Net Income (Loss) per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income (loss) attributable to common stockholders and participating securities for basic and diluted earnings per share	$ 21,404	$ 22,767	$ 20,684	$ 20,242	$ (38,296)	$ 32,282	$ 17,303	$ (36,304)	$ 85,097	$ (25,015)	$ (9,484)
Less: Dividends and undistributed earnings allocated to participating securities									312	377	1,045
Net income (loss) allocable to common stockholders - basic and diluted									$ 84,785	$ (25,392)	$ (10,529)
Denominator:
Weighted average common shares outstanding for basic earnings per share									41,817,455	41,688,950	41,490,556
Weighted average common shares outstanding for diluted earnings per share									41,817,455	41,688,950	41,490,556
Net income (loss) per common share - basic (in dollars per share)	$ 0.51	$ 0.54	$ 0.49	$ 0.48	$ (0.92)	$ 0.77	$ 0.41	$ (0.88)	$ 2.03	$ (0.61)	$ (0.25)
Net income (loss) per common share - diluted (in dollars per share)	$ 0.51	$ 0.54	$ 0.49	$ 0.48	$ (0.92)	$ 0.77	$ 0.41	$ (0.88)	$ 2.03	$ (0.61)	$ (0.25)