Debt - Debt (Details) £ in Millions, $ in Millions	Mar. 31, 2020 USD ($)	Mar. 31, 2020 GBP (£)	Mar. 30, 2020 USD ($)
Club Facility [Member]
Debt Instruments [Line Items]
Letters of Credit Outstanding, Amount	$ 165	£ 60
Bilateral Facility [Member]
Debt Instruments [Line Items]
Letters of Credit Outstanding, Amount	18
Debt Instrument, Unused Borrowing Capacity, Amount	$ 3
Senior Notes | Senior Note Six Point Zero Percent Due in Two Thousand Nineteen [Member]
Debt Instruments [Line Items]
Repayment of debt			$ 500