Fair Value Measurements - Insurance Contracts - Fair Value Option (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reinsurance balances recoverable on paid and unpaid losses
Beginning fair value	$ 321,973	$ 176,723
Assumed business	90,805	304,387
Incurred losses and LAE:
Change in fair value	(970)	21,369
Ending fair value	356,668	321,973	$ 176,723
Net
Beginning fair value	2,134,464	1,252,445
Assumed business	9,218	1,517,281
Incurred losses and LAE:
Reduction in estimates of ultimate losses	(29,732)	(78,388)
Reduction in unallocated LAE	(19,915)	(20,656)
Change in fair value	117,181	6,664	30,256
Total incurred losses and LAE	67,534	(92,380)
Paid losses	(324,625)	(428,774)
Effect of exchange rate movements	39,013	(114,108)
Ending fair value	1,925,604	2,134,464	1,252,445
Components of Changes in Fair Value
Change in fair value	117,181	6,664	30,256
Reinsurance balances recoverable
Reinsurance balances recoverable on paid and unpaid losses
Beginning fair value	739,591	542,224
Assumed business	0	372,780
Incurred losses and LAE:
Reduction in estimates of ultimate losses	(2,958)	(30,041)
Reduction in unallocated LAE	0	0
Change in fair value	43,449	21,181
Net incurred losses and LAE	40,491	(8,860)
Paid losses	(92,145)	(148,175)
Effect of exchange rate movements	7,581	(18,378)
Ending fair value	695,518	739,591	542,224
Losses and LAE liabilities
Liability for losses and LAE
Beginning fair value	2,874,055	1,794,669
Assumed business	9,218	1,890,061
Incurred losses and LAE:
Reduction in estimates of ultimate losses	(32,690)	(108,429)
Reduction in unallocated LAE	(19,915)	(20,656)
Change in fair value	160,630	27,845
Net incurred losses and LAE	108,025	(101,240)
Paid losses	(416,770)	(576,949)
Effect of exchange rate movements	46,594	(132,486)
Ending fair value	2,621,122	2,874,055	1,794,669
Duration
Incurred losses and LAE:
Change in fair value	22,719	74,011	41,332
Components of Changes in Fair Value
Change in fair value	22,719	74,011	41,332
Corporate bond yield
Incurred losses and LAE:
Change in fair value	94,462	(71,031)	(11,076)
Components of Changes in Fair Value
Change in fair value	94,462	(71,031)	(11,076)
Risk cost of capital
Incurred losses and LAE:
Change in fair value	0	3,684	0
Components of Changes in Fair Value
Change in fair value	$ 0	$ 3,684	$ 0