Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net proceeds		$ 345,324	$ 256,764	$ 278,040
Total allowable shares (in shares)		375,000,000	375,000,000
Total shares issued (in shares)		161,503,585	153,577,028
Shares of common stock (in shares)		40,371	35,456	31,807
Net proceeds		$ 1,375	$ 1,176	$ 1,148
Stock issuance costs		$ 3,947	$ 3,836	$ 15,204
DRIP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares permitted for issuance (in shares)	10,000,000
Shares of common stock (in shares)		311,048	229,696	187,626
Net proceeds		$ 13,264	$ 9,391	$ 8,340
2018 ATM Equity Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total allowable shares (in shares)		12,000,000
Total shares issued (in shares)		7,378,163
2016 ATM Equity Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total allowable shares (in shares)		12,000,000
Total shares issued (in shares)		10,044,656
Shares of common stock (in shares)		7,378,163	5,821,366	5,716,222
Average price per share (net) (in dollars per share)		$ 44.48	$ 41.88	$ 46.48
Net proceeds		$ 328,196	$ 243,822	$ 265,696
Stock issuance costs		$ 3,821	$ 3,782	$ 4,266
2015 ATM Equity Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total allowable shares (in shares)		10,000,000
Total shares issued (in shares)		9,852,465