Stockholders' Equity - Common Stock (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended		24 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Feb. 28, 2017
Stockholders' equity
Proceeds from issuance of common stock			$ 45,125
Payments of stock issuance costs			$ 784
Maximum | Share repurchase program
Stockholders' equity
Repurchase Program, maximum amount authorized for repurchase					$ 300,000
Common Stock | Share repurchase program
Stockholders' equity
Repurchase Program, number of shares repurchased (in shares)	432,464
Repurchase Program, value of shares repurchased	$ 5,700
Repurchase Program, remaining authorized capacity	$ 294,300	$ 294,300
Common Stock | At The Market
Stockholders' equity
Proceeds from issuance of common stock				$ 124,500
Payments of stock issuance costs				$ 2,300
ATM Program, number of shares sold or issued (in shares)	0	0		7,467,709
ATM Program, remaining amount authorized for issuance	$ 175,500	$ 175,500
Common Stock | Maximum | At The Market
Stockholders' equity
ATM Program, maximum amount authorized for issuance					$ 300,000