Selected Quarterly Data (Unaudited) - Summary of Selected Quarterly Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 138,065	$ 152,210	$ 132,932	$ 129,028	$ 128,024	$ 123,675	$ 94,132	$ 79,776
Operating Income (Loss)	29,228	33,946	27,308	22,617	28,015	16,789	3,184	10,047	$ 113,099	$ 58,035	$ 50,814
Net income (loss)	1,093	27,931	23,229	23,827	5,643	20,523	28	13,745	76,080	39,939	48,763
Net Income (Loss) Attributable to Common Stockholders	$ 77	$ 24,913	$ 20,986	$ 21,216	$ 3,414	$ 16,708	$ (2,389)	$ 10,943	$ 67,192	$ 28,676	$ 48,502
Earnings (loss) per share—Basic (in $ per share)	$ 0.01	$ 3.47	$ 2.91	$ 2.95	$ 0.48	$ 2.32	$ (0.34)	$ 1.67	$ 9.37	$ 4.09	$ 6.34
Earnings (loss) per share—Diluted (in $ per share)	$ 0.01	$ 3.19	$ 2.75	$ 2.77	$ 0.46	$ 2.21	$ (0.34)	$ 1.62	$ 8.86	$ 3.96	$ 6.20