Markets Insurance | Trade credit
Segment Reporting Information [Line Items]
Net premiums earned									17,623	14,099	9,471
Operating segments | Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned									909,028	856,846	740,517
Operating segments | Reinsurance | Property
Segment Reporting Information [Line Items]
Net premiums earned									411,968	336,009	273,167
Operating segments | Reinsurance | Specialty
Segment Reporting Information [Line Items]
Net premiums earned									321,538	349,044	283,415
Operating segments | Reinsurance | Casualty
Segment Reporting Information [Line Items]
Net premiums earned									$ 175,522	$ 171,793	$ 183,935