Losses and Loss Expenses - Information about incurred and paid claims development - Insurance - Professional (Details) $ in Thousands		Dec. 31, 2016 USD ($) Claim	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		$ 53,162,870
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		35,612,206
All outstanding liabilities prior to 2007, net of reinsurance		2,501,779
Liabilities for unpaid losses and loss expenses, net of reinsurance		20,052,443
Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		38,032,090
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		25,758,860
All outstanding liabilities prior to 2007, net of reinsurance		1,069,908
Liabilities for unpaid losses and loss expenses, net of reinsurance		13,343,138
Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		9,795,663
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		5,984,309
All outstanding liabilities prior to 2007, net of reinsurance		140,029
Liabilities for unpaid losses and loss expenses, net of reinsurance		3,951,383
Accident Year 2007
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,390,981	$ 4,432,150	$ 4,519,599	$ 4,541,978	$ 4,569,019	$ 4,689,256	$ 4,684,540	$ 4,718,961	$ 4,806,850	$ 4,632,657
IBNR (1)	[1]	246,827
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,894,280	3,822,207	3,744,872	3,612,934	3,397,370	3,126,834	2,587,918	2,055,288	1,289,961	521,083
Accident Year 2007 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,246,355	3,274,775	3,318,423	3,317,676	3,329,848	3,442,331	3,431,600	3,426,696	3,490,955	3,258,668
IBNR (1)	[1]	$ 158,351
Number of Claims | Claim		45,542
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,943,144	2,896,977	2,822,610	2,715,861	2,549,256	2,333,047	1,884,318	1,462,857	930,428	367,171
Accident Year 2007 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		928,677	944,024	1,000,256	993,007	995,364	1,070,028	1,061,995	1,106,630	1,130,926	911,695
IBNR (1)	[1]	$ 68,144
Number of Claims | Claim		4,371
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 801,141	776,445	738,301	692,492	612,498	540,255	421,654	240,287	111,289	$ 25,207
Accident Year 2008
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,322,204	5,357,648	5,417,072	5,449,364	5,479,907	5,524,325	5,503,422	5,575,015	5,504,796
IBNR (1)	[1]	307,738
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,700,567	4,582,409	4,450,875	4,240,640	3,947,203	3,446,415	2,857,474	1,994,235	788,760
Accident Year 2008 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,786,671	3,822,133	3,857,758	3,889,538	3,918,450	3,974,071	3,935,499	3,969,259	3,891,682
IBNR (1)	[1]	$ 186,723
Number of Claims | Claim		54,952
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,415,212	3,337,607	3,234,869	3,069,812	2,843,188	2,427,033	1,973,577	1,339,577	461,606
Accident Year 2008 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,114,924	1,136,720	1,167,289	1,174,486	1,195,931	1,221,906	1,131,685	1,087,436	1,033,982
IBNR (1)	[1]	$ 60,568
Number of Claims | Claim		5,187
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 969,698	911,634	853,405	782,381	697,071	516,426	346,923	185,149	$ 17,987
Accident Year 2009
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,392,904	4,422,167	4,550,542	4,550,362	4,556,902	4,542,408	4,627,569	4,622,547
IBNR (1)	[1]	317,583
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,663,224	3,551,474	3,366,917	3,117,922	2,655,163	2,192,791	1,530,295	638,179
Accident Year 2009 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,346,817	3,369,614	3,500,575	3,483,027	3,480,715	3,458,320	3,499,124	3,472,222
IBNR (1)	[1]	$ 190,193
Number of Claims | Claim		54,896
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,865,558	2,787,167	2,634,783	2,432,021	2,043,196	1,666,673	1,140,796	462,090
Accident Year 2009 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,164,825	1,169,232	1,202,994	1,196,078	1,158,091	1,094,824	1,036,140	968,336
IBNR (1)	[1]	$ 77,399
Number of Claims | Claim		5,589
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 948,015	922,453	838,990	745,836	507,629	337,568	161,125	$ 22,350
Accident Year 2010
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,981,939	4,957,474	4,974,782	5,026,101	5,002,061	5,111,634	4,983,044
IBNR (1)	[1]	464,306
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,084,244	3,851,827	3,521,163	3,144,278	2,645,868	1,855,407	840,030
Accident Year 2010 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,697,761	3,661,919	3,688,045	3,735,809	3,699,234	3,809,102	3,702,458
IBNR (1)	[1]	$ 313,815
Number of Claims | Claim		61,416
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,072,546	2,914,856	2,685,015	2,370,062	1,978,027	1,377,513	607,658
Accident Year 2010 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,061,340	1,016,013	996,024	1,035,571	957,486	984,780	989,432
IBNR (1)	[1]	$ 113,635
Number of Claims | Claim		5,418
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 803,916	735,626	623,975	480,475	324,580	155,845	$ 21,803
Accident Year 2011
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		6,083,796	6,116,046	5,980,635	6,111,779	6,187,474	6,236,775
IBNR (1)	[1]	558,008
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		5,021,000	4,737,827	4,242,053	3,664,970	2,872,319	1,246,596
Accident Year 2011 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,001,271	4,026,511	3,921,559	4,009,166	3,986,420	4,014,389
IBNR (1)	[1]	$ 379,349
Number of Claims | Claim		65,437
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,290,213	3,062,399	2,653,619	2,216,640	1,617,155	649,690
Accident Year 2011 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,111,524	1,104,107	1,027,504	1,012,458	960,895	967,386
IBNR (1)	[1]	$ 162,147
Number of Claims | Claim		5,253
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 815,129	683,043	480,248	315,130	144,365	$ 28,625
Accident Year 2012
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,663,926	5,754,387	5,640,576	5,660,616	5,697,958
IBNR (1)	[1]	715,827
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,199,766	3,802,042	3,149,842	2,256,973	952,158
Accident Year 2012 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,851,259	3,921,415	3,800,046	3,799,876	3,870,871
IBNR (1)	[1]	$ 468,002
Number of Claims | Claim		70,919
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,886,674	2,578,639	2,079,400	1,471,761	634,043
Accident Year 2012 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,117,970	1,124,437	1,011,363	976,593	976,450
IBNR (1)	[1]	$ 212,997
Number of Claims | Claim		5,320
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 713,165	591,820	378,059	195,088	$ 44,486
Accident Year 2013
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,426,144	5,523,695	5,554,791	5,558,316
IBNR (1)	[1]	1,082,491
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,614,076	3,049,907	2,194,912	976,023
Accident Year 2013 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,924,048	3,978,956	4,003,359	3,942,360
IBNR (1)	[1]	$ 802,611
Number of Claims | Claim		78,018
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,597,422	2,141,332	1,509,232	657,753
Accident Year 2013 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		922,634	942,022	963,634	965,875
IBNR (1)	[1]	$ 339,380
Number of Claims | Claim		5,595
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 457,890	304,503	152,052	$ 36,582
Accident Year 2014
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,397,264	5,369,393	5,183,056
IBNR (1)	[1]	1,396,883
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,090,343	2,250,446	957,525
Accident Year 2014 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,978,436	3,917,364	3,748,154
IBNR (1)	[1]	$ 1,052,314
Number of Claims | Claim		87,491
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,263,643	1,597,123	669,745
Accident Year 2014 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		843,003	799,952	747,671
IBNR (1)	[1]	$ 371,270
Number of Claims | Claim		5,969
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 312,548	138,832	$ 17,042
Accident Year 2015
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,498,206	5,354,494
IBNR (1)	[1]	1,962,006
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		2,152,569	970,277
Accident Year 2015 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,039,203	3,849,731
IBNR (1)	[1]	$ 1,418,476
Number of Claims | Claim		94,229
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 1,563,019	691,253
Accident Year 2015 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		800,568	755,087
IBNR (1)	[1]	$ 493,667
Number of Claims | Claim		6,391
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 139,345	$ 22,521
Accident Year 2016
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		6,005,506
IBNR (1)	[1]	3,425,518
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,192,137
Accident Year 2016 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,160,269
IBNR (1)	[1]	$ 2,358,172
Number of Claims | Claim		65,684
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 861,429
Accident Year 2016 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		730,198
IBNR (1)	[1]	$ 654,395
Number of Claims | Claim		3,704
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 23,462

[1] Total IBNR Liabilities Plus Expected Development on Reported Claims