Significant Accounting Policies (Schedule of Adoption of New Accounting Pronouncements) (Details) - USD ($)	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	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, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues	$ 355,972,000	$ 245,346,000	$ 308,530,000	$ 260,178,000	$ 291,750,000	$ 227,401,000	$ 252,613,000	$ 199,427,000	$ 294,830,000	$ 300,976,000	$ 275,667,000	$ 255,504,000	$ 1,170,026,000	$ 971,191,000	$ 1,126,977,000
Gross revenue, net of expenses															188,561,000
Financing cash flows													(134,107,000)	119,263,000	404,143,000
ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues														971,191,000	1,126,977,000
Gross revenue, net of expenses														531,504,000	500,333,000
ASU 2014-09 [Member] | Scenario, Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues														610,517,000	566,395,000
Direct expenses														(79,013,000)	(66,062,000)
Gross revenue, net of expenses														531,504,000	500,333,000
ASU 2014-09 [Member] | Scenario, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues														360,674,000	560,582,000
ASU 2016-15 [Member] | Scenario, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing cash flows													1,302,000
Service Revenues [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues													749,515,000	624,417,000	555,843,000
Direct expenses													(159,058,000)	(133,189,000)	(113,296,000)
Service Revenues [Member] | ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues														624,417,000	555,843,000
Direct expenses														(133,189,000)	(113,296,000)
Service Revenues [Member] | ASU 2014-09 [Member] | Scenario, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues														13,900,000	(10,552,000)
Direct expenses														(54,176,000)	(47,234,000)
Revenue from Inventory Sales [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues													420,511,000	346,774,000	571,134,000
Direct expenses													$ (374,339,000)	(306,498,000)	(513,348,000)
Revenue from Inventory Sales [Member] | ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues														346,774,000	571,134,000
Direct expenses														(306,498,000)	(513,348,000)
Revenue from Inventory Sales [Member] | ASU 2014-09 [Member] | Scenario, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues														346,774,000	571,134,000
Direct expenses														$ (306,498,000)	$ (513,348,000)