Shareholders' Equity (Details)	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($)	Jun. 23, 2015 USD ($) agreements
Class of Stock [Line Items]
Number of separate equity distribution agreements | agreements				4
Net proceeds from equity issuances		$ 113,225,000	$ 172,936,000
Shares issued under dividend reinvestment program		$ 2,482,000
Common Stock [Member]
Class of Stock [Line Items]
Capital shares reserved for future issuances				$ 200,000,000.0
Stock Issued During Period, Shares, New Issues | shares	1,500,000	3,587,000
Stock Issued During Period, Weighted Average Price | $ / shares	$ 32.89	$ 32.06
Net proceeds from equity issuances	$ 49,300,000	$ 113,200,000
Shares issued under dividend reinvestment program (in shares) | shares	20,884	77,000
Stock issued during period, weighted share price, Dividend Reinstated Plan | $ / shares	$ 32.79	$ 32.24
Shares issued under dividend reinvestment program	$ 700,000	$ 1,000