Debt (Revolving Credit Facility) (Narrative) (Details) - USD ($)	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Line of Credit Facility [Line Items]
Long-term Line of Credit	$ 0	$ 36,500,000
Bank of America Syndicate [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	500,000,000
Long-term Line of Credit	0	$ 36,500,000
Line of Credit Facility, Remaining Borrowing Capacity	493,700,000
Bank of America Syndicate [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	75,000,000
Contingent Liability under Letters of Credit	6,300,000
Bank of America Syndicate [Member] | Revolving Credit Facility, Swing Line Loan [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 30,000,000
London Interbank Offered Rate (LIBOR) [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.25%
London Interbank Offered Rate (LIBOR) [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.25%
Bank of America's Base Rate [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.25%
Bank of America's Base Rate [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.25%