Summary of Significant Accounting Policies - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Net income loss available to common stockholders
Net income	$ 17,079	$ 34,035	$ 22,183	$ 57,743
Net income applicable to preferred stock		3,151		5,452
Net income applicable to common stock	$ 17,079	$ 30,884	$ 22,183	$ 52,291
Determination of shares:
Weighted-average common shares outstanding (in shares)	90,928,000	81,647,000	90,840,000	81,308,000
Assumed conversion of dilutive employee stock-based awards (in shares)	2,230,000	1,474,000	1,629,000	1,459,000
Assumed conversion of restricted stock (in shares)	81,000	34,000	74,000	42,000
Diluted weighted-average common shares outstanding before participating security	93,239,000	83,155,000	92,543,000	82,809,000
Assumed conversion of preferred stock (in shares)		8,331,000		8,478,000
Diluted weighted-average common shares outstanding (in shares)	93,239,000	91,486,000	92,543,000	91,287,000
Calculation of basic EPS:
Net income applicable to common stock	$ 17,079	$ 30,884	$ 22,183	$ 52,291
Weighted-average common shares outstanding (in shares)	90,928,000	81,647,000	90,840,000	81,308,000
Basic EPS (in dollars per share)	$ 0.19	$ 0.38	$ 0.24	$ 0.64
Calculation of diluted EPS using two class method:
Net income applicable to common stock	$ 17,079	$ 30,884	$ 22,183	$ 52,291
Diluted weighted-average common shares outstanding before participating security	93,239,000	83,155,000	92,543,000	82,809,000
Diluted EPS (in dollars per share)	$ 0.18	$ 0.37	$ 0.24	$ 0.63
Anti-dilutive securities, options to purchase shares outstanding	55,062	1,696,858	1,598,500	2,889,501
Series C Preferred Stock
Earnings Per Share
Preferred stock, shares outstanding	0	7,447	0	7,447	0