Investment Information - Restricted assets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Restricted Assets [Line Items]
Restricted assets	$ 8,095,051	$ 7,553,903
Collateral or guarantees - affiliated transactions
Restricted Assets [Line Items]
Restricted assets	4,629,369	4,623,483
Collateral or guarantees - third party agreements
Restricted Assets [Line Items]
Restricted assets	2,605,587	2,181,682
Deposits with US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	788,295	689,114
Deposits with non-US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	71,800	59,624
Watford Holdings Ltd
Restricted Assets [Line Items]
Assets pledged as collateral	$ 1,060,000	$ 1,300,000