Derivative Financial Instruments - Changes in Fair Value of Derivative Contracts (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax:
Beginning balance	$ (2,030,647)	$ (1,839,388)	$ (1,966,478)	$ (1,724,954)	$ (1,724,954)
Other comprehensive income (loss)	1,241	72	5,181	(231)
Ending balance	106,320	$ (1,900,655)	106,320	(1,900,655)	(1,966,478)
Derivative contracts
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax:
Beginning balance			5,853	$ 6,937	6,937
Loss recognized in other comprehensive loss			(4,796)		669
Amounts reclassified from other comprehensive loss to earnings			(360)		(1,563)
Other comprehensive income (loss)			(5,156)		(894)
Ending balance	$ 477		477		5,853
Interest rate swap agreements
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax:
Gain recognized in earnings			$ (220)		$ (190)

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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