Summary of Significant Accounting Policies - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Determination of shares:
Weighted-average common shares outstanding (in shares)	91,191,000	90,751,000
Assumed conversion of dilutive employee stock-based awards (in shares)	3,260,000	1,105,000
Assumed conversion of restricted stock (in shares)	199,000	61,000
Diluted weighted-average common shares outstanding before participating security	94,650,000	91,917,000
Anti-dilutive securities, options to purchase shares outstanding	646,307	4,545,585
Calculation of basic EPS:
Net income applicable to common stock	$ 45,437	$ 5,104
Weighted-average common shares outstanding (in shares)	91,191,000	90,751,000
Basic EPS (in dollars per share)	$ 0.50	$ 0.06
Calculation of diluted EPS using two class method:
Net income applicable to common stock	$ 45,437	$ 5,104
Diluted weighted-average common shares outstanding before participating security	94,650,000	91,917,000
Diluted EPS (in dollars per share)	$ 0.48	$ 0.06