Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 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	24	$ 42
Accumulated other comprehensive loss at end of period	1,975,060
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	(678)	(817)
Other comprehensive income (loss) before reclassifications		0
Losses reclassified from net accumulated other comprehensive loss	9	42
Net current period other comprehensive income	9	42
Accumulated other comprehensive loss at end of period	(669)	(775)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(612)	(464)
Other comprehensive income (loss) before reclassifications		12
Losses reclassified from net accumulated other comprehensive loss	15	0
Net current period other comprehensive income	15	12
Accumulated other comprehensive loss at end of period	(597)	(452)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,290)	(1,281)
Other comprehensive income (loss) before reclassifications		12
Losses reclassified from net accumulated other comprehensive loss	24	42
Net current period other comprehensive income	24	54
Accumulated other comprehensive loss at end of period	$ (1,266)	$ (1,227)