Accumulated Other Comprehensive Income (Loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Reclassifications out of accumulated other comprehensive income (loss):
Interest expense	$ (20,206)	$ (18,969)	$ (39,807)	$ (37,681)
Income tax expense	(41,059)	(26,859)	(17,199)	8,770
Total reclassifications			840
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassifications out of accumulated other comprehensive income (loss):
Total reclassifications	420	410	840	820
Amortization of loss on interest rate hedge [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassifications out of accumulated other comprehensive income (loss):
Interest expense	441	442	883	883
Income tax expense	(172)	(174)	(345)	(348)
Total reclassifications	269	268	538	535
Amortization of deferred actuarial loss and prior service cost [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassifications out of accumulated other comprehensive income (loss):
Operating Expenses	247	235	494	470
Income tax expense	(96)	(93)	(192)	(185)
Total reclassifications	$ 151	$ 142	$ 302	$ 285