Fair Value Measurements (Fair Value of Company's Long-term Debt) (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt, including current maturities		$ 1,022,113	$ 1,096,010	$ 1,046,063
Fair value in excess of carrying value	[1]	$ 31,080	$ 8,257	$ 11,047

[1] Carrying value used for this purpose excludes unamortized debt issuance costs.