Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	$ 30	$ 25	61	$ 49
Accumulated other comprehensive loss at end of period	2,159,146		2,159,146
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	(764)	(669)	(776)	(678)
Losses reclassified from net accumulated other comprehensive loss	12	10	24	19
Net current period other comprehensive income	12	10	24	19
Accumulated other comprehensive loss at end of period	(752)	(659)	(752)	(659)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(672)	(597)	(691)	(612)
Losses reclassified from net accumulated other comprehensive loss	18	15	37	30
Net current period other comprehensive income	18	15	37	30
Accumulated other comprehensive loss at end of period	(654)	(582)	(654)	(582)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,436)	(1,266)	(1,467)	(1,290)
Losses reclassified from net accumulated other comprehensive loss	30	25	61	49
Net current period other comprehensive income	30	25	61	49
Accumulated other comprehensive loss at end of period	$ (1,406)	$ (1,241)	$ (1,406)	$ (1,241)