Equity - At-The-Market Offerings of Common Stock (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands			3 Months Ended				12 Months Ended
	Nov. 18, 2016	Mar. 28, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2017
ATM Program Activity
Number of shares	6,330	9,780
Gross proceeds	$ 80,900	$ 105,800					$ 179,882	$ 170,323	$ 200,402
Maximum
Class of Stock [Line Items]
Authorized aggregate offering price of common stock										$ 300,000
At-The-Market Offering
Class of Stock [Line Items]
Remaining offering amount available			$ 53,700				$ 53,700
ATM Program Activity
Number of shares			2,504	4,772	2,989	0	10,265
Average sales price per share (usd per share)			$ 19.98	$ 17.62	$ 16.13	$ 0.00	$ 17.76
Gross proceeds			$ 50,046	$ 84,077	$ 48,198	$ 0	$ 182,321
Commissions paid on stock issuance			$ 600	$ 1,100	$ 600