Derivative Financial Instruments (Details) - Sep. 30, 2018 £ in Millions, $ in Millions, $ in Millions, $ in Millions	GBP (£) item	AUD ($) item	CAD ($) item	USD ($) item
Foreign Exchange Contract
Derivative [Line Items]
Notional Amount			$ 5
Interest Rate Swap Contracts
Derivative [Line Items]
Number of contracts with credit risk related contingent features | item	0	0	0	0
Derivatives In Cash Flow Hedging Relationships [Member] | 2018 | U.S.
Derivative [Line Items]
Weighted average fixed rate	2.11%	2.11%	2.11%	2.11%
Derivative liability, notional amount				$ 1,450
Derivatives In Cash Flow Hedging Relationships [Member] | 2018 | U.K
Derivative [Line Items]
Weighted average fixed rate	0.82%	0.82%	0.82%	0.82%
Derivative liability, notional amount | £	£ 550
Derivatives In Cash Flow Hedging Relationships [Member] | 2019 | U.S.
Derivative [Line Items]
Weighted average fixed rate	2.06%	2.06%	2.06%	2.06%
Derivative liability, notional amount				$ 1,000
Derivatives In Cash Flow Hedging Relationships [Member] | 2019 | U.K
Derivative [Line Items]
Weighted average fixed rate	0.90%	0.90%	0.90%	0.90%
Derivative liability, notional amount | £	£ 550
Derivatives In Cash Flow Hedging Relationships [Member] | 2019 | Australia
Derivative [Line Items]
Weighted average fixed rate	2.98%	2.98%	2.98%	2.98%
Derivative liability, notional amount		$ 35
Derivatives In Cash Flow Hedging Relationships [Member] | 2020 | U.S.
Derivative [Line Items]
Weighted average fixed rate	2.06%	2.06%	2.06%	2.06%
Derivative liability, notional amount				$ 1,000
Derivatives In Cash Flow Hedging Relationships [Member] | 2020 | U.K
Derivative [Line Items]
Weighted average fixed rate	0.94%	0.94%	0.94%	0.94%
Derivative liability, notional amount | £	£ 500
Derivatives In Cash Flow Hedging Relationships [Member] | 2021 | U.S.
Derivative [Line Items]
Weighted average fixed rate	1.46%	1.46%	1.46%	1.46%
Derivative liability, notional amount				$ 400
Derivatives In Cash Flow Hedging Relationships [Member] | 2021 | U.K
Derivative [Line Items]
Weighted average fixed rate	0.94%	0.94%	0.94%	0.94%
Derivative liability, notional amount | £	£ 500
Derivatives In Cash Flow Hedging Relationships [Member] | 2022 | U.S.
Derivative [Line Items]
Weighted average fixed rate	1.46%	1.46%	1.46%	1.46%
Derivative liability, notional amount				$ 400
Derivatives In Cash Flow Hedging Relationships [Member] | 2022 | U.K
Derivative [Line Items]
Weighted average fixed rate	0.94%	0.94%	0.94%	0.94%
Derivative liability, notional amount | £	£ 500