Losses and Loss Adjustment Expenses - Total Losses and Loss Adjustment Expense Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Outstanding losses	$ 5,394,529	$ 5,168,900
IBNR	5,057,868	4,468,991
Fair value adjustments - acquired companies	(167,511)	(214,518)
Fair value adjustments - fair value option	(217,933)	(374,752)
ULAE	362,285	360,883
Total gross liability for unpaid losses and LAE	10,429,238	9,409,504	$ 7,398,088	$ 5,987,867
Losses and loss adjustment expenses	7,808,116	6,535,449
Losses and loss adjustment expenses, fair value (Note 10 and Note 12)	2,621,122	2,874,055
Non-life Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Outstanding losses	4,407,082	4,271,769
IBNR	3,945,407	3,527,767
Fair value adjustments - acquired companies	(170,689)	(217,527)
Fair value adjustments - fair value option	(217,933)	(374,752)
ULAE	331,494	333,405
Total gross liability for unpaid losses and LAE	8,295,361	7,540,662	5,949,472	4,716,363
Atrium
Liability for Claims and Claims Adjustment Expense [Line Items]
Outstanding losses	89,141	94,885
IBNR	136,543	140,521
Fair value adjustments - acquired companies	3,700	3,476
ULAE	2,288	2,402
Total gross liability for unpaid losses and LAE	231,672	241,284	240,873	212,122
StarStone
Liability for Claims and Claims Adjustment Expense [Line Items]
Outstanding losses	888,794	796,194
IBNR	962,353	787,894
Fair value adjustments - acquired companies	(522)	(467)
ULAE	28,503	25,076
Total gross liability for unpaid losses and LAE	1,879,128	1,608,697	$ 1,207,743	$ 1,059,382
Operating Segments | Non-life Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Outstanding losses	4,407,082	4,271,769
IBNR	3,945,407	3,527,767
Fair value adjustments - acquired companies	(170,689)	(217,527)
Fair value adjustments - fair value option	(217,933)	(374,752)
ULAE	331,494	333,405
Total gross liability for unpaid losses and LAE	8,295,361	7,540,662
Losses and loss adjustment expenses	5,674,239	4,666,607
Losses and loss adjustment expenses, fair value (Note 10 and Note 12)	2,621,122	2,874,055
Operating Segments | Atrium
Liability for Claims and Claims Adjustment Expense [Line Items]
Outstanding losses	89,141	94,885
IBNR	136,543	140,521
Fair value adjustments - acquired companies	3,700	3,476
Fair value adjustments - fair value option	0	0
ULAE	2,288	2,402
Total gross liability for unpaid losses and LAE	231,672	241,284
Losses and loss adjustment expenses	231,672	241,284
Losses and loss adjustment expenses, fair value (Note 10 and Note 12)	0	0
Operating Segments | StarStone
Liability for Claims and Claims Adjustment Expense [Line Items]
Outstanding losses	888,794	796,194
IBNR	962,353	787,894
Fair value adjustments - acquired companies	(522)	(467)
Fair value adjustments - fair value option	0	0
ULAE	28,503	25,076
Total gross liability for unpaid losses and LAE	1,879,128	1,608,697
Losses and loss adjustment expenses	1,879,128	1,608,697
Losses and loss adjustment expenses, fair value (Note 10 and Note 12)	0	0
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Outstanding losses	9,512	6,052
IBNR	13,565	12,809
Fair value adjustments - acquired companies	0	0
Fair value adjustments - fair value option	0	0
ULAE	0	0
Total gross liability for unpaid losses and LAE	23,077	18,861
Losses and loss adjustment expenses	23,077	18,861
Losses and loss adjustment expenses, fair value (Note 10 and Note 12)	$ 0	$ 0