Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period			$ 5,120,152
Accumulated other comprehensive loss at end of period	$ 5,279,263		5,279,263
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	(23,308)	$ (24,086)	(23,836)	$ (23,878)
Other comprehensive income (loss) before reclassifications	(1)	(17)	1	(14)
Gains reclassified from net accumulated other comprehensive loss	266	19	792	(192)
Net current period other comprehensive loss	265	2	793	(206)
Accumulated other comprehensive loss at end of period	(23,043)	$ (24,084)	(23,043)	$ (24,084)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(217)		0
Other comprehensive income (loss) before reclassifications	0		(219)
Gains reclassified from net accumulated other comprehensive loss	0		2
Net current period other comprehensive loss	0		(217)
Accumulated other comprehensive loss at end of period	(217)		(217)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(23,525)		(23,836)
Other comprehensive income (loss) before reclassifications	(1)		(218)
Gains reclassified from net accumulated other comprehensive loss	266		794
Net current period other comprehensive loss	265		576
Accumulated other comprehensive loss at end of period	$ (23,260)		$ (23,260)