Summary of Significant Accounting Policies - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Determination of shares:
Weighted-average common shares outstanding (in shares)	91,468,000	90,928,000	91,330,000	90,840,000
Assumed conversion of dilutive employee stock-based awards (in shares)	3,319,000	2,230,000	3,299,000	1,629,000
Assumed conversion of restricted stock (in shares)	208,000	81,000	205,000	74,000
Diluted weighted-average common shares outstanding before participating security	94,995,000	93,239,000	94,834,000	92,543,000
Anti-dilutive securities, options to purchase shares outstanding	660,958	55,062	653,673	1,598,500
Calculation of basic EPS:
Net income applicable to common stock	$ 53,988	$ 17,079	$ 99,425	$ 22,183
Weighted-average common shares outstanding (in shares)	91,468,000	90,928,000	91,330,000	90,840,000
Basic EPS (in dollars per share)	$ 0.59	$ 0.19	$ 1.09	$ 0.24
Calculation of diluted EPS using two class method:
Net income applicable to common stock	$ 53,988	$ 17,079	$ 99,425	$ 22,183
Diluted weighted-average common shares outstanding before participating security	94,995,000	93,239,000	94,834,000	92,543,000
Diluted EPS (in dollars per share)	$ 0.57	$ 0.18	$ 1.05	$ 0.24