Common Stock (Details Textual) - USD ($)		12 Months Ended
	May. 06, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Mar. 31, 2015
Class of Stock [Line Items]
Stock Issued During Period, Value, New Issues		$ 92,259,597	$ 73,302,109	$ 93,393,049
Proceeds From Issuance Of Common Stock		92,259,603	$ 73,302,116	$ 93,393,056
ATM Program [Member]
Class of Stock [Line Items]
Gross Amount Remaining To Be Issued		$ 60,000,000
Common Stock Average Price		$ 30.31
Common Stock [Member]
Class of Stock [Line Items]
Registration Payment Arrangement, Maximum Potential Consideration					$ 500,000,000
Shares, Issued		1,725,000	2,587,500
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)		$ 52,950,000	$ 71,511,000
Common Stock [Member] | ATM Program [Member]
Class of Stock [Line Items]
Stock Issued During Period, Value, New Issues		1,318,812
Proceeds From Issuance Of Common Stock		$ 40,000,000
Value Of Equity Instruments To Be Issued	$ 100,000,000