Derivative Financial Instruments (Details) £ in Millions, AUD in Millions, $ in Millions	Feb. 27, 2017 AUD	Jan. 06, 2017 AUD	Mar. 31, 2017 GBP (£) item	Mar. 31, 2017 AUD item	Mar. 31, 2017 USD ($) item
Interest Rate Swap Contracts
Derivative [Line Items]
Number of contracts with credit risk related contingent features | item			0	0	0
Derivatives In Cash Flow Hedging Relationships | 2017 | U.S.
Derivative [Line Items]
Derivative liability, notional amount | $					$ 1,000
Weighted average fixed rate			2.53%	2.53%	2.53%
Derivatives In Cash Flow Hedging Relationships | 2017 | U.K.
Derivative [Line Items]
Derivative liability, notional amount | £			£ 550
Weighted average fixed rate			0.82%	0.82%	0.82%
Derivatives In Cash Flow Hedging Relationships | 2018 | U.S.
Derivative [Line Items]
Derivative liability, notional amount | $					$ 1,150
Weighted average fixed rate			2.17%	2.17%	2.17%
Derivatives In Cash Flow Hedging Relationships | 2018 | U.K.
Derivative [Line Items]
Derivative liability, notional amount | £			£ 550
Weighted average fixed rate			0.82%	0.82%	0.82%
Derivatives In Cash Flow Hedging Relationships | 2018 | AUSTRALIA
Derivative [Line Items]
New agreements during the period, notional amount | AUD	AUD 50.0	AUD 50.0
Derivative fixed interest rate	3.20%	2.75%
Derivatives In Cash Flow Hedging Relationships | 2019 | U.S.
Derivative [Line Items]
Derivative liability, notional amount | $					$ 1,000
Weighted average fixed rate			2.06%	2.06%	2.06%
Derivatives In Cash Flow Hedging Relationships | 2019 | U.K.
Derivative [Line Items]
Derivative liability, notional amount | £			£ 550
Weighted average fixed rate			0.90%	0.90%	0.90%
Derivatives In Cash Flow Hedging Relationships | 2019 | AUSTRALIA
Derivative [Line Items]
New agreements during the period, notional amount | AUD		AUD 35.0
Derivative fixed interest rate		2.98%
Derivatives In Cash Flow Hedging Relationships | 2020 | U.S.
Derivative [Line Items]
Derivative liability, notional amount | $					$ 1,000
Weighted average fixed rate			2.06%	2.06%	2.06%
Derivatives In Cash Flow Hedging Relationships | 2020 | U.K.
Derivative [Line Items]
Derivative liability, notional amount | £			£ 500
Weighted average fixed rate			0.94%	0.94%	0.94%
Derivatives In Cash Flow Hedging Relationships | 2021 | U.S.
Derivative [Line Items]
Derivative liability, notional amount | $					$ 400
Weighted average fixed rate			1.46%	1.46%	1.46%
Derivatives In Cash Flow Hedging Relationships | 2021 | U.K.
Derivative [Line Items]
Derivative liability, notional amount | £			£ 500
Weighted average fixed rate			0.94%	0.94%	0.94%
Derivatives In Cash Flow Hedging Relationships | 2022 | U.S.
Derivative [Line Items]
Derivative liability, notional amount | $					$ 400
Weighted average fixed rate			1.46%	1.46%	1.46%
Derivatives In Cash Flow Hedging Relationships | 2022 | U.K.
Derivative [Line Items]
Derivative liability, notional amount | £			£ 500
Weighted average fixed rate			0.94%	0.94%	0.94%
Derivatives In Cash Flow Hedging Relationships | April 1, 2017 – February 27, 2018 | AUSTRALIA
Derivative [Line Items]
Derivative liability, notional amount | AUD				AUD 135.0
Weighted average fixed rate			2.98%	2.98%	2.98%
Derivatives In Cash Flow Hedging Relationships | February 28, 2018 - September 28, 2018 | AUSTRALIA
Derivative [Line Items]
Derivative liability, notional amount | AUD				AUD 85.0
Weighted average fixed rate			3.11%	3.11%	3.11%
Derivatives In Cash Flow Hedging Relationships | September 29, 2018 - February 28, 2019 | AUSTRALIA
Derivative [Line Items]
Derivative liability, notional amount | AUD				AUD 35.0
Weighted average fixed rate			2.98%	2.98%	2.98%