Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 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	$ 1,883,662		1,883,662
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	(732)	$ (904)	(817)	$ (989)
Other comprehensive income (loss) before reclassifications	0		0
Losses reclassified from net accumulated other comprehensive loss	44	44	129	129
Net current period other comprehensive income	44	44	129	129
Accumulated other comprehensive loss at end of period	(688)	$ (860)	(688)	$ (860)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(441)		(464)
Other comprehensive income (loss) before reclassifications	12		35
Losses reclassified from net accumulated other comprehensive loss	0		0
Net current period other comprehensive income	12		35
Accumulated other comprehensive loss at end of period	(429)		(429)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,173)		(1,281)
Other comprehensive income (loss) before reclassifications	12		35
Losses reclassified from net accumulated other comprehensive loss	44		129
Net current period other comprehensive income	56		164
Accumulated other comprehensive loss at end of period	$ (1,117)		$ (1,117)