Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	$ 25	$ 43	49	$ 85
Accumulated other comprehensive loss at end of period	1,985,433		1,985,433
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	(669)	(775)	(678)	(817)
Other comprehensive income (loss) before reclassifications		0		0
Losses reclassified from net accumulated other comprehensive loss	10	43	19	85
Net current period other comprehensive income	10	43	19	85
Accumulated other comprehensive loss at end of period	(659)	(732)	(659)	(732)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(597)	(452)	(612)	(464)
Other comprehensive income (loss) before reclassifications		11		23
Losses reclassified from net accumulated other comprehensive loss	15	0	30	0
Net current period other comprehensive income	15	11	30	23
Accumulated other comprehensive loss at end of period	(582)	(441)	(582)	(441)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,266)	(1,227)	(1,290)	(1,281)
Other comprehensive income (loss) before reclassifications		11		23
Losses reclassified from net accumulated other comprehensive loss	25	43	49	85
Net current period other comprehensive income	25	54	49	108
Accumulated other comprehensive loss at end of period	$ (1,241)	$ (1,173)	$ (1,241)	$ (1,173)