Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	$ 5,828,323
Losses reclassified from net accumulated other comprehensive loss	302	$ 247
Accumulated other comprehensive loss at end of period	5,866,996
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(26,465)	(22,780)
Losses reclassified from net accumulated other comprehensive loss	300	246
Net current period other comprehensive loss	300	246
Accumulated other comprehensive loss at end of period	(26,165)	(22,534)
Defined Benefit and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(267)	(220)
Losses reclassified from net accumulated other comprehensive loss	2	1
Net current period other comprehensive loss	2	1
Accumulated other comprehensive loss at end of period	(265)	(219)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(26,732)	(23,000)
Losses reclassified from net accumulated other comprehensive loss	302	247
Net current period other comprehensive loss	302	247
Accumulated other comprehensive loss at end of period	$ (26,430)	$ (22,753)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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