Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period			$ 1,931,696
Losses reclassified from net accumulated other comprehensive loss	$ 26	$ 44	75	$ 129
Accumulated other comprehensive loss at end of period	2,025,733		2,025,733
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	(659)	(732)	(678)	(817)
Other comprehensive income (loss) before reclassifications		0		0
Losses reclassified from net accumulated other comprehensive loss	10	44	29	129
Net current period other comprehensive income	10	44	29	129
Accumulated other comprehensive loss at end of period	(649)	(688)	(649)	(688)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(582)	(441)	(612)	(464)
Other comprehensive income (loss) before reclassifications		12		35
Losses reclassified from net accumulated other comprehensive loss	16	0	46	0
Net current period other comprehensive income	16	12	46	35
Accumulated other comprehensive loss at end of period	(566)	(429)	(566)	(429)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,241)	(1,173)	(1,290)	(1,281)
Other comprehensive income (loss) before reclassifications		12		35
Losses reclassified from net accumulated other comprehensive loss	26	44	75	129
Net current period other comprehensive income	26	56	75	164
Accumulated other comprehensive loss at end of period	$ (1,215)	$ (1,117)	$ (1,215)	$ (1,117)