Segment Information - Gross Premiums Written by Geographical Area (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Gross premiums written	$ 1,105,277
Gross premiums written (percent)	100.00%
Non-Life Run-Off
Segment Reporting Information [Line Items]
Gross premiums written	$ 38,704
Gross premiums written (percent)	100.00%
Atrium
Segment Reporting Information [Line Items]
Gross premiums written	$ 149,082
Gross premiums written (percent)	100.00%
StarStone Segment
Segment Reporting Information [Line Items]
Gross premiums written	$ 824,714
Gross premiums written (percent)	100.00%
Life and Annuities
Segment Reporting Information [Line Items]
Gross premiums written	$ 92,777
Gross premiums written (percent)	100.00%
United States
Segment Reporting Information [Line Items]
Gross premiums written	$ 726,163
Gross premiums written (percent)	65.70%
United States | Non-Life Run-Off
Segment Reporting Information [Line Items]
Gross premiums written	$ 38,704
Gross premiums written (percent)	100.00%
United States | Atrium
Segment Reporting Information [Line Items]
Gross premiums written	$ 82,821
Gross premiums written (percent)	55.50%
United States | StarStone Segment
Segment Reporting Information [Line Items]
Gross premiums written	$ 526,981
Gross premiums written (percent)	63.90%
United States | Life and Annuities
Segment Reporting Information [Line Items]
Gross premiums written	$ 77,657
Gross premiums written (percent)	83.70%
United Kingdom
Segment Reporting Information [Line Items]
Gross premiums written	$ 91,064
Gross premiums written (percent)	8.20%
United Kingdom | Non-Life Run-Off
Segment Reporting Information [Line Items]
Gross premiums written	$ 0
Gross premiums written (percent)	0.00%
United Kingdom | Atrium
Segment Reporting Information [Line Items]
Gross premiums written	$ 8,774
Gross premiums written (percent)	5.90%
United Kingdom | StarStone Segment
Segment Reporting Information [Line Items]
Gross premiums written	$ 81,200
Gross premiums written (percent)	9.80%
United Kingdom | Life and Annuities
Segment Reporting Information [Line Items]
Gross premiums written	$ 1,090
Gross premiums written (percent)	1.20%
Europe
Segment Reporting Information [Line Items]
Gross premiums written	$ 119,580
Gross premiums written (percent)	10.80%
Europe | Non-Life Run-Off
Segment Reporting Information [Line Items]
Gross premiums written	$ 0
Gross premiums written (percent)	0.00%
Europe | Atrium
Segment Reporting Information [Line Items]
Gross premiums written	$ 8,729
Gross premiums written (percent)	5.80%
Europe | StarStone Segment
Segment Reporting Information [Line Items]
Gross premiums written	$ 109,058
Gross premiums written (percent)	13.20%
Europe | Life and Annuities
Segment Reporting Information [Line Items]
Gross premiums written	$ 1,793
Gross premiums written (percent)	1.90%
Asia
Segment Reporting Information [Line Items]
Gross premiums written	$ 41,645
Gross premiums written (percent)	3.80%
Asia | Non-Life Run-Off
Segment Reporting Information [Line Items]
Gross premiums written	$ 0
Gross premiums written (percent)	0.00%
Asia | Atrium
Segment Reporting Information [Line Items]
Gross premiums written	$ 7,889
Gross premiums written (percent)	5.30%
Asia | StarStone Segment
Segment Reporting Information [Line Items]
Gross premiums written	$ 33,756
Gross premiums written (percent)	4.10%
Asia | Life and Annuities
Segment Reporting Information [Line Items]
Gross premiums written	$ 0
Gross premiums written (percent)	0.00%
Rest of World
Segment Reporting Information [Line Items]
Gross premiums written	$ 126,825
Gross premiums written (percent)	11.50%
Rest of World | Non-Life Run-Off
Segment Reporting Information [Line Items]
Gross premiums written	$ 0
Gross premiums written (percent)	0.00%
Rest of World | Atrium
Segment Reporting Information [Line Items]
Gross premiums written	$ 40,869
Gross premiums written (percent)	27.50%
Rest of World | StarStone Segment
Segment Reporting Information [Line Items]
Gross premiums written	$ 73,719
Gross premiums written (percent)	9.00%
Rest of World | Life and Annuities
Segment Reporting Information [Line Items]
Gross premiums written	$ 12,237
Gross premiums written (percent)	13.20%