Losses and Loss Adjustment Expenses - Net Loss Reserves By Acquisition And Year (Details) - Non-life Run-off $ in Thousands	Dec. 31, 2019 USD ($)
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	$ 6,656,097
Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	1,740,810
Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	308,613
General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	813,541
Workers' compensation/personal accident
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	1,710,528
Marine, aviation and transit
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	342,989
Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	124,132
Professional indemnity/Directors & Officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	655,260
Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	480,599
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	178,215
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	301,410
2009 and Prior
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	354,415
2009 and Prior | Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	180,887
2009 and Prior | Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	44,461
2009 and Prior | General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	60,400
2009 and Prior | Workers' compensation/personal accident
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	3,362
2009 and Prior | Marine, aviation and transit
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	8,890
2009 and Prior | Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2009 and Prior | Professional indemnity/Directors & Officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	9,438
2009 and Prior | Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	25,131
2009 and Prior | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	3,957
2009 and Prior | Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	17,889
2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	141,306
2010 | Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	36,371
2010 | Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	10,364
2010 | General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	13,409
2010 | Workers' compensation/personal accident
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	49,828
2010 | Marine, aviation and transit
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	2,638
2010 | Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	18
2010 | Professional indemnity/Directors & Officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	5,021
2010 | Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	5,756
2010 | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	4,058
2010 | Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	13,843
2011
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	182,914
2011 | Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2011 | Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2011 | General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	24,072
2011 | Workers' compensation/personal accident
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	145,975
2011 | Marine, aviation and transit
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	3,256
2011 | Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	55
2011 | Professional indemnity/Directors & Officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	8,619
2011 | Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	217
2011 | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	298
2011 | Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	422
2012
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	57,336
2012 | Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2012 | Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2012 | General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	12,082
2012 | Workers' compensation/personal accident
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	3,986
2012 | Marine, aviation and transit
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2012 | Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2012 | Professional indemnity/Directors & Officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	28,274
2012 | Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	500
2012 | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	6,199
2012 | Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	6,295
2013
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	85,005
2013 | Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	7,973
2013 | Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2013 | General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	14,695
2013 | Workers' compensation/personal accident
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	58,316
2013 | Marine, aviation and transit
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2013 | Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2013 | Professional indemnity/Directors & Officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2013 | Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	660
2013 | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2013 | Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	3,361
2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	99,834
2014 | Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2014 | Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2014 | General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	30,313
2014 | Workers' compensation/personal accident
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2014 | Marine, aviation and transit
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	13,762
2014 | Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2014 | Professional indemnity/Directors & Officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	35,795
2014 | Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	250
2014 | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	14,127
2014 | Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	5,587
2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	453,355
2015 | Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	673
2015 | Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2015 | General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	46,838
2015 | Workers' compensation/personal accident
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	331,383
2015 | Marine, aviation and transit
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	1,589
2015 | Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	46,906
2015 | Professional indemnity/Directors & Officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2015 | Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	14,785
2015 | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	3,581
2015 | Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	7,600
2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	885,176
2016 | Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	387,808
2016 | Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	101,649
2016 | General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	5,389
2016 | Workers' compensation/personal accident
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	266,932
2016 | Marine, aviation and transit
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2016 | Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	18,982
2016 | Professional indemnity/Directors & Officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	77,685
2016 | Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	203
2016 | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2016 | Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	26,528
2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	1,116,945
2017 | Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	729,749
2017 | Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	26,798
2017 | General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	59,432
2017 | Workers' compensation/personal accident
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	73,069
2017 | Marine, aviation and transit
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	81,037
2017 | Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	22,973
2017 | Professional indemnity/Directors & Officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2017 | Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	4,453
2017 | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	455
2017 | Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	118,979
2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	1,724,191
2018 | Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	60,424
2018 | Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	17,247
2018 | General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	305,370
2018 | Workers' compensation/personal accident
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	385,223
2018 | Marine, aviation and transit
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	139,908
2018 | Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	0
2018 | Professional indemnity/Directors & Officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	326,157
2018 | Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	406,545
2018 | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	66,755
2018 | Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	16,562
2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	1,555,620
2019 | Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	336,925
2019 | Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	108,094
2019 | General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	241,541
2019 | Workers' compensation/personal accident
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	392,454
2019 | Marine, aviation and transit
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	91,909
2019 | Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	35,198
2019 | Professional indemnity/Directors & Officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	164,271
2019 | Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	22,099
2019 | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	78,785
2019 | Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	$ 84,344