Fair Value Measurements (Fair Value of Company's Long-term Debt) (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
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		$ 1,078,185	$ 517,998	$ 444,821
Fair value in excess of carrying value	[1]	$ 4,495	$ 5,813	$ (8,063)

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