Reconciliation of Common Shareholders' Equity, Common Shares and Earnings Per Share (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, December 31, 2015			$ 605,023
Common Stock Issuances, Net of Expenses			41,456
Common Stock Retirements			(104)
Net Income	$ 14,616	$ 15,392	44,811	$ 44,763
Other Comprehensive Income			322
Employee Stock Incentive Plans Expense			1,163
Common Dividends ($0.9375 per share)			(35,952)
Balance, September 30, 2016	656,719		656,719
Common Shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, December 31, 2015			189,286
Common Stock Issuances, Net of Expenses			6,855
Common Stock Retirements			(18)
Balance, September 30, 2016	196,123		196,123
Premium on Common Shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, December 31, 2015			293,610
Common Stock Issuances, Net of Expenses			34,601
Common Stock Retirements			(86)
Employee Stock Incentive Plans Expense			1,163
Balance, September 30, 2016	329,288		329,288
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, December 31, 2015			126,025
Net Income			44,811
Common Dividends ($0.9375 per share)			(35,952)
Balance, September 30, 2016	134,884		134,884
Accumulated Other Comprehensive Income/(Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, December 31, 2015			(3,898)
Other Comprehensive Income			322
Balance, September 30, 2016	$ (3,576)		$ (3,576)