Significant New Business - 2018 Significant Business (Details) $ in Thousands, £ in Millions, $ in Millions												12 Months Ended
	Oct. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 27, 2018 USD ($)	Aug. 01, 2018 USD ($)	Feb. 23, 2018 AUD ($)	Feb. 23, 2018 USD ($)	Feb. 16, 2018 USD ($)	Feb. 16, 2018 GBP (£)	Jan. 29, 2018 USD ($)	Jan. 29, 2018 GBP (£)	Dec. 28, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Feb. 23, 2018 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Investments [Line Items]
Assumed business												$ 1,586,307	$ 1,772,104	$ 1,525,703
Funds held by reinsured companies		$ 321,267										408,735	321,267
Deferred charge		86,585										$ 272,462	$ 86,585	$ 80,192		$ 94,551
Allianz
Schedule of Investments [Line Items]
Reinsurance premium consideration		$ 70,000
Funds held by reinsured companies											$ 81,400
Assumed business, net											$ 81,400
Maiden Re North America
Schedule of Investments [Line Items]
Assumed business			$ 72,100
Funds held by reinsured companies			70,400
Deferred charge			$ 1,700
Coca-Cola
Schedule of Investments [Line Items]
Assumed business				$ 120,800
Deferred charge				17,200
Cash consideration				$ 103,600
Zurich
Schedule of Investments [Line Items]
Reinsurance premium consideration	$ 465,500				$ 343.9	$ 268,700
Assumed business	$ 622,900				359.4	$ 280,800
Fair value adjustment on gross reserves					$ 15.5										$ 12,100
Neon
Schedule of Investments [Line Items]
Reinsurance premium consideration							$ 445,100	£ 329.1
Assumed business							546,300	403.9
Fair value adjustment on gross reserves							20,600
Assumed business, net							462,600	£ 342.1
Fair value adjustment on net reserves							$ 17,500
Novae Transaction
Schedule of Investments [Line Items]
Reinsurance premium consideration									$ 803,500	£ 594.1
Assumed business									1,163,200	860.1
Fair value adjustment on gross reserves									67,500
Assumed business, net									853,000	£ 630.7
Fair value adjustment on net reserves									$ 49,500