Segment Information - Net premiums written by geographical location (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	$ 10,242,683	$ 7,951,063	$ 5,944,841
Life operations
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	12,047	62,239	177,632
Life operations | Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	12,194	62,276	91,979
Life operations | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	(147)	(37)	85,653
Operating Segments [Member] | Property and casualty operations
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	10,230,636	7,888,824	5,767,209
Operating Segments [Member] | Property and casualty operations | Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	1,267,613	781,618	636,109
Operating Segments [Member] | Property and casualty operations | United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	3,575,969	3,045,031	2,528,196
Operating Segments [Member] | Property and casualty operations | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	4,778,299	3,628,258	2,124,117
Operating Segments [Member] | Property and casualty operations | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	$ 608,755	$ 433,917	$ 478,787