Summary of Significant Accounting Policies - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Determination of shares:
Weighted-average common shares outstanding (in shares)	91,948,000	90,913,000	91,538,000	90,865,000
Assumed conversion of dilutive employee stock-based awards (in shares)	3,163,000	2,582,000	3,264,000	1,957,000
Assumed conversion of restricted stock (in shares)	223,000	94,000	213,000	81,000
Diluted weighted-average common shares outstanding (in shares)	95,334,000	93,589,000	95,015,000	92,903,000
Anti-dilutive securities, options to purchase shares outstanding	660,436	36,934	655,952	87,160
Calculation of basic EPS:
Net income applicable to common stock	$ 36,125	$ 789,340	$ 135,550	$ 811,523
Weighted-average common shares outstanding (in shares)	91,948,000	90,913,000	91,538,000	90,865,000
Basic EPS (in dollars per share)	$ 0.39	$ 8.68	$ 1.48	$ 8.93
Calculation of diluted EPS using two class method:
Net income applicable to common stock	$ 36,125	$ 789,340	$ 135,550	$ 811,523
Diluted EPS (in dollars per share)	$ 0.38	$ 8.43	$ 1.43	$ 8.74