Adoption of New Accounting Standards (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Recognized a discrete tax benefit	$ 600,000		$ 2,600,000
Tax benefit (expense) recognized as additional paid-in capital		$ 1,400,000		$ 1,300,000
Reclassified tax benefits to operating activities			256,472,971	173,135,287
Cash flows from operating activities			$ 352,556,094	356,416,654
Additional Paid-in Capital [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Tax benefit (expense) recognized as additional paid-in capital		1,400,000		1,300,000
Tax benefit recognized as additional paid-in capital		1,700,000		2,700,000
Tax expense recognized as additional paid-in capital		$ 300,000		1,400,000
Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effect on earnings per share (in dollars per share)	$ 0.01		$ 0.01
Reclassified tax benefits to operating activities				3,400,000
Cash flows from operating activities				$ 3,400,000