Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period			$ 1,807,949
Accumulated other comprehensive loss at end of period	$ 1,842,759		1,842,759
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	(775)	$ (947)	(817)	$ (989)
Other comprehensive income (loss) before reclassifications	0		0
Losses reclassified from net accumulated other comprehensive loss	43	43	85	85
Net current period other comprehensive income	43	43	85	85
Accumulated other comprehensive loss at end of period	(732)	$ (904)	(732)	$ (904)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(452)		(464)
Other comprehensive income (loss) before reclassifications	11		23
Losses reclassified from net accumulated other comprehensive loss	0		0
Net current period other comprehensive income	11		23
Accumulated other comprehensive loss at end of period	(441)		(441)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,227)		(1,281)
Other comprehensive income (loss) before reclassifications	11		23
Losses reclassified from net accumulated other comprehensive loss	43		85
Net current period other comprehensive income	54		108
Accumulated other comprehensive loss at end of period	$ (1,173)		$ (1,173)