Losses and Loss Expenses - Information about incurred and paid claims development - Insurance - Specialty (Details) $ in Thousands		Dec. 31, 2017 USD ($) Claim	Dec. 31, 2016 USD ($)	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 ($)	Dec. 31, 2008 USD ($)
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		$ 58,562,228
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		38,846,447
All outstanding liabilities prior to 2008, net of reinsurance		2,451,126
Liabilities for unpaid losses and loss expenses, net of reinsurance		22,166,907
Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		41,174,961
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		27,809,329
All outstanding liabilities prior to 2008, net of reinsurance		988,847
Liabilities for unpaid losses and loss expenses, net of reinsurance		14,354,479
Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		9,619,350
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		7,909,210
All outstanding liabilities prior to 2008, net of reinsurance		132,594
Liabilities for unpaid losses and loss expenses, net of reinsurance		1,842,734
Accident Year 2008
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,410,125	$ 5,471,441	$ 5,510,523	$ 5,572,963	$ 5,603,554	$ 5,631,593	$ 5,674,761	$ 5,651,657	$ 5,735,428	$ 5,644,208
IBNR (1)	[1]	213,925
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,897,567	4,827,462	4,704,623	4,568,478	4,352,467	4,049,900	3,539,510	2,934,270	2,051,520	810,334
Accident Year 2008 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,859,215	3,898,706	3,934,337	3,970,698	4,000,716	4,027,990	4,083,129	4,042,042	4,082,887	3,980,331
IBNR (1)	[1]	$ 125,683
Number of Claims | Claim		48,668
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,561,228	3,512,792	3,431,608	3,325,230	3,156,209	2,922,681	2,498,765	2,032,058	1,383,392	476,792
Accident Year 2008 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		839,244	851,009	856,842	837,617	864,078	876,491	890,963	910,335	968,492	891,890
IBNR (1)	[1]	$ (2,387)
Number of Claims | Claim		19,242
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 821,249	821,401	817,093	809,870	791,525	765,430	722,183	659,645	529,159	$ 253,344
Accident Year 2009
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,542,173	4,525,234	4,555,586	4,685,356	4,680,173	4,689,064	4,668,243	4,755,362	4,763,117
IBNR (1)	[1]	293,117
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,844,074	3,767,363	3,650,893	3,460,344	3,204,371	2,731,005	2,254,270	1,573,893	657,811
Accident Year 2009 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,467,258	3,447,870	3,469,966	3,601,916	3,580,582	3,579,030	3,551,370	3,589,904	3,566,518
IBNR (1)	[1]	$ 179,179
Number of Claims | Claim		48,912
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,002,238	2,946,407	2,864,085	2,706,905	2,498,379	2,101,695	1,712,626	1,172,186	477,677
Accident Year 2009 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		821,551	822,085	831,410	887,959	897,121	909,680	936,862	1,008,485	1,014,909
IBNR (1)	[1]	$ 1,578
Number of Claims | Claim		18,360
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 796,935	790,078	787,163	776,899	758,202	733,742	680,060	568,559	$ 321,635
Accident Year 2010
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,053,432	5,138,545	5,109,862	5,127,836	5,178,036	5,158,742	5,272,265	5,141,028
IBNR (1)	[1]	300,001
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,358,655	4,212,573	3,971,942	3,634,321	3,252,357	2,740,308	1,926,916	873,754
Accident Year 2010 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,737,724	3,807,684	3,769,113	3,794,776	3,842,324	3,805,979	3,920,528	3,806,180
IBNR (1)	[1]	$ 190,030
Number of Claims | Claim		55,114
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,273,536	3,163,363	3,001,709	2,764,877	2,444,498	2,041,175	1,423,331	627,377
Accident Year 2010 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		860,409	863,284	863,079	875,049	901,020	918,623	987,403	1,030,506
IBNR (1)	[1]	$ 11,198
Number of Claims | Claim		21,791
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 816,193	807,477	798,099	780,483	736,611	680,947	566,060	$ 327,917
Accident Year 2011
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		6,226,953	6,276,173	6,311,990	6,175,271	6,306,319	6,392,279	6,430,567
IBNR (1)	[1]	433,196
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		5,375,172	5,175,876	4,883,707	4,373,375	3,778,977	2,963,941	1,285,733
Accident Year 2011 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,083,928	4,127,808	4,154,898	4,049,553	4,139,325	4,120,075	4,140,334
IBNR (1)	[1]	$ 292,990
Number of Claims | Claim		59,373
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,535,878	3,394,821	3,159,121	2,739,159	2,289,261	1,675,220	674,219
Accident Year 2011 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		922,492	934,612	940,624	925,539	962,256	1,000,374	999,603
IBNR (1)	[1]	$ 24,446
Number of Claims | Claim		22,704
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 873,203	865,691	842,024	818,112	759,238	622,701	$ 303,685
Accident Year 2012
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,827,240	5,837,626	5,930,172	5,808,941	5,836,701	5,869,907
IBNR (1)	[1]	566,059
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,665,888	4,312,155	3,903,535	3,232,741	2,317,760	979,204
Accident Year 2012 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,973,579	3,984,388	4,055,833	3,923,936	3,931,116	3,991,564
IBNR (1)	[1]	$ 364,324
Number of Claims | Claim		64,064
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,228,221	2,976,781	2,660,153	2,145,780	1,520,685	658,877
Accident Year 2012 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		907,145	920,827	936,682	946,199	986,668	1,039,752
IBNR (1)	[1]	$ 7,976
Number of Claims | Claim		25,341
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 848,480	830,272	793,659	722,133	610,222	$ 323,488
Accident Year 2013
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,526,922	5,632,776	5,739,200	5,777,217	5,767,068
IBNR (1)	[1]	652,927
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,207,557	3,755,115	3,171,205	2,285,432	1,009,683
Accident Year 2013 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,982,810	4,064,032	4,125,518	4,148,375	4,072,919
IBNR (1)	[1]	$ 449,668
Number of Claims | Claim		70,379
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,039,989	2,691,262	2,220,578	1,567,382	683,757
Accident Year 2013 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		963,962	973,662	973,591	1,011,519	969,561
IBNR (1)	[1]	$ 23,499
Number of Claims | Claim		28,501
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 859,290	818,201	741,972	617,540	$ 343,578
Accident Year 2014
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,574,378	5,580,769	5,549,389	5,368,693
IBNR (1)	[1]	987,715
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,765,670	3,188,259	2,322,853	985,112
Accident Year 2014 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,126,635	4,111,762	4,049,578	3,873,356
IBNR (1)	[1]	$ 759,437
Number of Claims | Claim		80,946
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,771,499	2,338,259	1,650,664	692,365
Accident Year 2014 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		962,638	971,827	988,726	972,197
IBNR (1)	[1]	$ 28,881
Number of Claims | Claim		35,063
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 831,653	768,421	635,917	$ 348,944
Accident Year 2015
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,864,953	5,678,720	5,540,823
IBNR (1)	[1]	1,405,335
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,277,189	2,217,824	997,289
Accident Year 2015 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,348,859	4,168,619	3,975,009
IBNR (1)	[1]	$ 1,058,392
Number of Claims | Claim		94,968
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,396,659	1,613,855	715,111
Accident Year 2015 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,054,762	1,054,941	995,703
IBNR (1)	[1]	$ 76,262
Number of Claims | Claim		43,706
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 828,146	675,618	$ 346,794
Accident Year 2016
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		6,427,610	6,182,999
IBNR (1)	[1]	2,013,460
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		2,769,701	1,233,032
Accident Year 2016 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,526,453	4,294,206
IBNR (1)	[1]	$ 1,459,995
Number of Claims | Claim		102,466
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 1,923,192	889,975
Accident Year 2016 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,174,955	1,107,572
IBNR (1)	[1]	$ 120,726
Number of Claims | Claim		48,416
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 800,160	$ 424,069
Accident Year 2017
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		8,108,442
IBNR (1)	[1]	4,386,436
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,684,974
Accident Year 2017 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,068,500
IBNR (1)	[1]	$ 2,435,748
Number of Claims | Claim		73,518
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 1,076,889
Accident Year 2017 | Specialty | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,112,192
IBNR (1)	[1]	$ 403,497
Number of Claims | Claim		33,402
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 433,901

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