Taxation - Tax rates by jurisdiction (Details)		3 Months Ended	9 Months Ended	12 Months Ended
		Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Ireland | Active income
Income Tax Disclosure [Line Items]
Applicable statutory tax rate, by jurisdiction	[1]				12.50%
Ireland | Passive Income [Member]
Income Tax Disclosure [Line Items]
Applicable statutory tax rate, by jurisdiction	[1]				25.00%
Bermuda
Income Tax Disclosure [Line Items]
Applicable statutory tax rate, by jurisdiction					0.00%
United States
Income Tax Disclosure [Line Items]
Applicable statutory tax rate, by jurisdiction	[2]				35.00%
United Kingdom
Income Tax Disclosure [Line Items]
Applicable statutory tax rate, by jurisdiction	[3]	20.00%	19.00%		19.25%
Switzerland | Active income
Income Tax Disclosure [Line Items]
Applicable statutory tax rate, by jurisdiction	[4]				21.15%
Germany
Income Tax Disclosure [Line Items]
Applicable statutory tax rate, by jurisdiction	[5]				15.00%
Germany | Including surcharge and other taxes
Income Tax Disclosure [Line Items]
Applicable statutory tax rate, by jurisdiction					33.00%
France
Income Tax Disclosure [Line Items]
Applicable statutory tax rate, by jurisdiction	[6]				33.33%
France | Including surcharge and other taxes
Income Tax Disclosure [Line Items]
Applicable statutory tax rate, by jurisdiction	[6]				34.43%
Subsequent Event [Member] | United States
Income Tax Disclosure [Line Items]
Applicable statutory tax rate, by jurisdiction	[2]			21.00%

[1] The 12.50% statutory corporate income tax rate applies to active income from the conduct of a trade in Ireland. For passive income or income from other defined activities the statutory rate increases to 25%.

[2] The statutory tax rate is 35% for the 2017 tax year. Under the Tax Cuts and Jobs Act of 2017, the federal corporate tax rate has been reduced to 21% for taxable years beginning on or after January 1, 2018.

[3] The statutory rate is 19% effective April 1, 2017. However, from January 1, through March 31, 2017 the statutory rate is 20%. Due to this change of rate, the average statutory tax rate for 2017 is 19.25%.

[4] Represents the combined federal and cantonal rate primarily applicable to the Company's Swiss entities.

[5] The statutory corporate income tax rate is 15.00%. However, including applicable surcharges and local trade tax, which can vary by location, would increase the combined statutory rate to approximately 33%.

[6] The statutory corporate income tax rate is 33.33%. However, with the mandatory social surcharge, the combined statutory rate would increase to 34.43%.