Fair Value Measurements and Derivative Instruments - Estimated Fair Value (Details) - Fair Value, Measurements, Nonrecurring - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Level 1
Assets:
Cash and cash equivalents	[1],[2]	$ 235,001	$ 287,852
Total Assets	[2]	235,001	287,852
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]	10,337,397	10,244,214
Total Liabilities	[4]	10,337,397	10,244,214
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]	235,001	287,852
Total Assets		235,001	287,852
Liabilities:
Long-term debt (including current portion of long-term debt)	[3]	9,625,559	9,871,267
Total Liabilities		9,625,559	9,871,267
Total Fair Value
Assets:
Cash and cash equivalents	[1]	235,001	287,852
Total Assets		235,001	287,852
Liabilities:
Long-term debt (including current portion of long-term debt)	[3]	10,337,397	10,244,214
Total Liabilities		$ 10,337,397	$ 10,244,214

[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. These amounts do not include our capital lease obligations or commercial paper.

[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 June 30, 2019 and December 31, 2018.