Debt (2018 Revolving Credit Facility - Narrative) (Details) - USD ($)	1 Months Ended	12 Months Ended
	Apr. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Oct. 08, 2019
Debt Instrument [Line Items]
Subsidiary ownership		100.00%	100.00%
Letters of credit outstanding		$ 172,000,000
Line of Credit | 2018 Revolving Credit Facility | Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 3,500,000,000
Current borrowing capacity	2,000,000,000.0			$ 2,100,000,000
Debt instrument limit of securing indebtedness	$ 2,000,000,000.0
Debt instrument limit of securing indebtedness, percent of consolidated net tangible assets	25.00%
Minimum current ratio	1.00
Leverage ratio, percentage of credit limit	10.00%
Leverage ratio, amount of credit limit	$ 150,000,000
Line of Credit | 2018 Revolving Credit Facility | Revolving Credit Facility | June 30, 2018 Through March 31, 2019
Debt Instrument [Line Items]
Leverage ratio	4.50
Line of Credit | 2018 Revolving Credit Facility | Revolving Credit Facility | June 30, 2019 Through March 31, 2020
Debt Instrument [Line Items]
Leverage ratio	4.25
Line of Credit | 2018 Revolving Credit Facility | Revolving Credit Facility | On Or After June 30, 2020
Debt Instrument [Line Items]
Leverage ratio	4.00
Line of Credit | 2018 Revolving Credit Facility | Revolving Credit Facility | Eurodollar | Minimum
Debt Instrument [Line Items]
Basis points	1.50%
Line of Credit | 2018 Revolving Credit Facility | Revolving Credit Facility | Eurodollar | Maximum
Debt Instrument [Line Items]
Basis points	2.50%
Line of Credit | 2018 Revolving Credit Facility | Revolving Credit Facility | Base Rate | Minimum
Debt Instrument [Line Items]
Basis points	0.50%
Line of Credit | 2018 Revolving Credit Facility | Revolving Credit Facility | Base Rate | Maximum
Debt Instrument [Line Items]
Basis points	1.50%