Significant New Business - 2019 Significant Business (Details) $ in Thousands, £ in Millions, $ in Millions								12 Months Ended
	Oct. 01, 2019 USD ($)	Aug. 05, 2019 USD ($)	Apr. 11, 2019 USD ($)	Feb. 14, 2019 USD ($) Agreement	Feb. 14, 2019 GBP (£) Agreement	Feb. 23, 2018 AUD ($)	Feb. 23, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Investments [Line Items]
Assumed business								$ 1,586,307	$ 1,772,104	$ 1,525,703
Deferred charge asset								$ 272,462	$ 86,585	$ 80,192	$ 94,551
Zurich
Schedule of Investments [Line Items]
Assumed business	$ 622,900					$ 359.4	$ 280,800
Reinsurance premium consideration	465,500					$ 343.9	$ 268,700
Deferred charge asset	$ 115,800
Maiden Re Bermuda
Schedule of Investments [Line Items]
Assumed business		$ 530,200
Reinsurance premium consideration		445,000
Deferred charge asset		85,200
Amerisure
Schedule of Investments [Line Items]
Assumed business			$ 48,300
Reinsurance premium consideration			45,500
Deferred charge asset			$ 2,900
Percentage of quota share assumed			50.00%
AmTrust RITC
Schedule of Investments [Line Items]
Assumed business				$ 897,100	£ 703.8
Deferred charge asset				$ 20,600
Number of quota share agreements | Agreement				4	4
Assumed business, net				$ 620,400	£ 486.8
Cash consideration				$ 688,200	£ 539.9
Minimum | Maiden Re Bermuda
Schedule of Investments [Line Items]
Assumed business		2,200,000
Maximum | Maiden Re Bermuda
Schedule of Investments [Line Items]
Assumed business		$ 600,000
Zurich | EnhanzedRe
Schedule of Investments [Line Items]
Percentage ceded								10.00%