Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income taxes	$ 113,700	$ 252,200	$ 273,900
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	5,828,300
Net current period other comprehensive income (loss)	1,200	1,000
Adoption of ASU No. 2018-02		0
Accumulated other comprehensive loss at end of period	6,298,200	5,828,300
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(26,500)	(22,800)
Net current period other comprehensive income (loss)	1,200	1,000
Adoption of ASU No. 2018-02		(4,700)
Accumulated other comprehensive loss at end of period	(25,300)	(26,500)	(22,800)
Defined Benefit Pension and Postretirement Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(200)	(200)
Net current period other comprehensive income (loss)	0	0
Adoption of ASU No. 2018-02		0
Accumulated other comprehensive loss at end of period	(200)	(200)	(200)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(26,700)	(23,000)
Adoption of ASU No. 2018-02		(4,700)
Accumulated other comprehensive loss at end of period	(25,500)	(26,700)	$ (23,000)
Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
(Gains) losses reclassified from net accumulated other comprehensive loss	1,200	1,000
Interest Rate Swap \| Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
(Gains) losses reclassified from net accumulated other comprehensive loss	1,200	1,000
Interest Rate Swap \| Defined Benefit Pension and Postretirement Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
(Gains) losses reclassified from net accumulated other comprehensive loss	0	0
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	$ 400	$ 600

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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