Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	$ 24
Accumulated other comprehensive loss at end of period	2,130,238
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	(776)	(678)
Losses reclassified from net accumulated other comprehensive loss	12	9
Net current period other comprehensive income	12	9
Accumulated other comprehensive loss at end of period	(764)	(669)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(691)	(612)
Losses reclassified from net accumulated other comprehensive loss	19	15
Net current period other comprehensive income	19	15
Accumulated other comprehensive loss at end of period	(672)	(597)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,467)	(1,290)
Losses reclassified from net accumulated other comprehensive loss	31	24
Net current period other comprehensive income	31	24
Accumulated other comprehensive loss at end of period	$ (1,436)	$ (1,266)