Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period			$ 2,130,363
Losses reclassified from net accumulated other comprehensive loss	$ 31	$ 26	92	$ 75
Accumulated other comprehensive loss at end of period	2,380,749		2,380,749
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	(752)	(659)	(776)	(678)
Losses reclassified from net accumulated other comprehensive loss	13	10	37	29
Net current period other comprehensive income	13	10	37	29
Accumulated other comprehensive loss at end of period	(739)	(649)	(739)	(649)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(654)	(582)	(691)	(612)
Losses reclassified from net accumulated other comprehensive loss	18	16	55	46
Net current period other comprehensive income	18	16	55	46
Accumulated other comprehensive loss at end of period	(636)	(566)	(636)	(566)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,406)	(1,241)	(1,467)	(1,290)
Losses reclassified from net accumulated other comprehensive loss	31	26	92	75
Net current period other comprehensive income	31	26	92	75
Accumulated other comprehensive loss at end of period	$ (1,375)	$ (1,215)	$ (1,375)	$ (1,215)