Long-term Debt (Revolving Credit Facility) (Details) - Revolving Credit Facility - USD ($)	12 Months Ended
	Dec. 31, 2018	Jul. 31, 2017
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		$ 500,000,000
Line of credit facility, additional maximum borrowing capacity		$ 250,000,000
Commitment fee percentage	0.10%
London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.75%
Federal Funds Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
One-Month LIBOR Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%
One-Month LIBOR Rate | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.00%