Summary of Significant Accounting Policies - Basic and Diluted Net Income Per Share Calculations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic net income per share calculation:
Numerator - Net income	$ 100,321	$ 86,865	$ 194,281	$ 170,375
Denominator - Weighted average common shares outstanding (in shares)	76,343,608	77,123,854	76,572,120	77,101,816
Basic net income per common share (in dollars per share)	$ 1.31	$ 1.13	$ 2.54	$ 2.21
Diluted net income per share calculation:
Numerator – Net income	$ 100,321	$ 86,865	$ 194,281	$ 170,375
Denominator - Weighted average common shares outstanding (in shares)	76,344,000	77,124,000	76,572,000	77,102,000
Dilutive shares contingently issuable (in shares)	268,000	344,000	294,000	316,000
Weighted average diluted common shares outstanding (in shares)	76,611,657	77,468,082	76,866,163	77,417,930
Diluted net income per common share (in dollars per share)	$ 1.31	$ 1.12	$ 2.53	$ 2.20