Fair Value - Estimated fair value of long-term debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Carrying Value
Grouping Financial Statement Captions
Long-term debt	$ 8,423,001	$ 8,232,703
Total Fair Value
Grouping Financial Statement Captions
Long-term debt	$ 8,947,616	$ 9,155,942

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

No definition available.

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant