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 loss at beginning of period	$ 5,120,152
(Gains) losses reclassified from net accumulated other comprehensive loss	1,059	$ 72
Accumulated other comprehensive loss at end of period	5,269,455	5,120,152
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(23,836)	(23,878)
Other comprehensive income (loss) before reclassifications	0	(30)
(Gains) losses reclassified from net accumulated other comprehensive loss	1,056	72
Net current period other comprehensive income (loss)	1,056	42
Accumulated other comprehensive loss at end of period	(22,780)	(23,836)
Defined Benefit Pension and Postretirement Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	0
Other comprehensive income (loss) before reclassifications	(223)
(Gains) losses reclassified from net accumulated other comprehensive loss	3
Net current period other comprehensive income (loss)	(220)
Accumulated other comprehensive loss at end of period	(220)	0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(23,836)
Other comprehensive income (loss) before reclassifications	(223)
(Gains) losses reclassified from net accumulated other comprehensive loss	1,059
Net current period other comprehensive income (loss)	836
Accumulated other comprehensive loss at end of period	$ (23,000)	$ (23,836)