Shareholders' Equity (Common Stock) (Details) - USD ($)	1 Months Ended	9 Months Ended		26 Months Ended
	Aug. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018
Class of Stock [Line Items]
Proceeds from issuance of common stock, net of issuance costs		$ 0	$ 139,414,000
At The Market Program
Class of Stock [Line Items]
Aggregate amount of shares the company may sell (up to)	$ 400,000,000
Percentage of commission to be paid on gross sales price of shares sold (up to)	2.00%
Percentage of commission to be paid of sale price of borrowed shares of common stock (up to)	2.00%
Issuance of common stock (in shares)		0	3,864,872	5,186,871
Weighted average price of shares issued (in dollars per share)			$ 36.22	$ 35.91
Proceeds from new issuance of common stock			$ 140,000,000	$ 186,300,000
Proceeds from issuance of common stock, net of issuance costs			$ 139,400,000	185,000,000
Amount of shares remaining for issuance		$ 213,700,000		$ 213,700,000