Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Measurements
Marketable Securities	$ 380,242	$ 503,503	[1]
Fair Value of Financial Instruments
Carrying amount of long-term debt	3,109,049	2,687,024
Recurring
Fair Value Measurements
Cash, Cash Equivalents	324,561	181,792
Marketable Securities	380,242	503,503
Total Assets Measured at Fair Value	704,803	685,295
Cost of cash and cash equivalents and available for sale securities	704,800	685,500
Fair value of cash and cash equivalents and available for sale securities	704,800	685,300
Recurring | Bonds and bond funds
Fair Value Measurements
Marketable Securities	159,942	344,251
Recurring | Commercial paper
Fair Value Measurements
Marketable Securities	220,300	159,252
Recurring | Level 1
Fair Value Measurements
Cash, Cash Equivalents	324,561	181,792
Total Assets Measured at Fair Value	324,561	181,792
Recurring | Level 2
Fair Value Measurements
Marketable Securities	380,242	503,503
Total Assets Measured at Fair Value	380,242	503,503
Fair Value of Financial Instruments
Fair value of long-term debt	3,150,000	2,700,000
Carrying amount of long-term debt	3,130,000	2,710,000
Recurring | Level 2 | Bonds and bond funds
Fair Value Measurements
Marketable Securities	159,942	344,251
Recurring | Level 2 | Commercial paper
Fair Value Measurements
Marketable Securities	$ 220,300	$ 159,252

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.