Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period			$ 5,269,455
Losses reclassified from net accumulated other comprehensive loss	$ 258	$ 266	756	$ 792
Accumulated other comprehensive loss at end of period	5,626,852		5,626,852
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	(22,284)	(23,308)	(22,780)	(23,836)
Other comprehensive income (loss) before reclassifications		(1)		1
Losses reclassified from net accumulated other comprehensive loss	257	266	753	792
Net current period other comprehensive loss	257	265	753	793
Accumulated other comprehensive loss at end of period	(22,027)	(23,043)	(22,027)	(23,043)
Defined Benefit and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(218)	(217)	(220)	0
Other comprehensive income (loss) before reclassifications		0		(219)
Losses reclassified from net accumulated other comprehensive loss	1	0	3	2
Net current period other comprehensive loss	1	0	3	(217)
Accumulated other comprehensive loss at end of period	(217)	(217)	(217)	(217)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(22,502)	(23,525)	(23,000)	(23,836)
Other comprehensive income (loss) before reclassifications		(1)		(218)
Losses reclassified from net accumulated other comprehensive loss	258	266	756	794
Net current period other comprehensive loss	258	265	756	576
Accumulated other comprehensive loss at end of period	$ (22,244)	$ (23,260)	$ (22,244)	$ (23,260)