Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	$ 2,130,300
(Gains) losses reclassified from net accumulated other comprehensive loss	0	$ (100)
Net current period other comprehensive income (loss)	100	100
Adoption of ASU No. 2018-02		0
Accumulated other comprehensive loss at end of period	2,536,600	2,130,300
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, Tax	0	0	$ (100)
Income Tax Expense (Benefit)	38,900	68,400	82,100
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,500)	(1,300)
Adoption of ASU No. 2018-02		(300)
Accumulated other comprehensive loss at end of period	(1,400)	(1,500)	(1,300)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(700)	(600)
(Gains) losses reclassified from net accumulated other comprehensive loss	0	(100)
Net current period other comprehensive income (loss)	0	100
Adoption of ASU No. 2018-02		(200)
Accumulated other comprehensive loss at end of period	(700)	(700)	(600)
Reclassification from AOCI, Current Period, Tax	0	0
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(800)	(700)
(Gains) losses reclassified from net accumulated other comprehensive loss	0	0
Net current period other comprehensive income (loss)	100	0
Adoption of ASU No. 2018-02		(100)
Accumulated other comprehensive loss at end of period	(700)	(800)	$ (700)
Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
(Gains) losses reclassified from net accumulated other comprehensive loss	(100)	0
Interest Rate Swap | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
(Gains) losses reclassified from net accumulated other comprehensive loss	0	0
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	(100)	0
Reclassification out of Accumulated Other Comprehensive Income [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Income Tax Expense (Benefit)	0	0
Reclassification from AOCI, Current Period, Tax	0	0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Income Tax Expense (Benefit)	$ 0	$ 100