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Stock-Based Compensation (Narrative) (Details) - USD ($) shares in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for awards to employees and outside directors	10
RSU awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total grant date fair value of shares vested	$ 49,000,000	$ 43,000,000	$ 42,000,000
Unrecognized compensation cost	$ 30,000,000
Unrecognized compensation cost, period of recognition, years	23 months
RSU awards | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Performance awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Total grant date fair value of shares vested	$ 23,000,000	$ 13,000,000	$ 0
Unrecognized compensation cost	$ 27,000,000
Unrecognized compensation cost, period of recognition, years	22 months
Risk Free Interest Rate	2.50%
Expected Volatility Rate	14.80%
Performance awards | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percent	200.00%
Piedmont | RSU awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Piedmont | Performance awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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