Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents - carrying amount		$ 456,535	$ 21,218
Restricted cash - carrying amount		1,697	2,056
Notes payable		76,800	75,000
Long-term debt, including current maturities - carrying amount		1,866,866	1,542,589
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents - carrying amount		456,535	21,218
Restricted cash - carrying amount		1,697	2,056
Notes payable		76,800	75,000
Long-term debt, including current maturities - carrying amount		1,866,866	1,542,589
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents - fair value	[1]	456,535	21,218
Restricted Cash - fair value	[1]	1,697	2,056
Notes payable - fair value	[1]	76,800	75,000
Long-term debt, including current maturities - fair value	[2]	$ 1,992,274	$ 1,734,555

[1] Carrying value approximates fair value due to either short-term length of maturity or variable interest rates that approximate prevailing market rates and therefore is classified in Level 1 in the fair value hierarchy.

[2] Long-term debt is valued based on observable inputs available either directly or indirectly for similar liabilities in active markets and therefore is classified in Level 2 in the fair value hierarchy.