Accumulated Other Comprehensive Income (Loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassifications out of accumulated other comprehensive income (loss):
Interest expense	$ (27,213)	$ (20,206)	$ (48,430)	$ (39,807)
Income tax expense	(36,054)	(41,059)	1,537	(17,199)
Total reclassifications			532
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassifications out of accumulated other comprehensive income (loss):
Total reclassifications	224	420	532	840
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	158	441	451	883
Income tax expense	(62)	(172)	(176)	(345)
Total reclassifications	96	269	275	538
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	210	247	421	494
Income tax expense	(82)	(96)	(164)	(192)
Total reclassifications	$ 128	$ 151	$ 257	$ 302