Losses and Loss Expenses - Information about incurred and paid claims development - Reinsurance (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Short-duration Insurance Contracts, Historical Claims Duration [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
Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		17,387,267
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		11,037,118
All outstanding liabilities prior to 2008, net of reinsurance		1,462,279
Liabilities for unpaid losses and loss expenses, net of reinsurance		7,812,428
Accident Year 2008
Short-duration Insurance Contracts, Historical Claims Duration [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 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,550,910	1,572,735	1,576,186	1,602,265	1,602,838	1,603,603	1,591,632	1,609,615	1,652,541	1,663,877
IBNR (1)	[1]	88,242
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,336,339	1,314,670	1,273,015	1,243,248	1,196,258	1,127,219	1,040,745	902,212	668,128	$ 333,542
Accident Year 2009
Short-duration Insurance Contracts, Historical Claims Duration [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 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,074,915	1,077,364	1,085,620	1,083,440	1,099,591	1,110,034	1,116,873	1,165,458	1,196,599
IBNR (1)	[1]	113,938
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		841,836	820,956	786,808	753,439	705,992	629,310	541,644	401,707	$ 180,134
Accident Year 2010
Short-duration Insurance Contracts, Historical Claims Duration [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 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,315,708	1,330,861	1,340,749	1,333,060	1,335,712	1,352,763	1,351,737	1,334,848
IBNR (1)	[1]	109,971
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,085,119	1,049,210	970,233	869,444	807,859	699,133	503,585	$ 246,377
Accident Year 2011
Short-duration Insurance Contracts, Historical Claims Duration [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 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		2,143,025	2,148,365	2,157,092	2,125,718	2,166,994	2,272,204	2,290,233
IBNR (1)	[1]	140,206
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,839,294	1,781,055	1,724,586	1,634,216	1,489,716	1,288,721	$ 611,514
Accident Year 2012
Short-duration Insurance Contracts, Historical Claims Duration [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 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,853,661	1,853,238	1,874,339	1,885,005	1,905,585	1,878,343
IBNR (1)	[1]	201,735
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,437,667	1,335,374	1,243,382	1,086,961	797,075	$ 320,327
Accident Year 2013
Short-duration Insurance Contracts, Historical Claims Duration [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 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,544,112	1,568,744	1,613,682	1,628,842	1,694,149
IBNR (1)	[1]	203,259
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,167,568	1,063,853	950,627	718,050	$ 325,926
Accident Year 2014
Short-duration Insurance Contracts, Historical Claims Duration [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 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,447,743	1,469,007	1,499,811	1,495,337
IBNR (1)	[1]	228,278
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		994,171	850,000	672,189	$ 292,747
Accident Year 2015
Short-duration Insurance Contracts, Historical Claims Duration [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 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,516,094	1,510,101	1,565,814
IBNR (1)	[1]	346,943
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		880,530	603,969	$ 282,178
Accident Year 2016
Short-duration Insurance Contracts, Historical Claims Duration [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 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,901,157	1,888,793
IBNR (1)	[1]	553,465
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		846,509	$ 343,057
Accident Year 2017
Short-duration Insurance Contracts, Historical Claims Duration [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 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,039,942
IBNR (1)	[1]	1,950,688
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 608,085

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