Derivative Instruments - Interest Rate Swap Instruments (Details) - Designated as Hedging Instrument	Sep. 30, 2018 USD ($)
Interest Rate Swap 1
Derivative [Line Items]
Notional Amount	$ 750,000,000
Interest Rate Swap 2
Derivative [Line Items]
Notional Amount	750,000,000
Interest Rate Swap 3
Derivative [Line Items]
Notional Amount	1,100,000,000
Interest Rate Swap 4
Derivative [Line Items]
Notional Amount	595,000,000
Interest Rate Swap 5
Derivative [Line Items]
Notional Amount	325,000,000
Interest Rate Swap 6
Derivative [Line Items]
Notional Amount	550,000,000
Interest Rate Swap 7
Derivative [Line Items]
Notional Amount	800,000,000
Interest Rate Swap 8
Derivative [Line Items]
Notional Amount	800,000,000
Interest Rate Swap 9
Derivative [Line Items]
Notional Amount	450,000,000
Interest Rate Swap 10
Derivative [Line Items]
Notional Amount	550,000,000
Interest Rate Swap 11
Derivative [Line Items]
Notional Amount	400,000,000
Interest Rate Swap 12
Derivative [Line Items]
Notional Amount	800,000,000
Interest Rate Swap 13
Derivative [Line Items]
Notional Amount	400,000,000
Interest Rate Swap 14
Derivative [Line Items]
Notional Amount	400,000,000
Interest Rate Swap 15
Derivative [Line Items]
Notional Amount	450,000,000
Interest Rate Swap 16
Derivative [Line Items]
Notional Amount	550,000,000
Interest Rate Swap 17
Derivative [Line Items]
Notional Amount	400,000,000
Interest Rate Swap 18
Derivative [Line Items]
Notional Amount	325,000,000
Interest Rate Swap 19
Derivative [Line Items]
Notional Amount	595,000,000
Interest Rate Swap 20
Derivative [Line Items]
Notional Amount	450,000,000
Interest Rate Swap 21
Derivative [Line Items]
Notional Amount	1,100,000,000
Interest Rate Swap 22
Derivative [Line Items]
Notional Amount	$ 550,000,000
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 1
Derivative [Line Items]
Strike Rate	1.97%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 2
Derivative [Line Items]
Strike Rate	1.97%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 3
Derivative [Line Items]
Strike Rate	1.59%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 4
Derivative [Line Items]
Strike Rate	1.63%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 5
Derivative [Line Items]
Strike Rate	1.60%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 6
Derivative [Line Items]
Strike Rate	1.58%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 7
Derivative [Line Items]
Strike Rate	1.10%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 8
Derivative [Line Items]
Strike Rate	1.06%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 9
Derivative [Line Items]
Strike Rate	1.12%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 10
Derivative [Line Items]
Strike Rate	1.93%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 11
Derivative [Line Items]
Strike Rate	2.86%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 12
Derivative [Line Items]
Strike Rate	2.21%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 13
Derivative [Line Items]
Strike Rate	2.85%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 14
Derivative [Line Items]
Strike Rate	2.86%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 15
Derivative [Line Items]
Strike Rate	1.30%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 16
Derivative [Line Items]
Strike Rate	2.13%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 17
Derivative [Line Items]
Strike Rate	2.90%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 18
Derivative [Line Items]
Strike Rate	2.99%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 19
Derivative [Line Items]
Strike Rate	2.99%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 20
Derivative [Line Items]
Strike Rate	1.47%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 21
Derivative [Line Items]
Strike Rate	2.90%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap 22
Derivative [Line Items]
Strike Rate	2.23%