Fair Value Measurements Fair Value Measurements - Interest Rage Swap Derivatives (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Derivative [Line Items]
Derivative, Notional Amount	$ 3,000
Designated as Hedging Instrument [Member] | Interest Rate Swap at 2.274% [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 250
Derivative, Fixed Interest Rate	2.274%
Derivative, Variable Interest Rate	2.522%
Derivative, Maturity Date	Dec. 31, 2022
Designated as Hedging Instrument [Member] | Interest Rate Swap at 2.828% effective 12/31/2018 [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 200
Derivative, Fixed Interest Rate	2.828%
Derivative, Variable Interest Rate	2.522%
Derivative, Maturity Date	Dec. 31, 2022
Designated as Hedging Instrument [Member] | Interest Rate Swap at 2.739% effective 12/31/2018 [Member] [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 600
Derivative, Fixed Interest Rate	2.739%
Derivative, Variable Interest Rate	2.522%
Derivative, Maturity Date	Dec. 31, 2022
Designated as Hedging Instrument [Member] | Interest Rate Swap at 2.153% [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 250
Derivative, Fixed Interest Rate	2.153%
Derivative, Maturity Date	Dec. 31, 2020
Designated as Hedging Instrument [Member] | Interest Rate Swap at 2.196% [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 250
Derivative, Fixed Interest Rate	2.196%
Derivative, Maturity Date	Dec. 31, 2021
Designated as Hedging Instrument [Member] | Interest Rate Swap at 2.788% [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 400
Derivative, Fixed Interest Rate	2.788%
Derivative, Maturity Date	Dec. 31, 2021
Designated as Hedging Instrument [Member] | Interest Rate Swap at 2.828% effective 01/01/2019 [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 200
Derivative, Fixed Interest Rate	2.828%
Derivative, Maturity Date	Dec. 31, 2022
Designated as Hedging Instrument [Member] | Interest Rate Swap at 2.172% [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 250
Derivative, Fixed Interest Rate	2.172%
Derivative, Maturity Date	Dec. 31, 2020
Designated as Hedging Instrument [Member] | Interest Rate Swap @ 2.731% [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 200
Derivative, Fixed Interest Rate	2.731%
Derivative, Maturity Date	Dec. 31, 2020
Designated as Hedging Instrument [Member] | Interest Rate Swap at 2.707% [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 400
Derivative, Fixed Interest Rate	2.707%
Derivative, Maturity Date	Dec. 31, 2021