Long-Term Debt (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Aug. 07, 2015 USD ($)
Line of Credit Facility [Line Items]
Letters of Credit, Renewable Term	1 year
Line of Credit [Member]
Line of Credit Facility [Line Items]
Unsecured debt, credit agreement		$ 1,500,000,000.0
Credit facility, amount outstanding	$ 0
Margin for LIBOR based loans, percentage	1.00%
Margin for base rate loans, percentage	0.00%
Line of credit facility commitment fee amount, percentage	0.125%
Covenant ratio debt EBITDA maximum numerator	3.25
Covenant ratio coverage maximum numerator	4.00
Line of Credit [Member] | LIBOR [Member]
Line of Credit Facility [Line Items]
Credit facility interest rate, percentage	1.00%
Line of Credit [Member] | Federal Funds [Member]
Line of Credit Facility [Line Items]
Credit facility interest rate, percentage	0.50%
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 10,600,000