Losses and Loss Expenses - Information about incurred and paid claims development - Insurance - Specialty (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
Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		9,042,561
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		7,380,486
All outstanding liabilities prior to 2007, net of reinsurance		139,887
Liabilities for unpaid losses and loss expenses, net of reinsurance		1,801,962
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 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		799,817	803,587	782,043	802,825	813,822	832,003	837,517	847,085	852,759	825,002
IBNR (1)		$ 4,787
Number of Claims | Claim		18,554
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 779,786	776,816	774,511	758,150	739,870	711,472	675,749	608,006	468,074	$ 249,996
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 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		826,097	830,905	813,581	839,237	851,059	863,902	884,249	934,797	875,540
IBNR (1)	[1]	$ 3,738
Number of Claims | Claim		22,083
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 797,195	791,620	786,003	767,795	742,474	699,636	637,898	511,167	$ 244,084
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 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		798,315	807,644	862,563	872,379	882,725	909,964	978,410	987,010
IBNR (1)		$ 1,770
Number of Claims | Claim		20,875
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 767,565	764,637	754,577	736,233	712,561	660,099	552,213	$ 311,190
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 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		837,473	838,086	849,972	875,392	894,249	962,021	1,004,165
IBNR (1)		$ 12,588
Number of Claims | Claim		24,437
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 783,693	774,635	757,382	714,045	660,232	548,616	$ 317,829
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 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		908,417	915,622	901,762	945,212	974,012	975,525
IBNR (1)		$ 34,269
Number of Claims | Claim		25,070
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 840,552	817,521	794,906	738,976	603,843	$ 294,082
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 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		890,061	901,214	917,430	950,530	1,009,999
IBNR (1)	[1]	$ 19,757
Number of Claims | Claim		27,855
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 804,007	767,416	698,093	589,660	$ 310,705
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 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		941,582	943,553	979,459	940,906
IBNR (1)		$ 45,919
Number of Claims | Claim		31,103
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 792,004	716,825	596,425	$ 331,485
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 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		948,242	964,163	945,355
IBNR (1)		$ 66,235
Number of Claims | Claim		36,659
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 749,446	618,150	$ 338,376
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 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,027,349	967,064
IBNR (1)		$ 137,686
Number of Claims | Claim		39,552
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 656,573	$ 335,018
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 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,065,208
IBNR (1)		$ 386,279
Number of Claims | Claim		28,418
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 409,665

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