Fair Value Measurements and Derivative Instruments (Details) - Fair Value, Measurements, Nonrecurring - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Level 1
Assets:
Cash and cash equivalents	[1],[2]	$ 111,245	$ 120,112
Total Assets	[2]	111,245	120,112
Liabilities:
Long-term debt (including current portion of long-term debt)	[2],[3]	0	0
Total Liabilities	[2]	0	0
Level 2
Assets:
Cash and cash equivalents	[1],[4]	0	0
Total Assets	[4]	0	0
Liabilities:
Long-term debt (including current portion of long-term debt)	[3],[4]	9,333,745	8,038,092
Total Liabilities	[4]	9,333,745	8,038,092
Level 3
Assets:
Cash and cash equivalents	[1],[5]	0	0
Total Assets	[5]	0	0
Liabilities:
Long-term debt (including current portion of long-term debt)	[3],[5]	0	0
Total Liabilities	[5]	0	0
Total Carrying Amount
Assets:
Cash and cash equivalents	[1]	111,245	120,112
Total Assets		111,245	120,112
Liabilities:
Long-term debt (including current portion of long-term debt)	[3]	8,762,052	7,506,312
Total Liabilities		8,762,052	7,506,312
Total Fair Value
Assets:
Cash and cash equivalents	[1]	111,245	120,112
Total Assets		111,245	120,112
Liabilities:
Long-term debt (including current portion of long-term debt)	[3]	9,333,745	8,038,092
Total Liabilities		$ 9,333,745	$ 8,038,092

[1] Consists of cash and marketable securities with original maturities of less than 90 days.

[2] Inputs based on quoted prices (unadjusted) in active markets for identical assets or liabilities that we have the ability to access. Valuation of these items does not entail a significant amount of judgment.

[3] Consists of unsecured revolving credit facilities, senior notes, senior debentures and term loans. This does not include our capital lease obligations.

[4] Inputs other than quoted prices included within Level 1 that are observable for the liability, either directly or indirectly. For unsecured revolving credit facilities and unsecured term loans, fair value is determined utilizing the income valuation approach. This valuation model takes into account the contract terms of our debt such as the debt maturity and the interest rate on the debt. The valuation model also takes into account the creditworthiness of the Company.

[5] Inputs that are unobservable. The Company did not use any Level 3 inputs as of March 31, 2018 and December 31, 2017.