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 income (loss) at beginning of period	$ 11,020,849
(Gains) losses reclassified from net accumulated other comprehensive loss	1,773	$ 1,802
Accumulated other comprehensive income (loss) at end of period	11,070,111
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	(51,151)	(54,862)
Other comprehensive income (loss) before reclassifications		(4)
(Gains) losses reclassified from net accumulated other comprehensive loss	825	938
Net current period other comprehensive income (loss)	825	934
Accumulated other comprehensive income (loss) at end of period	(50,326)	(53,928)
Unrealized Gains and Losses on Marketable Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	110	110
Other comprehensive income (loss) before reclassifications		0
(Gains) losses reclassified from net accumulated other comprehensive loss	0	0
Net current period other comprehensive income (loss)	0	0
Accumulated other comprehensive income (loss) at end of period	110	110
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	(59,313)	(55,001)
Other comprehensive income (loss) before reclassifications		(653)
(Gains) losses reclassified from net accumulated other comprehensive loss	948	864
Net current period other comprehensive income (loss)	948	211
Accumulated other comprehensive income (loss) at end of period	(58,365)	(54,790)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(110,354)	(109,753)
Other comprehensive income (loss) before reclassifications		(657)
(Gains) losses reclassified from net accumulated other comprehensive loss	1,773	1,802
Net current period other comprehensive income (loss)	1,773	1,145
Accumulated other comprehensive income (loss) at end of period	$ (108,581)	$ (108,608)