Summary of Significant Accounting Policies - Basic and Diluted Net Income Per Share Calculations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Basic net income per common share calculation:
Net income available to common shareholders	$ 106,847	$ 110,521	$ 197,647	$ 304,802
Denominator - Weighted average common shares outstanding (in shares)	75,240,664	76,301,010	75,417,663	76,480,757
Basic net income per common share (in dollars per share)	$ 1.42	$ 1.45	$ 2.62	$ 3.99
Diluted net income per common share calculation:
Numerator – Net income available to common shareholders	$ 106,847	$ 110,521	$ 197,647	$ 304,802
Denominator - Weighted average common shares outstanding (in shares)	75,241,000	76,301,000	75,418,000	76,481,000
Dilutive shares contingently issuable (in shares)	119,000	255,000	127,000	280,000
Weighted average diluted common shares outstanding (in shares)	75,360,033	76,556,309	75,544,677	76,761,167
Diluted net income per common share (in dollars per share)	$ 1.42	$ 1.44	$ 2.62	$ 3.97