Segment Information - Gross Premiums Written by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Gross, premiums written	$ 1,103,393	$ 1,316,097	$ 1,068,453
United States
Segment Reporting Information [Line Items]
Gross, premiums written	643,109
United Kingdom
Segment Reporting Information [Line Items]
Gross, premiums written	128,144
Europe
Segment Reporting Information [Line Items]
Gross, premiums written	134,188
Asia
Segment Reporting Information [Line Items]
Gross, premiums written	81,425
Rest of World
Segment Reporting Information [Line Items]
Gross, premiums written	$ 116,527
Geographic concentration risk | Gross premiums written
Segment Reporting Information [Line Items]
Gross premiums written (percent)	100.00%
Geographic concentration risk | Gross premiums written | United States
Segment Reporting Information [Line Items]
Gross premiums written (percent)	58.20%
Geographic concentration risk | Gross premiums written | United Kingdom
Segment Reporting Information [Line Items]
Gross premiums written (percent)	11.60%
Geographic concentration risk | Gross premiums written | Europe
Segment Reporting Information [Line Items]
Gross premiums written (percent)	12.20%
Geographic concentration risk | Gross premiums written | Asia
Segment Reporting Information [Line Items]
Gross premiums written (percent)	7.40%
Geographic concentration risk | Gross premiums written | Rest of World
Segment Reporting Information [Line Items]
Gross premiums written (percent)	10.60%
Operating Segments | Non-life Run-off
Segment Reporting Information [Line Items]
Gross, premiums written	$ (25,069)	(8,910)	14,102
Operating Segments | Atrium
Segment Reporting Information [Line Items]
Gross, premiums written	192,373	171,494	153,472
Operating Segments | StarStone
Segment Reporting Information [Line Items]
Gross, premiums written	917,555	1,121,135	895,160
Operating Segments | United States | Non-life Run-off
Segment Reporting Information [Line Items]
Gross, premiums written	(25,012)
Operating Segments | United States | Atrium
Segment Reporting Information [Line Items]
Gross, premiums written	102,199
Operating Segments | United States | StarStone
Segment Reporting Information [Line Items]
Gross, premiums written	548,683
Operating Segments | United Kingdom | Non-life Run-off
Segment Reporting Information [Line Items]
Gross, premiums written	350
Operating Segments | United Kingdom | Atrium
Segment Reporting Information [Line Items]
Gross, premiums written	12,972
Operating Segments | United Kingdom | StarStone
Segment Reporting Information [Line Items]
Gross, premiums written	114,822
Operating Segments | Europe | Non-life Run-off
Segment Reporting Information [Line Items]
Gross, premiums written	(265)
Operating Segments | Europe | Atrium
Segment Reporting Information [Line Items]
Gross, premiums written	17,677
Operating Segments | Europe | StarStone
Segment Reporting Information [Line Items]
Gross, premiums written	115,481
Operating Segments | Asia | Non-life Run-off
Segment Reporting Information [Line Items]
Gross, premiums written	(48)
Operating Segments | Asia | Atrium
Segment Reporting Information [Line Items]
Gross, premiums written	5,700
Operating Segments | Asia | StarStone
Segment Reporting Information [Line Items]
Gross, premiums written	75,773
Operating Segments | Rest of World | Non-life Run-off
Segment Reporting Information [Line Items]
Gross, premiums written	(94)
Operating Segments | Rest of World | Atrium
Segment Reporting Information [Line Items]
Gross, premiums written	53,825
Operating Segments | Rest of World | StarStone
Segment Reporting Information [Line Items]
Gross, premiums written	$ 62,796
Operating Segments | Geographic concentration risk | Gross premiums written | Non-life Run-off
Segment Reporting Information [Line Items]
Gross premiums written (percent)	100.00%
Operating Segments | Geographic concentration risk | Gross premiums written | Atrium
Segment Reporting Information [Line Items]
Gross premiums written (percent)	100.00%
Operating Segments | Geographic concentration risk | Gross premiums written | StarStone
Segment Reporting Information [Line Items]
Gross premiums written (percent)	100.00%
Operating Segments | Geographic concentration risk | Gross premiums written | United States | Non-life Run-off
Segment Reporting Information [Line Items]
Gross premiums written (percent)	99.70%
Operating Segments | Geographic concentration risk | Gross premiums written | United States | Atrium
Segment Reporting Information [Line Items]
Gross premiums written (percent)	53.10%
Operating Segments | Geographic concentration risk | Gross premiums written | United States | StarStone
Segment Reporting Information [Line Items]
Gross premiums written (percent)	59.80%
Operating Segments | Geographic concentration risk | Gross premiums written | United Kingdom | Non-life Run-off
Segment Reporting Information [Line Items]
Gross premiums written (percent)	(1.40%)
Operating Segments | Geographic concentration risk | Gross premiums written | United Kingdom | Atrium
Segment Reporting Information [Line Items]
Gross premiums written (percent)	6.70%
Operating Segments | Geographic concentration risk | Gross premiums written | United Kingdom | StarStone
Segment Reporting Information [Line Items]
Gross premiums written (percent)	12.50%
Operating Segments | Geographic concentration risk | Gross premiums written | Europe | Non-life Run-off
Segment Reporting Information [Line Items]
Gross premiums written (percent)	1.10%
Operating Segments | Geographic concentration risk | Gross premiums written | Europe | Atrium
Segment Reporting Information [Line Items]
Gross premiums written (percent)	9.20%
Operating Segments | Geographic concentration risk | Gross premiums written | Europe | StarStone
Segment Reporting Information [Line Items]
Gross premiums written (percent)	12.60%
Operating Segments | Geographic concentration risk | Gross premiums written | Asia | Non-life Run-off
Segment Reporting Information [Line Items]
Gross premiums written (percent)	0.20%
Operating Segments | Geographic concentration risk | Gross premiums written | Asia | Atrium
Segment Reporting Information [Line Items]
Gross premiums written (percent)	3.00%
Operating Segments | Geographic concentration risk | Gross premiums written | Asia | StarStone
Segment Reporting Information [Line Items]
Gross premiums written (percent)	8.30%
Operating Segments | Geographic concentration risk | Gross premiums written | Rest of World | Non-life Run-off
Segment Reporting Information [Line Items]
Gross premiums written (percent)	0.40%
Operating Segments | Geographic concentration risk | Gross premiums written | Rest of World | Atrium
Segment Reporting Information [Line Items]
Gross premiums written (percent)	28.00%
Operating Segments | Geographic concentration risk | Gross premiums written | Rest of World | StarStone
Segment Reporting Information [Line Items]
Gross premiums written (percent)	6.80%
Other
Segment Reporting Information [Line Items]
Gross, premiums written	$ 18,534	$ 32,378	$ 5,719
Other | United States
Segment Reporting Information [Line Items]
Gross, premiums written	17,239
Other | United Kingdom
Segment Reporting Information [Line Items]
Gross, premiums written	0
Other | Europe
Segment Reporting Information [Line Items]
Gross, premiums written	1,295
Other | Asia
Segment Reporting Information [Line Items]
Gross, premiums written	0
Other | Rest of World
Segment Reporting Information [Line Items]
Gross, premiums written	$ 0
Other | Geographic concentration risk | Gross premiums written
Segment Reporting Information [Line Items]
Gross premiums written (percent)	100.00%
Other | Geographic concentration risk | Gross premiums written | United States
Segment Reporting Information [Line Items]
Gross premiums written (percent)	93.00%
Other | Geographic concentration risk | Gross premiums written | United Kingdom
Segment Reporting Information [Line Items]
Gross premiums written (percent)	0.00%
Other | Geographic concentration risk | Gross premiums written | Europe
Segment Reporting Information [Line Items]
Gross premiums written (percent)	7.00%
Other | Geographic concentration risk | Gross premiums written | Asia
Segment Reporting Information [Line Items]
Gross premiums written (percent)	0.00%
Other | Geographic concentration risk | Gross premiums written | Rest of World
Segment Reporting Information [Line Items]
Gross premiums written (percent)	0.00%