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 income (loss) at beginning of period	$ 11,455,000
(Gains) losses reclassified from net accumulated other comprehensive loss	1	$ 2
Accumulated other comprehensive income (loss) at end of period	11,561,000
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(58)	(51)
(Gains) losses reclassified from net accumulated other comprehensive loss	0	1
Net current period other comprehensive income (loss)	0	1
Accumulated other comprehensive income (loss) at end of period	(58)	(50)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(67)	(59)
(Gains) losses reclassified from net accumulated other comprehensive loss	1	1
Net current period other comprehensive income (loss)	1	1
Accumulated other comprehensive income (loss) at end of period	(66)	(58)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(125)	(110)
(Gains) losses reclassified from net accumulated other comprehensive loss	1	2
Net current period other comprehensive income (loss)	1	2
Accumulated other comprehensive income (loss) at end of period	$ (124)	$ (108)