Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net proceeds		$ 256,764	$ 278,040	$ 332,117
Total allowable shares (in shares)		375,000,000	375,000,000
Total shares issued as of December 31, 2017 (in shares)		153,577,028	147,149,945
Shares of common stock (in shares)		35,456	31,807	34,230
Net proceeds		$ 1,176	$ 1,148	$ 991
Stock issuance costs		$ 3,836	$ 15,204	$ 4,198
DRIP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance, dividend reinvestment plan (in shares)	16,000,000
Shares of common stock (in shares)		229,696	187,626	196,584
Net proceeds		$ 9,391	$ 8,340	$ 7,182
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 as of December 31, 2017 (in shares)		10,044,656
Shares of common stock (in shares)		5,821,366	5,716,222	8,573,533
Average stock price per share (net) (in USD per share)		$ 41.88	$ 46.48	$ 37.45
Net proceeds		$ 243,822	$ 265,696	$ 321,067
Stock issuance costs		$ 3,782	$ 4,266	$ 4,016
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 as of December 31, 2017 (in shares)		9,852,465
2013 ATM Equity Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total allowable shares (in shares)		9,000,000
Total shares issued as of December 31, 2017 (in shares)		6,252,812