Significant New Business - 2017 Significant Business (Details) $ in Thousands, £ in Millions					12 Months Ended
	Dec. 28, 2017 USD ($)	Feb. 07, 2017 USD ($)	Feb. 07, 2017 GBP (£)	Jan. 11, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Feb. 07, 2017 GBP (£)
Schedule of Investments [Line Items]
Funds held by reinsured companies					$ 408,735	$ 321,267
Assumed business					$ 1,586,307	$ 1,772,104	$ 1,525,703
Allianz
Schedule of Investments [Line Items]
Percentage of reinsurance agreements entered	50.00%
Assumed business, net	$ 81,400
Funds held by reinsured companies	81,400
Funds transferred to a reinsurance trust	8,100
Funds held by reinsured companies	$ 162,800
RSA
Schedule of Investments [Line Items]
Assumed business, net		$ 1,153,900	£ 927.5
Assumed business		1,301,800	£ 1,046.4
Reinsurance premium receivable		997,200						£ 801.6
Fair value adjustment on gross reserves		174,100
Fair value adjustment on net reserves		$ 156,700
QBE
Schedule of Investments [Line Items]
Assumed business, net				$ 447,000
Assumed business				1,019,000
Reinsurance premium receivable				403,800
Fair value adjustment on gross reserves				180,000
Fair value adjustment on net reserves				43,200
Restricted cash				227,600
Funds held				$ 176,200