Stockholders' Equity - Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		6 Months Ended	12 Months Ended
	May 31, 2016	Mar. 31, 2016	Jun. 30, 2019	Dec. 31, 2018
Series E Cumulative Redeemable Preferred Stock
Stockholders' equity
Number of shares of preferred stock sold (in shares)		4,600,000
Preferred stock, Cumulative Redeemable Preferred Stock, dividend rate (as a percent)		6.95%	6.95%	6.95%
Liquidation preference (in dollars per share)		$ 25.00	$ 25.00	$ 25.00
Proceeds from preferred stock offerings		$ 115.0
Payment of preferred stock offering costs		$ 4.0
Redemption price (in dollars per share)		$ 25.00
Series F Cumulative Redeemable Preferred Stock
Stockholders' equity
Number of shares of preferred stock sold (in shares)	3,000,000
Preferred stock, Cumulative Redeemable Preferred Stock, dividend rate (as a percent)	6.45%		6.45%	6.45%
Liquidation preference (in dollars per share)	$ 25.00		$ 25.00	$ 25.00
Proceeds from preferred stock offerings	$ 75.0
Payment of preferred stock offering costs	$ 2.6
Redemption price (in dollars per share)	$ 25.00