Fair Value Measurements Fair Value Measurements - Major Terms of Interest Rate Swap Agreements (Details) - Cash Flow Hedging [Member] $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Derivative [Line Items]
Derivative, Notional Amount	$ 2,000
Interest Rate Swap at 2.274% [Member]
Derivative [Line Items]
Derivative, Inception Date	Dec. 31, 2018
Derivative, Notional Amount	$ 250
Derivative, Fixed Interest Rate	2.274%
Derivative, Maturity Date	Dec. 31, 2022
Interest Rate Swap at 2.828% effective 12/31/2018 [Member]
Derivative [Line Items]
Derivative, Inception Date	Dec. 31, 2018
Derivative, Notional Amount	$ 200
Derivative, Fixed Interest Rate	2.828%
Derivative, Maturity Date	Dec. 31, 2022
Interest Rate Swap at 2.153% [Member]
Derivative [Line Items]
Derivative, Inception Date	Jan. 01, 2019
Derivative, Notional Amount	$ 250
Derivative, Fixed Interest Rate	2.153%
Derivative, Maturity Date	Dec. 31, 2020
Interest Rate Swap at 2.196% [Member]
Derivative [Line Items]
Derivative, Inception Date	Jan. 01, 2019
Derivative, Notional Amount	$ 250
Derivative, Fixed Interest Rate	2.196%
Derivative, Maturity Date	Dec. 31, 2021
Interest Rate Swap at 2.788% [Member]
Derivative [Line Items]
Derivative, Inception Date	Jan. 01, 2019
Derivative, Notional Amount	$ 400
Derivative, Fixed Interest Rate	2.788%
Derivative, Maturity Date	Dec. 31, 2021
Interest Rate Swap at 2.828% effective 01/01/2019 [Member]
Derivative [Line Items]
Derivative, Inception Date	Jan. 01, 2019
Derivative, Notional Amount	$ 200
Derivative, Fixed Interest Rate	2.828%
Derivative, Maturity Date	Dec. 31, 2022
Interest Rate Swap at 2.172% [Member]
Derivative [Line Items]
Derivative, Inception Date	Jan. 02, 2019
Derivative, Notional Amount	$ 250
Derivative, Fixed Interest Rate	2.172%
Derivative, Maturity Date	Dec. 31, 2020
Interest Rate Swap @ 2.731% [Member]
Derivative [Line Items]
Derivative, Inception Date	Jan. 02, 2019
Derivative, Notional Amount	$ 200
Derivative, Fixed Interest Rate	2.731%
Derivative, Maturity Date	Dec. 31, 2020