Fair Value Measurements - Major Terms of Interest Rate Swap Agreements (Details) - 3 months ended Mar. 31, 2019 - Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Total	Total	Total
Derivative [Line Items]
Derivative, Notional Amount	$ 3,000
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% [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 2.739% [Member]
Derivative [Line Items]
Derivative, Inception Date	Dec. 31, 2018
Derivative, Notional Amount	$ 600
Derivative, Fixed Interest Rate			2.739%
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% [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 at 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
Interest Rate Swap at 2.707% [Member]
Derivative [Line Items]
Derivative, Inception Date	Jan. 02, 2019
Derivative, Notional Amount	$ 400
Derivative, Fixed Interest Rate			2.707%
Derivative, Maturity Date	Dec. 31, 2021
Interest Rate Swap [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, Variable Interest Rate		2.493%	2.493%