Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income taxes	$ 19,100	$ 39,000
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	6,298,200
Net current period other comprehensive loss	300	300
Accumulated other comprehensive loss at end of period	6,388,500
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	(25,300)	(26,500)
Net current period other comprehensive loss	300	300
Accumulated other comprehensive loss at end of period	(25,000)	(26,200)
Defined Benefit and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(200)	(200)
Net current period other comprehensive loss	0	0
Accumulated other comprehensive loss at end of period	(200)	(200)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(25,500)	(26,700)
Accumulated other comprehensive loss at end of period	(25,200)	(26,400)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income taxes	0	0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income taxes	100	100
Interest Rate Swap
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Losses reclassified from net accumulated other comprehensive loss	300	300
Interest Rate Swap | Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Losses reclassified from net accumulated other comprehensive loss	300	300
Interest Rate Swap | Defined Benefit and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Losses reclassified from net accumulated other comprehensive loss	$ 0	$ 0