Losses and Loss Expenses - Information about incurred and paid claims development - Insurance - Property (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
Property | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		8,320,290
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		6,403,947
All outstanding liabilities prior to 2008, net of reinsurance		13,430
Liabilities for unpaid losses and loss expenses, net of reinsurance		1,929,773
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 | Property | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		784,699	782,156	783,904	785,673	786,429	793,740	797,027	815,929	849,039	850,761
IBNR (1)	[1]	$ 12,884
Number of Claims | Claim		7,860
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 768,092	767,073	764,451	772,250	769,740	751,128	673,976	619,072	480,476	$ 156,169
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 | Property | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		356,760	350,438	352,427	354,888	354,587	360,747	368,148	384,987	409,525
IBNR (1)	[1]	$ 5,625
Number of Claims | Claim		6,736
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 346,694	341,939	338,609	319,161	316,849	295,786	262,119	198,738	$ 88,549
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 | Property | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		581,492	581,195	584,289	576,699	582,803	600,272	629,454	614,975
IBNR (1)	[1]	$ 8,919
Number of Claims | Claim		7,604
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 546,255	541,777	535,734	556,204	544,973	495,909	396,953	$ 172,596
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 | Property | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		871,842	875,589	882,602	892,650	927,416	950,884	1,015,292
IBNR (1)	[1]	$ 71
Number of Claims | Claim		9,239
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 858,866	850,727	849,147	819,464	777,503	637,440	$ 222,071
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 | Property | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		645,148	648,211	656,688	655,395	680,608	706,411
IBNR (1)	[1]	$ 6,639
Number of Claims | Claim		10,197
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 625,089	614,891	594,798	550,774	404,469	$ 168,359
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 | Property | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		716,067	718,804	725,484	748,537	766,897
IBNR (1)	[1]	$ 10,823
Number of Claims | Claim		11,381
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 683,854	666,485	613,722	474,473	$ 172,866
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 | Property | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		756,646	757,109	766,516	729,439
IBNR (1)	[1]	$ 12,039
Number of Claims | Claim		13,954
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 708,562	674,517	523,596	$ 186,475
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 | Property | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		839,474	846,954	822,971
IBNR (1)	[1]	$ 38,686
Number of Claims | Claim		16,747
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 666,765	503,459	$ 231,415
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 | Property | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,113,153	989,575
IBNR (1)	[1]	$ 51,908
Number of Claims | Claim		20,372
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 699,497	$ 334,272
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 | Property | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,655,009
IBNR (1)	[1]	$ 275,986
Number of Claims | Claim		17,088
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 500,273

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