Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	$ 10,600,920
Losses reclassified from net accumulated other comprehensive loss	7,179	$ 6,362
Accumulated other comprehensive income (loss) at end of period	11,020,849	10,600,920
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(54,862)	(57,628)
Other comprehensive income (loss) before reclassifications	3	(70)
Losses reclassified from net accumulated other comprehensive loss	3,708	2,836
Net current period other comprehensive income (loss)	3,711	2,766
Accumulated other comprehensive income (loss) at end of period	(51,151)	(54,862)
Unrealized Gains and Losses on Marketable Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	110	110
Other comprehensive income (loss) before reclassifications	0	0
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
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(55,001)	(50,621)
Other comprehensive income (loss) before reclassifications	(7,783)	(7,906)
Losses reclassified from net accumulated other comprehensive loss	3,471	3,526
Net current period other comprehensive income (loss)	(4,312)	(4,380)
Accumulated other comprehensive income (loss) at end of period	(59,313)	(55,001)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(109,753)	(108,139)
Other comprehensive income (loss) before reclassifications	(7,780)	(7,976)
Losses reclassified from net accumulated other comprehensive loss	7,179	6,362
Net current period other comprehensive income (loss)	(601)	(1,614)
Accumulated other comprehensive income (loss) at end of period	$ (110,354)	$ (109,753)