Capital Stock AOCI (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance				$ 26,758					$ 26,758
Change in fair value of derivative instruments designated as cash flow hedges									(407)	$ 30,405	$ 0
Reclassification of realized loss on interest rate swap agreements designated as cash flow hedges									440	777	0
Foreign currency translation adjustment									151	(758)	(408)
Other comprehensive income									353,710	24,639	127,261
Ending balance	$ 26,942				$ 26,758				26,942	26,758
Amounts Reclassified From Accumulated Other Comprehensive Income (Loss)
Income tax (benefit)									(93,274)	(48)	4,048
Amounts reclassified from accumulated other comprehensive (loss), net of tax	207,160	$ 57,156	$ 45,671	34,611	22,778	$ (51,211)	$ 6,174	$ 8,742	344,598	(13,517)	111,089
Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance				31,182				0	31,182	0
Change in fair value of derivative instruments designated as cash flow hedges									(407)	30,405
Reclassification of realized loss on interest rate swap agreements designated as cash flow hedges									440	777
Foreign currency translation adjustment									0	0
Other comprehensive income									33	31,182
Ending balance	31,215				31,182				31,215	31,182	0
Foreign Currency Translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance				(4,424)				(3,666)	(4,424)	(3,666)
Change in fair value of derivative instruments designated as cash flow hedges									0	0
Reclassification of realized loss on interest rate swap agreements designated as cash flow hedges									0	0
Foreign currency translation adjustment									151	(758)
Other comprehensive income									151	(758)
Ending balance	(4,273)				(4,424)				(4,273)	(4,424)	(3,666)
Accumulated Other Comprehensive (Loss) Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance				$ 26,758				$ (3,666)	26,758	(3,666)
Change in fair value of derivative instruments designated as cash flow hedges									(407)	30,405
Reclassification of realized loss on interest rate swap agreements designated as cash flow hedges									440	777
Foreign currency translation adjustment									151	(758)
Other comprehensive income									184	30,424
Ending balance	$ 26,942				$ 26,758				26,942	26,758	$ (3,666)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedges
Amounts Reclassified From Accumulated Other Comprehensive Income (Loss)
Amounts reclassified from accumulated other comprehensive (loss) before income tax									611	1,200
Income tax (benefit)									(171)	(423)
Amounts reclassified from accumulated other comprehensive (loss), net of tax									$ 440	$ 777